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Trees, People and the Built Environment - Proceedings of the Urban Trees Research Conference, 13-14 April 2011

Research Report
Trees, people and the built
Proceedings of the Urban Trees Research
Conference 13–14 April 2011
Trees, people and the
built environment
Proceedings of the Urban Trees
Research Conference 1314 April 2011
Hosted by
The Institute of Chartered Foresters
The Clarendon Suites,
Edgbaston, Birmingham, UK
Edited by
Mark Johnston and Glynn Percival
Forestry Commission: Edinburgh
Research Report
© Crown Copyright 2012
You may re-use this information (not including logos) free of charge in any format or medium, under
the terms of the Open Government Licence. To view this licence, visit: or write to the Information Policy Team,
The National Archives, Kew, London TW9 4DU, or e-mail:
First published in 2012 by Forestry Commission, Silvan House, 231 Corstorphine Road, Edinburgh EH12 7AT.
ISBN 978-0-85538-849-2
Johnston, M. and Percival, G. eds. (2012).
Trees, people and the built environment.
Forestry Commission Research Report.
Forestry Commission, Edinburgh. i–vi + 1–258 pp.
Keywords: Trees; urban forests; green infrastructure; sustainability; built environment; ecosystem services.
Enquiries relating to this publication should be addressed to:
Forestry Commission
231 Corstorphine Road
EH12 7AT
T: 0131 334 0303
If you need this publication in an alternative format, for example in large print or
in another language, please contact the Forestry Commission Diversity Team at the
above address. Telephone: 0131 314 6575 or email:
The editors can be contacted at:
General enquiries relating to the conference can be sent to:
Institute of Chartered Foresters
59 George Street
T: 0131 240 1425
Introduction to the Conference by Mark Johnston, Conference Chair 1
Message to delegates from HRH The Prince of Wales 3
Opening address by Pam Warhurst, Chair of the Forestry Commission 5
Plenary session 1 – Management of the urban forest
Using urban forestry research in New York City 9
Matthew Wells
Measuring the ecosystem services of Torbay’s trees: the Torbay i-Tree Eco pilot project 18
Kenton Rogers, David Hansford, Tim Sunderland, Andrew Brunt and Neil Coish
A framework for strategic urban forest management planning and monitoring 29
Philip van Wassenaer, Alexander Satel, Andrew Kenneyand Margot Ursic
Parallel session 1a – Tree planting and establishment
Results of a long-term project using controlled mycorrhization with specific fungal strains on different urban trees
Francesco Ferrini and Alessio Fini
Fundamentals of tree establishment: a review 51
Andrew Hirons and Glynn Percival
Fifteen years of urban tree planting and establishment research 63
Gary Watson
Parallel session 1b – Promoting green networks and human wellbeing
Exploring the role of street trees in the improvement and expansion of green networks 73
Norman Dandy, Mariella Marzano, Darren Moseley, Amy Stewart and Anna Lawrence
Promoting wellbeing through environment: the role of urban forestry 84
Kathryn Gilchrist
Flourishing trees, flourishing minds: nearby trees may improve mental wellbeing among housing association 94
Adam Winson
Parallel session 2a – Trees and urban climate challenges
The use of trees in urban stormwater management 104
Elizabeth Denman, Peter May and Gregory Moore
Quantifying the cooling benefits of urban trees 113
Roland Ennos
Parallel session 2b – Energy supplies and other management challenges
Advances in utility arboriculture research and the implications for the amenity and urban forestry sectors 119
Dealga O’Callaghan
Challenges and problems of urban forest development in Addis Ababa, Ethiopia 130
Eyob Tenkir Shikur
Plenary session 2 – Governance of the urban forest
Innovations in urban forest governance in Europe 141
Cecil Konijnendijk
Governance and the urban forest 148
Anna Lawrence and Norman Dandy
Parallel session 3a – Trees and urban design
Does beauty still matter? Experiential and utilitarian values of urban trees 159
Herbert Schroeder
Urban trees and the green infrastructure agenda 166
Martin Kelly
Parallel session 3b – Multipurpose management and urban futures
‘Natives versus aliens’: the relevance of the debate to urban forest management in Britain 181
Mark Johnston, Sylvie Nail and Sue James
Strategies for exploring urban futures in, and across, disciplines 192
Robert MacKenzie, Thomas Pugh, Matthew Barnes, James Hale and the EPSRC Urban Futures Team
Parallel session 4a – The value of communities in successful urban greening
Working with communities to realise the full potential of urban tree planting: a sustainable legacy
(The research is ongoing and a paper was not available for publication)
Katie Roberts
Community participation in urban tree cover in the UK 202
Mike Townsend, Sian Atkinson and Nikki Williams
Parallel session 4b – Resolving conflicts with urban infrastructure
Investigation into the interactions between closed circuit television and urban forest vegetation in Wales 210
Stuart Body
A review of current research relating to domestic building subsidence in the UK: what price tree retention? 219
Stephen Planteand Margaret MacQueen
Closing address by Peter Head, Consultant to Arup 228
Urban/rural ecology in the transition to the ‘ecological age’
Appendix 1: Conference organisation 232
Appendix 2: Biographies for speakers and chairs 233
Appendix 3: Poster exhibition 243
Appendix 4: Delegates list 246
Appendix 5: Conference programme 254
Appendix 6: Conference sponsors 258
Introduction to the Conference
Our urban forests, the trees and woodlands in and around our cities, have a vital role to
play in promoting sustainable communities. As the most important single component of
green infrastructure these trees can provide numerous environmental, economic and
social benefits, contributing enormously to the health and welfare of everyone who lives
and works in the urban environment. As concerns grow about the quality of the urban
environment in many towns and cities throughout the world, the importance of
protecting and expanding our urban forests can only increase.
Urban forestry itself can be defined as a planned, systematic and integrated approach to
the management of our urban trees and woodlands. It was a desire to emphasise that
third element, the integrated approach, which was the initial driving force behind the
development of this conference. Let me explain the background.
Back in the 1980s and 1990s, a series of Arboricultural Research Conferences were held in Britain, supported by the Forestry
Commission. I was fortunate to attend some of those events along with many tree officers, tree consultants, academics,
researchers and others. Although widely regarded as providing arboriculturists and some landscape practitioners with highly
relevant information about current research on both urban and rural trees, for some reason they did not continue. However,
in those research conferences and in many other arboricultural events I have attended in recent years, there was one
fundamental weakness. Invariably at these events, it was just ‘tree people’ talking to ourselves. Those professionals who really
had such an impact our work – the landscape architects, engineers, surveyors, architects, ecologists, conservationists and
others – were just not there or at least very thin on the ground.
I have always been keen on the idea of resurrecting those early research conferences but this time with some crucial
differences. After sharing my thoughts on this with a few close colleagues, a small group of us decided to make our ideas a
reality. Right from the outset, we agreed on two crucial points about our proposed research conference. First, we believed
the focus should be specifically on urban trees, to reflect the vital role that our urban forests can play in creating healthy and
sustainable town and cities. The conference would ‘showcase’ the very latest research on the subject of urban trees and the
management of the urban forest. Secondly, and most importantly, we needed to reach out to all those other professionals,
apart from arboriculturists, that have such a major impact on the urban forest. Fortunately, the recently formed Trees and
Design Action Group (TDAG) had already made a significant start down that road by providing a forum where natural and
built environment professionals could engage with each other on issues relating to trees in the urban environment. Building
on TDAG’s established contacts, we invited a wide range of relevant organisations to nominate representatives to join a
steering group to lead the development of the proposed conference.
The first meeting of the Conference Steering Group took place in Birmingham in January 2010 attended by 12
representatives of relevant professional bodies and other organisations. There was considerable enthusiasm for the idea
of the conference from all present and some very useful suggestions on how to develop the research aspects of this.
However, there was no consensus on how the event could be organised or when it could be held. After the meeting,
support for the proposed conference continued to grow rapidly but no individual organisation appeared keen to take a
lead and offer substantial material support to ensure it would happen. It was at this point that the Institute of Chartered
Foresters (ICF) stepped forward. The then President of ICF, Bill MacDonald, was quick to recognise the importance of
holding this conference, and the value of the partnership of organisations that had already agreed to support it.
Consequently, ICF made an offer to the Steering Group to host the event as its National Conference for 2011. The
Steering Group would continue to be responsible for deciding the conference programme and other academic aspects
of the event, while ICF would provide the administrative and other support required. The Steering Group readily agreed
to this proposal.
Another important factor in enabling the Steering Group to deliver the conference was the early and significant support of
the Forestry Commission. Not only did it play a crucial role in facilitating the event itself, it also undertook to publish the
conference proceedings, thus ensuring that there would be a permanent record of all the vital research that was being presented.
We were also fortunate in gaining support for the conference from HRH The Prince of Wales, a very prominent champion for
trees and a sustainable urban environment. Although HRH was unable to attend the event in person, due to other commitments
around that time, he was able to send a very pertinent and personal message of support to the conference delegates.
When the conference was eventually held in April 2011 it was an outstanding success. With nearly 400 delegates, it was
one of the largest tree conferences ever held in Britain. Most importantly, the conference achieved its main aim of
including the other relevant non-tree professional bodies, particularly from the built environment sector. A number of
senior figures from these bodies acted as Session Chair for parts of the conference and there were a significant number of
their members as delegates.
The success of the conference was due to the efforts of many different organisations and individuals, and too numerous to
mention everyone individually. However, I want to thank the members of the Conference Steering Group who represented
the various partner organisations. Without their support, commitment and hard work, we would not have been able to
maintain that unique partnership of relevant organisations. And without their efforts to promote the conference to their
members we would not have had anything like the number of delegates we achieved.
On behalf of the Conference Steering Group, I want to thank the ICF whose vision and leadership in offering to host the
event was pivotal in ensuring it actually happened. In particular, we want to thank Allison Lock and her team at ICF for the
very professional way in which they delivered the organisational aspects of the conference. For many of those attending, this
was their first experience of an ICF organised event and a great many subsequently commented on how well the event
reflected on the standing and professionalism of the ICF.
Lastly, on a personal note, I want to thank two individuals who played a vital role in the success of the whole conference.
They are Keith Sacre of Barcham Trees and Sue James of TDAG. Without their enthusiasm, commitment and expertise, much
of what we achieved would not have been possible. They not only played a crucial role as members of the Steering Group,
they also gave me invaluable support and encouragement at those times when I was in danger of being overwhelmed by the
task of ‘keeping the show on the road’.
There can be no doubt that this urban trees research conference was a remarkable success. The event itself and the quality of
the papers in the conference proceedings are testament to that. However, ultimately, it should be judged on what lasting
impact it has on developing a more integrated approach to the planning and management of our urban forests. An excellent
start has been made but everyone involved in the conference must ensure that those gains are consolidated and built on.
One way might be to organise another research conference in the future. Another is to support the continuing work of TDAG.
Mark Johnston
Conference Chair and Chair of the Conference Steering Group
Message to delegates from HRH The Prince of Wales
Opening address
I’m really, really pleased to be here because this is heart and mind stuff for me.
When I spoke at your [the ICF conference] dinner last year, I said I believe that
we’ve got a huge opportunity if collectively we pull together around this
environmental agenda, across the sector. Forget our differences and play to our
strengths. Try and influence the way people are thinking so that they buy-in to
the importance of trees in society, to the importance of diverting funding to
make sure that we have a greener world – a better world to pass on to our kids.
Well, 12 months ago who would have thought we’d have had the few months
that we’ve just had? Who would have thought that trees, forest and woodlands
would have been front page, the biggest item in any MP’s mail, interviews right and left and centre. The passion of the
people coming through? Who would have thought that we’d have seen people collecting together in really cold conditions
in their thousands to make their point and say: ‘trees, woodlands and forests matter to us’? Who would have thought that
forestry would be the debate around bars and coffee shops as well as around Westminster to the extent that it has been?
Who would have thought that we could have ignited that degree of passion in a nation around our trees?
I’m so pleased that that happened. I’m delighted that the nation spoke. It was the start of a conversation, but it was also only
the beginning, because for me one of the really important outcomes that has to come from that sort of national focus is the
change in what we spend our money on, in our personal lives, in our everyday lives, in our working lives, and at a national
budget level.
For me, what really matters is that we don’t only think of our heritage forests – really important though our heritage forests
are, though I defy you to define that – but also about those woodlands, and those trees in our parks, on our streets, and on
the edges of our towns and cities. They are the heritage woodlands for the people that live there. Where was the debate
around that? I didn’t hear much of it.
I think what I’d like to hear at the end of these two days is a consensus in the room that we are going to cruise on that
fabulous wave of national support that we have for woodlands, trees and forests and push it like mad, personally and
professionally, to make sure that this is a watershed moment in how we think about our environment and trees within that
environment from now on.
I come from the north of England, you can tell. I’ve worked with people in the Mersey Forest and the Red Rose Forest, and
very recently in the White Rose Forest. I used to be a leader of a council pressing for more green spaces in our towns before
it was fashionable to do that.
I also used to be the Chair of a health trust which made me passionate about the work that we are doing at the Forestry
Commission with the NHS Forest, to make sure that our health centres are also environmental health centres. That the charitable
monies held within those fabulous institutions aren’t only spent on what’s happening inside, but what’s happening outside.
I chair something called ‘Incredible, Edible Todmorden’. I have to mention that. We want more orchards. We want all our
schools to have trees surrounding them. We want to make sure that every health centre is surrounded by orchards. We want
to make sure that every tenant on every estate has access to land to grow what that tenant wants to grow. We want to bring
the woodland into the heart of our towns and our cities wherever they might be.
In all these organisations I have seen the importance of the environment to all our lives. At the Forestry Commission I’m
terribly proud of the work that we do: the work that we do on education, the work that we do on reconnecting people to
our environment, and the standards that we set, and help others to work to, to make sure that we are delivering sustainable
woodland and forestry management across the piece.
We’re not going to stop doing that. That is our core business. To make sure that we work effectively in the future in
partnership across our public forest estate so that those wonderful woodlands and forests that people stood up and were
counted for are maintained in perpetuity for our children and continue to deliver the public benefits that they do today.
We will continue to do that but, more and more, we need to have a dialogue with many more people across the length and
breadth of this country. It’s really important that we take the message about rethinking investment plans, rethinking
management plans from the very heart of our cities right out into our deepest countryside, beyond the bodies represented
in this room today.
Whilst we’re here together, environmentalist, tree people, we get a real buzz. We think it’s really funky, and that most people
think the environment is great. Well that’s not how the world is because there’s a load of people out there who don’t share
our passion. There’s a load of people out there who have a deficit to deal with. There’s a load of people out there who’ve had
to make a lot of people redundant. There’s a load of people who think there are more important things to deal with than
trees. We need to show them that the environment and these difficult challenges are not mutually exclusive.
We’ll be hearing lots today about examples all over the globe where passion for trees on our streets in our towns and cities
can lead to a better understanding of the environment, and that’s what we need. More people understanding environmental
wellbeing equates to their own wellbeing. If there’s one thing that drives me at the moment, it’s not the aesthetic; it’s the
survival of this planet.
At the end of the day we need ideas of how we can inspire more people from tenements, from our villages, our hamlets,
from the Manchesters, the Birminghams and the Cardiffs of this world, to get the importance of their environment. I would
like people to sign up to a 38 Degree poll that asks what are we doing about climate change? What are we doing about
investing in the smartest, greenest resource we have? How will we make a difference to our kids’ futures?
What are we actually doing about that? Taking the heart, marrying it with the minds and creating a drive and a movement
that says collectively we have a real opportunity to make a difference to our quality of life, not just today, but tomorrow.
We all know that trees, woodlands, forests, orchards, whatever they might be, have a fabulous impact on the way we feel.
We’re mapping happiness at the moment. Did you hear about that the other day: ‘mappiness’? It’s really great. You map how
people feel in different areas and then you ask: ‘What sort of area was that?’ Do you know when people feel great? When
they see trees, when they’re in forests, when they’re in woodlands, when they’re in parks. That’s when they feel great. It might
sound a bit tree-huggy for some of you in this room, but the thing for me that’s important is that David Cameron [Prime
Minister] thinks it’s great, and that’s good.
We need to recognise that and not be too snobby about it. Recognise that we need a hook into mappiness when we’re
telling our story. What we are missing is that drive and passion at a grass roots level over and beyond the 38 Degrees.
People don’t live their life in silos. If they feel good about something, if they feel great about a product, that’ll affect their
spend. If something makes them happy and they want to repeat that experience, that will change what they vote for, and
what they vote for will allow us to put the environment centre stage, and have the sorts of uplift that Professor Read in
his report on climate change demands of us, of all of us. It’s not, ‘well I would if I could but I’m really pressed at the
moment’. While our personal circumstances are being challenged, the planet, the ability for us to survive, our
environment, is slipping through our fingers.
So, what really matters is we listen to the people. We see the opportunity to build on that passion. We extend that dialogue
collectively with them. We help them to see it’s not just about the heritage forest, but it is about the woodlands and it is
about the town centre places, and it is about the community forest.
And it’s not all about money. I have never worked in a public body – and I’ve worked in them for 20 years – that ever had
any money whether it was a local authority or whatever. Of course it was really hard, but it was also great because I would
say to somebody, what would be really fabulous is if you came along with me and I used a bit of your budget and you used
a bit of my budget and that led to us thinking differently. We each gave a little bit, and we got a really creative solution.
I need to see change. We need to see change. We know everything we need to know about what needs doing. We just need
the will to do it.
So, for me, what’s really important today is that you, the ICF, have had the leadership and the foresight to bring together
people from a range of backgrounds whose common focus is their passion and their knowledge and their experience about
trees and their importance and how to manage them sustainably.
We are, in this room, one sector. We need to talk with one voice. We need to be clear what our message is to those with
influence. We need to be clear how we are going to communicate that message to the general public. We have the
advocates in this room. Some can do it at a government level. Some can do it in an area forum. Some can do it at planning
committee. There’s all sorts of champions in this room. We need during the course of the next two days to find the
mechanisms to allow them to function, to allow them to inspire, to allow them to make the difference.
I believe that we can do it. I believe we have to do it. I think we have examples of great practice all over the place that
instead of just packing and putting on a shelf, we need to share proactively.
There’s no certainty in these things, but the one thing that is certain is that we cannot miss the opportunity to come up with
some really positive messages at the end of these two days. To say: ‘Do you know what they’re doing in New York, know
what they’re doing in Canada, why can’t we do that? I’m going to go back and speak to the leader of council or the chair and
do something about that’. If we missed that opportunity to really raise our games individually, then collectively we will have
let a truly historic moment slip through our fingers.
There are several programmes at present that can help us. We’ve got the Woodland Carbon Task Force looking at ways of
getting more investment in our woodlands. We’ve got The Big Tree Plant. So needed, but also so in need of funding.
We’ve got the Independent Panel on Forestry. I’m a big fan of the Independent Panel actually. That might seem a strange
thing for me to say, but I believe we have an important platform in the panel to raise the profile of trees again and help
continue the public dialogue we all want. And I think we stand a chance of having some really interesting recommendations
that we can start to work on together.
So, well done for calling this conference together; it’s been a long time in the coming.
The Forestry Commission has been through the mill, as have many of you in this room in the last few months. But we are as
committed and as passionate as we always have been to make sure that the importance of trees becomes centre stage in
people’s lives, and that the knowledge that we have and the experience that we have is shared collectively, not just on the
Forestry Estate but throughout the sector. Not just with traditional friends, but through the International Year of the Forest
with a much broader church. I am committed to make that happen.
From local government countryside officers, landscape planners, foresters, from deliverers of community forests, from
politicians to policymakers, without you standing up and being counted on this issue, it simply won’t happen.
What I said last year is: ‘I’m up for it if you’re up for it’. If you want to make a difference, want to have your messages heard, I
want to help you deliver those. We can deliver those. It isn’t politically contentious. It’s a survival plan. So, let’s get on with
some great futures, and let’s make sure that we see this as the watershed moment that it is.
Thank you very much.
Pam Warhurst
Chair, Forestry Commission
Using urban forestry research in New York City
Until recently the benefits of trees were well known but not well defined or quantified. The US Forest Service has released a
number of exceptional analytical tools that allow urban forest managers to generate dollar figures for the benefits being
generated by their city or town’s trees. The New York City Department of Parks & Recreation (NYC DPR) successfully used
two of these tools, Urban Forest Effects Model (UFORE) and Street Tree Resource Analysis Tool for Urban Forest
Managers (STRATUM) to calculate the benefits provided to New Yorkers by the estimated 5.2 million trees in the city.
These figures persuaded Mayor Bloomberg that trees should be a vital component of PlaNYC, his plan for a greener,
greater New York. Initiatives involving trees are included in three of the plan’s five key policy areas for the urban
environment. Trees have instrumental roles to play in greening the landscape, cleaning the air, reducing energy use and
capturing stormwater. Consequently, PlaNYC led to massive increases in the urban forestry budget as NYC DPR is tasked
with planting 220000 streets trees and reforesting 809 hectares of parkland. Aside from justifying greater urban forestry
resources, research has also played a crucial role in setting policy and directing programming to ensure that these
resources are deployed to maximum effect.
Urban forestry managers have continually strived to find the precarious equilibrium between
the needs of trees and the needs of people. Often the pressures of liability and limited resources
have forced these managers to focus solely on tree maintenance and tree removals. There
has been some excellent research completed in the fields of tree mechanics and hazard tree
evaluation. This research has been coupled with numerous studies on the social and
psychological benefits of humans interacting with their natural environment. However, this
arboricultural and social research has a limited use for urban forest managers battling to
holistically manage a diverse resource at a city or town level. Only recently have urban forest
managers had more to help them secure funding and guide urban forest programming.
The US Forest Service has recently released a number of free useful tools for urban forest
managers. These tools allow urban forest managers to quantify the annual environmental
benefits provided to their town or city by their urban forest. These quantified environmental
benefits have allowed policy makers to understand and appreciate the urban forest. These
tools have very much put trees on the policy map.
The New York City Department of Parks and Recreation (NYC DPR) has used two of these
tools to analyse the city’s urban forest. The Urban Forest Effects Model (UFORE) calculated
the environmental benefits of the entire urban forest, while the Street Tree Resource Analysis
Tool for Urban Forest Managers (STRATUM) focused solely on the street tree population.
NYC DPR coupled the results of these tools with other pertinent research to justify the
inclusion of trees into Mayor Bloomberg’s sustainability plan for New York City (NYC) called
PlaNYC. In PlaNYC, trees play a major role in greening the landscape and are also being
actively deployed in helping to capture stormwater and cleaning the air. Their inclusion was
only possible through NYC DPR being able to prove and quantify the annual environmental
benefits provided by them. However, the research did not only justify why additional
resources should be allocated into the urban forest. This research also provided key
information that allowed proper attainable urban forest goals, policies and strategies to be
established to maximize the benefits of New York’s urban forest.
benefits, PlaNYC, quantify,
Plenary session 1: Management of the urban forest 9
Matthew P. Wells
Central Forestry and
Horticulture, New York City
Department of Parks &
Recreation, USA
10 Trees, people and the built environment
This paper will look at the key research studies and how they
have been used to justify and focus urban forestry
programming in NYC. Alongside this central theme will be
the importance and power of in-house collection of
administrative data and its analysis. NYC DPR has very
successfully used in-house resources, volunteers and interns
to help perform vital research.
The social value of the urban
forest and urban trees
The social value of the urban forest has been well
researched, although these studies have not been able to
quantify this value in dollars. It is understood that views of
trees and nature are known to help improve mental
wellbeing (Kaplan and Kaplan, 1989) and also help with
recovery from illness (Ulrich, 1984). It has been shown that
humans derive pleasure from trees (Lewis, 1996). Other
research has also shown that outdoor spaces with trees
facilitate greater interactions among local residents, which
improves neighbourhood socializing (Kou et al., 1998). This
research is fascinating and very valuable and reinforces what
many of us have always instinctively believed about trees
and the urban forest. However, these social values alone do
not provide the strongest justification or argument for urban
foresters trying to preserve existing trees or find resources to
plant new ones, especially if liability is also a concern.
Only when more recent research emerged that started to
quantify the environmental benefits and the associated
financial value provided by the urban forest did trees become
an essential element in a city rather than just a feel-good luxury
item. A great deal of this research has been done by the US
Forest Service (McPherson et al., 2007; Nowak et al., 2007;
Peper et al., 2007). They provide a number of free tools for
urban forest managers via their i-Tree software suite. Two of
these tools, the Urban Forest Effects Model (UFORE) and the
Street Tree Resource Analysis Tool for Urban Forest Managers
(STRATUM), have been invaluable to urban foresters in NYC,
especially when combined with other relevant research.
Research on the entire urban
forest in New York City
New York City (NYC) is America’s largest metropolis and
home to an estimated 8.2 million people (US Census
Bureau, 2006). NYC is extremely urban in its environment
and even though it is home to one of the most famous
parks in the world, Central Park, it is not otherwise known
for its trees and open spaces.
The Urban Forest Effects Model (UFORE)
The U.S. Forest Service completed a UFORE (now called i-
Tree Eco) survey and analysis of NYC’s entire urban forest
in 1996, and estimated that it contained 5.2 million trees
(Nowak et al., 2007). This was somewhat of a surprise.
Furthermore, the UFORE study put the structural value of
NYC’s urban forest at $5.2 billion and estimated that 50%
of the urban forest fell under the jurisdiction of the New
York City Department of Parks and Recreation (NYC DPR).
UFORE also estimated that NYC had a 20.9% tree cover,
with 42.7% of the trees being over 6 inches (15.25 cm) in
diameter. But perhaps the most interesting findings were
the environmental benefits the urban forest was delivering
to New Yorkers. The urban forest worked to remove 1998
tonnes of air pollution each year at an annual value of
$10.6 million and stored 1.22 million tonnes of carbon at
an estimated value of $24.9 million. Finally, the urban
forest was sequestrating 38 374 tonnes of carbon annually
at an annual value of $779 000. It should be noted that
despite all this impressive data, the UFORE study
acknowledged that additional social and environmental
benefits were not included. These key figures about NYC’s
urban forest immediately provided NYC DPR with a reason
to request additional resources for forestry. Ultimately, a
federal agency had proved that NYC’s urban forest was
providing substantial and valuable environmental benefits
to the city.
The UFORE report was more that just a report on
environmental benefits. It also provided essential data to
aid in the correct management of the urban forest. It
identified the most common species as being tree of
heaven (Ailanthus altissima) at 9.0%, black cherry (Prunus
serotina) at 8.1% and sweetgum (Liquidambar styraciflua) at
7.9% (Nowak et al., 2007). It also confirmed what many
already assumed, that large-canopied trees, provide the
greatest benefits, with ironically the London plane (Platanus
xhispanica) having the greatest importance in NYC based
on total leaf area and abundance. UFORE also helped us
understand the potential threat of the invasive Asian
longhorned beetle (ALB) to NYC. ALB was discovered in the
NYC borough of Brooklyn in 1996 and this was actually the
first time it had been discovered on the US mainland. ALB is
a beetle that destroys certain species of trees through
boring damage. UFORE concluded that 43.1% of the urban
forest was potentially at risk from ALB. This knowledge
made federal, state and city agencies very aware of the
implications of ALB for NYC as $2.25 billion worth of trees
were potentially at risk.
Plenary session 1: Management of the urban forest 11
Urban tree canopy coverage
In April 2006, NYC DPR commissioned the US Forest
Service and the University of Vermont’s Spatial Analysis
Laboratory to conduct an analysis of urban tree canopy
(UTC) coverage in the city. NYC DPR wanted to understand
if achieving an UTC goal of 30% by 2030 was possible. The
completed research established that 24% (17 972 hectares)
of NYC’s total land area was already covered by UTC (Grove
et al., 2006). The study also calculated that 42% (32 052
hectares) of the city’s total land area had the potential to be
covered by UTC because no roads or buildings were
present. The report concluded that a goal of 30% UTC by
2030 was achievable if 4856 hectares were added. The
report also recommended that progress towards attaining
this UTC goal should be monitored by using remote
sensing at five-year intervals.
Research on street trees in New
York City
Street trees are perhaps the most visible and easily defined
component of any urban forest. They are the trees outside
people’s homes and places of work that touch their lives on
a day-to-day basis. Street trees therefore usually require the
most intensive management by urban foresters and their
location tends to make them the ones that people are most
interested in for either positive or negative reasons. They are
the public face of trees.
The 2005–2006 street tree census
Every decade the NYC DPR undertakes a census of the street
tree population. The last census undertaken in 2005–2006
was called ‘Trees Count’. The census was conducted with the
help of more than 1100 volunteers logging over 30000
hours (New York City Department of Parks & Recreation,
2007). This level of participation represented a 57% increase
from the previous census in 1995–1996 where only 700
volunteers participated. Volunteers were required to attend
a three-hour training session and collected 42% of the
census data. The remainder was completed by in-house staff
and by an urban forestry consultant.
The census collected over 15 million pieces of data across
the five boroughs. To facilitate the data collection, the city
was divided into 1649 survey zones that were assigned to
the individuals taking part in the census. For each tree
counted, the surveyor recorded information such as
location, species, diameter at breast height (dbh), condition,
tree pit type, soil level, sidewalk condition, presence of
overhead wires and infrastructure conflicts. Survey results
were reported back to NYC DPR using an interactive census
website application or on paper.
The published results of the tree census identified 592130
street trees in NYC; this represented a 19% increased from
the census a decade earlier (New York City Department of
Parks & Recreation, 2007). London plane was the most
prominent species making up 15.3% of the population with
Norway maple (Acer platanoides) not far behind at 14.1%.
Other important species were Callery pear (Pyrus calleryana)
at 10.9%, honey locust (Gleditsia triacanthos) at 8.9% and pin
oak (Quercus palustris) at 7.5%. This data immediately
highlighted that NYC needed greater species diversification
and no one species should really exceed 10% of the total
population (Peper et al., 2007).
Table 1 shows the tree condition results of the census and
Table 2 shows the size of the trees. The census data provided
a good snapshot of the entire street tree population within a
relatively small time band. This is not achieved when
surveying a portion of the street tree population on an
annual basis over multiple years.
Other interesting information that came out of the census
was that 15% of the tree population suffered from trunk
wounds and 5.3% had a cavity of some type. Finally, the
census highlighted some of the key conflicts that NYC’s tree
population has with infrastructure (see Table 3).
Tree condition Percentage of the population
Excellent 23.9%
Good 66.4%
Poor 8.3%
Dead 1.4%
Tree size Percentage of the population
Small (0–15 cm) 25%
Medium (15–46 cm) 50%
Large (46–76 cm) 20%
Extra large (over 76cm) 5%
Table 1 Tree condition results of the 2005–2006 tree census (New York
City Department of Parks & Recreation, 2007).
Table 2 Tree size results of the 2005–2006 tree census (New York City
Department of Parks & Recreation, 2007).
12 Trees, people and the built environment
The number of trees impacted by urban conflicts in NYC is
considerable (Table 3). Therefore, mitigating these street
tree conflicts with infrastructure, as far as reasonably
possible, is a key challenge for NYC DPR. The census
recorded that nearly 36% of the population was under wires
and could be subjected to utility clearance pruning. The
census also identified that 17.3% of the trees surveyed had
raised adjacent sidewalk and 11.2% of the population had
cracked adjacent sidewalk. In NYC property owners are
responsible for the maintenance of the sidewalk adjacent to
their land (New York City Department of Transportation,
2008). Damaged sidewalks and the disturbance of utility
wires are often cited as a reason for requesting removal of
a tree or protesting against the planting of a new one. The
authors of recent research analysed complaints to NYC DPR
about the placement of new tree planting. A total of 33% of
these complainants objected because of the potential of the
tree to cause utility service disturbance and 14% objected
because of the potential of future sidewalk damage (Rae et
al., 2010). These are obviously both significant factors when
considering urban forestry programming and the concerns
of property owners.
The tree census data allowed NYC DPR to consider their
street tree inventory at a borough level and the change in
that inventory since the census in 1995–1996 (Table 4).
The census clearly showed that certain boroughs had
considerably more trees than others, as detailed in Table 4.
It can be seen that Staten Island had the greatest rise in its
street tree population since 1995–1996 with a 33%
increase. Manhattan had the least with just a 9% increase
and Queens was not far behind at only a 10% increase. The
census data also identified that London plane was the most
common species citywide, but is only the dominant species
in Brooklyn (24%) compared to honey locust in the Bronx
(13%) and Manhattan (23%), Callery pear in Staten Island
Urban conflict Number of trees Percentage of the
Overhead wires 209 171 35.8%
Raised sidewalks 100 829 17.3%
Cracked sidewalks 65 299 11.2%
Close paving 43 409 7.4%
Choking wires 13 865 2.4%
guard/grate 3918 0.7%
Tree lights 3918 0.4%
(25%) and Norway maple in Queens (18%)
(New York City
Department of Parks & Recreation, 2007).
The tree census also identified other borough trends in tree
health and infrastructure conflicts. The Bronx’s tree
population was in the worst condition, with 12% of trees
falling into the dead or poor condition categories, followed
closely by Manhattan at 11.3% and Queens at 10% (New
York City Department of Parks & Recreation, 2007). Staten
Island’s trees were recorded as being in the best condition
with only 6% of trees falling outside the good and excellent
tree condition categories. As stated previously, 36% of the
total citywide tree street population was recorded as being
under utility wires. However, when we look at this
percentage at a borough level, it rises significantly to 48% in
Queens but falls back to 23% in Staten Island and is lower
still in the Bronx at 12%. In summary, management policies
should account for the distinct differences in the urban
forest even within a single city or town.
Street Tree Resource Analysis Tool for
Urban Forest Managers (STRATUM)
STRATUM (Street Tree Assessment Tool for Urban Forest
Managers) is now known as i-Tree Streets and is another
application available from the US Forest Service. STRATUM
uses street tree inventory data to calculate the annual
environmental and aesthetic benefits generated. It is
distinctly different from UFORE because it does not
consider the urban forest in its entirety. The STRATUM
model is more accurate in its results compared to UFORE
because the size, species and condition of each and every
tree is known. It is possible to perform a STRATUM analysis
using just a sample of the street tree population (Kling,
2008), although this was not done in NYC. The quantified
benefits calculated by STRATUM include energy
conservation, air quality improvement, carbon dioxide
Borough 1995–1996
census % increase
Bronx 47 995 60 004 25%
Brooklyn 112 400 142 747 27%
Manhattan 45 793 49 858 9%
Queens 217111 239 882 10%
Staten Island 75 171 99 639 33%
Totals 498470 592 130 19%
Table 3 Trees with urban conflict results of the 2005–2006 tree census
(New York City Department of Parks & Recreation, 2007).
Table 4 Number of trees recorded per borough in the 2005–2006 tree
census versus 1995–1996 (New York City Department of Parks &
Recreation, 2007).
Plenary session 1: Management of the urban forest 13
reduction, and stormwater catchment. The model also
looks at the aesthetic contribution of street trees in terms of
increasing property value.
STRATUM analysis for a city could cost more than $100 000
to survey and analyse growth data for 800 trees (Kling,
2008). So that this cost would not be prohibitive, the US
Forest Service split the USA mainland into 16 climatic
zones. Within each zone, an in-depth analysis has taken
place at a single reference city. The reference city research
involves detailed data collection on 30–60 trees for each of
the predominant 20 species. NYC is the reference city for
the Northeast region. The concept is that any city or town
within a particular zone can then feed their street tree
inventory data into the model to produce a fairly accurate
calculation of the aesthetic and environmental benefits of
their tree stock without the associated cost of having their
own individual analysis done by the US Forest Service
(Kling, 2008).
In 2007, the US Forest Service’s Center for Urban Forest
Research produced a STRATUM report for NYC DPR’s
Commissioner Adrian Benepe (Peper et al., 2007). This
STRATUM analysis calculated that the street tree population
of NYC, identified in the 2005–2006 tree census, provided
an estimated $121.9 million in annual benefits. This
translates to $209 per tree. These benefits are broken down
in Table 5 below:
At the time of the report NYC DPR estimated that it spent
$21.8 million annually on planting new trees and maintaining
existing street trees (Peper et al., 2007). Therefore, the street
tree population provides $100.2 million or $172 per tree in
net annual benefits to the city. It can also therefore be
deduced that for every $1 spent on tree care operations, the
city receives $5.60 in benefits. Aside from these benefits,
STRATUM also estimated the replacement costs of the NYC
street tree population at $2.3 billion or $3938 per tree
(Peper et al., 2007).
Justifying greater resources
through research
A greater appreciation of the value and functions of an
urban forest can be used to justify increased support and
resources for its correct management (McPhearson et al.,
2010). In NYC the quantified figures for environmental
benefits produced by UFORE and STRATUM have been
invaluable and very influential. NYC DPR’s Commissioner
Benepe said of STRATUM, ‘It was probably the single most
important sales tool we used to convince policy makers to
put money into trees’ (McIntyre, 2008). Putting dollars
figures on trees perhaps does not sit well with all parties,
but, just as with proper tree valuation, it is essential. David
Nowak said on this subject ‘the monetizing (of trees) is a
necessary evil. We know trees have great value but they’re
intrinsically underrated. You have to talk the language of the
people who make decisions’ (Jonnes, 2011). In essence the
establishing of the benefits of an urban forest will become a
vital, if not mandatory, duty of any manager trying to
convince policy makers to invest in trees.
Mayor Bloomberg invests in trees through
The knock-on effects of UFORE and STRATUM were
dramatic in NYC. On Earth Day 2007, Major Bloomberg
launched a comprehensive sustainable development plan
for greener, greater NYC called PlaNYC (City of New York,
2007). PlaNYC lays out initiatives for the city to strive
towards in five key dimensions of the urban environment.
Trees play a significant role in 60% of those areas: namely
land, water and air. The role of trees in this plan can be
directly attributed to policy makers now understanding the
vast potential that trees offer in combating many of the
most worrying urban environmental challenges. UFORE data
is actually quoted in PlaNYC as justification for the inclusion
of trees in the initiatives. Furthermore, trees are relatively
inexpensive, easy to access and return far more than is
needed to be invested in them. Table 6 is a breakdown of
the PlaNYC initiatives involving trees.
Annual benefits Total value ($) Value ($) per tree
Energy $27 818 220 $47.63
Air quality $5 269 572 $9.02
catchment $35 628 224 $61.00
Carbon dioxide
reduction $754 947 $1.29
Aesthetic/other $52 492 384 $89.88
Total $121963 347 $208.82
Table 5 Annual benefits provided by New York City’s street tree population
as estimated by STRATUM (Peper et al., 2007).
14 Trees, people and the built environment
To achieve the PlaNYC initiatives involving trees, Mayor
Bloomberg massively increased NYC DPR’s annual urban
forestry budget. $118 million was listed in the Capital
budget (FY 2008–2017) for the 809 hectares of new forest
and $247 million for the estimated 220000 street trees
needed to obtain 100% stocking level (City of New York,
2007). Prior to PlaNYC, NYC DPR was annually planting
around 6000 trees; with PlaNYC, this figure sky-rocketed to
22000 trees. It should be noted that the 220000 street trees
and those planted through the reforestation initiative will
make up the majority of the city’s 60% commitment to the
million tree goal. The remaining 40% (400000 trees) will be
planted by private and community organizations and
homeowners (MillionTreesNYC, 2007a, 2007b).
In conclusion, Mayor Bloomberg planned to invest $365
million alone in tree planting over a decade because science
and research had shown they play such a key role in
producing a healthier and more sustainable environment for
New Yorkers.
Using research to direct urban
forestry programmes
In addition to research being used to justify and secure
resources for trees, it also should play a vital role in
determining how those resources are used, or else the
potential benefits of those additional resources may be
squandered or lost. Research can be used to help set up
programmes and monitor the progress of these programmes
once operational. It can also be used to give insight into the
outcomes of certain management decisions. Overall, research
should be used to establish achievable goals and to formulate
the most effective and efficient urban forestry programmes to
reach them. Urban foresters should endeavour to run
research driven programmes to guarantee success.
The 2006 report by the US Forest Service and the
University of Vermont’s Spatial Analysis Laboratory on the
present and possible urban tree canopy (UTC) in NYC was
clearly a key reference for Mayor Bloomberg’s staff when
formulating realistic initiatives and goals for PlaNYC. As
stated before, the research established that NYC’s UTC
could be increased from 24% to as high as 42% (Grove et
al., 2006). The report identified numerous opportunities
where this UTC increase could be realized based on land
use type. For example, UTC on the Public Right of Way
could be increased from 6% (4317 hectares) to 9% (6497
hectares). Therefore these figures reinforce the
management decision in PlaNYC to plant an additional
220 000 street trees to reach a 100% stocking to take full
advantage of this potential 3% UTC. In terms of other land
uses, the report established that there was around 2000
hectares of car parks in NYC, approximately 1% of the NYC
land area, and these were covered by 76 hectares of UTC.
The report estimated that this land use had the potential to
contain as much as 478 hectares of UTC, so this
represented another significant opportunity to add around
402 hectares of UTC. PlaNYC included an initiative for
changing planning regulations mandating perimeter
landscaping and adjacent street tree planting for
commercial and community run parking lots over 557
square metres (City of New York, 2007). In addition, for
parking lots over 1115 square metres, a specific number of
canopy trees would be required inside those lots in
planting islands.
UFORE made recommendations relating directly to air
quality because the study had shown that the urban forest
was taking in 38374 tonnes of carbon each year and also
removing 1998 tonnes of pollutants (Nowak et al., 2007). The
UFORE report for NYC included a tree planting index map
that used census data and tree stocking data to identify
areas of high population with low tree stocking densities.
UFORE recommended that these areas should be prioritized
for planting first. This management concept has been taken
forward and evolved in PlaNYC. In PlaNYC it states that the
planting of the 220000 street trees by NYC DPR will prioritize
neighborhoods with the lowest UTC levels and the highest
air quality concerns (City of New York, 2007). In practice
NYC DPR has identified six neighbourhoods with lower than
average tree stocking but higher than average asthma rates
among young people (MillionTreesNYC, 2007a, 2007b).
of the
Initiative Goal
Land Fill every street tree
opportunity in NYC to
achieve 100% stocking
Plant 22000 street
trees annually to fill
the estimated 220 000
open planting
opportunities by 2017
Water Plant trees with
improved pit designs
Maximize the ability of
tree pits to capture
Air Reforest 809 hectares
of parkland
reforestation project
by 2017
Air Partner with
stakeholders to help
plant one million trees
Plant one million trees
in the city on both
private and public
property by 2017
Table 6
PlaNYC initiatives involving trees (City of New York, 2007).
Plenary session 1: Management of the urban forest 15
These geographical areas are called Trees for Public Health
(TPH) neighbourhoods and they are being prioritized first
for tree planting.
In-house urban forestry research
NYC DPR also has a rich history of performing its own
research and analysis. The tree census is a great example of a
relatively simple research project using predominantly
volunteers and in-house staff to produce a vast wealth of
invaluable information about the street tree inventory. This
information was not only used to run the STRATUM analysis
but is also used on a regular basis to help guide urban
forestry programming. A clear understanding of every aspect
of a resource can only aid in its successful management.
Young tree mortality study
Perhaps some of the most impressive research undertaken
by NYC DPR is a young street tree mortality study using in-
house staff and interns. This study randomly selected and
surveyed 13405 street trees that had been in the ground
between three and nine years (Lu et al., 2010). The survey
was completed in the summers of 2006 and 2007 and
examined how biological, social and urban design factors
affected young street tree mortality. The results showed that
74.3% of the trees surveyed were alive, with the rest either
dead or missing. This percentage was raised to 82.7% for
trees planted in one and two-family residential areas and
dropped to 60.3% for trees in areas with heavy traffic. This
number dropped even further to a 53.1% survival rate for
trees located in central street medians. The research also
highlighted some other very interesting data on the impacts
of species, tree guards and the tree pit type on mortality
rates. Alarmingly, the London plane tree had the lowest
survival rate when compared to 19 other species, especially
when STRATUM identified it as the most important tree in
the urban forest in terms of environmental benefits
delivered (Peper et al., 2007). Surprisingly, this study also
concluded that tree pit size had little impact on survival rates
and that the presence of animal waste was actually
associated with a higher survival rate. This in-house research
is obviously an invaluable resource in helping guide NYC
DPR in reaching 100% stocking of live trees in its streets.
September 2010 tornado
Another example of the use of in-house research is perhaps
less obvious and occurred when a tornado passed through
NYC on 16 September 2010. After any storm event, gaining
situational awareness of the type and location of damage is
vital. This information is usually not available until qualified
staffers have completed comprehensive field inspections,
which could take several days if not weeks. Within two hours
after the tornado, NYC DPR had received around 1000 calls
reporting storm damage and had incorporated this into
their forestry management system, ForMS. Using the
addresses of these calls, NYC DPR was able to produce an
initial map of the areas in the city that had suffered the brunt
of the tree damage. This allowed for NYC DPR to provide
key situational awareness data to the Mayor’s Office and also
the city’s Office of Emergency Management. Valid situational
awareness is essential in tempering an appropriate response
to a tornado both in terms of requesting help and also in
activating emergency debris clearance contracts.
Eventually, just under 10000 calls had been made to NYC
DPR to report storm damage. NYC DPR used 15 years of
previous storm data to explain to decision makers how
severe this event was compared to previous storms and
hurricanes. This provided the justification for a vast increase in
the resources available for cleaning up the damage and for
the help that was asked from other entities including the
Federal Emergency Management Agency.
NYC DPR also used previous storm data to extrapolate from
the confirmed number of uprooted trees how many of
those had potentially caused sidewalk damaged when they
fell. This data was then provided directly to the New York
City Department of Design and Construction (NYC DDC)
who were tasked with repairing these damaged sidewalks.
This allowed DDC to start the process of bidding out
emergency contracts without having to wait for all the field
inspections to be completed.
Essentially, NYC DPR used research and analysis to give
rapid situational awareness of the storm damage. This
allowed for a far quicker gathering and deployment of
appropriate resources needed to perform the clean-up
operation and also communicating the severity of the
damage to policy makers.
Conclusions and future research
This paper has endeavoured to illustrate the vital role of
research in shaping the NYC urban forest and the programs
of NYC DPR. Urban forestry research has placed trees into
the toolbox of urban planners battling to mitigate the
negative impacts of city life and also take a responsible
stance on the wider issue of climate change. Research
should be an essential component of any urban forestry
programme. Even in-house research of existing programmes
16 Trees, people and the built environment
can provide vital data and guidance for maximizing the
benefits generated by those efforts. Research is a compass
to guide urban forestry efforts as well as to help justify
additional resources. NYC DPR has recently opened an
urban field station in partnership with the US Forest Service
at Fort Totten in Queens. This facility supports research by
providing a fully equipped base for researchers to carry out
their studies within NYC’s urban forest. NYC DPR intends to
use this resource to continually identify, pursue and
undertake urban forestry research that assists the agency in
its goal of providing the highest quality, hardest working
and most sustainable urban forest to New Yorkers it
possibly can.
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Plenary session 1: Management of the urban forest 17
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18 Trees, people and the built environment
Measuring the ecosystem services of Torbay’s
trees: the Torbay i-Tree Eco pilot project
Trees are an integral part of urban ecosystems. They provide a myriad of services that benefit urban communities, such
as offsetting carbon emissions, improving air quality by filtering pollutants and regulating local climate. These services
improve the environmental quality of urban areas as well as human health and wellbeing.
This paper presents a quantitative valuation of a range of benefits delivered by Torbay’s urban forest. Using collected field
data, the i-Tree Eco model and existing scientific literature the value of Torbay’s urban forest was estimated. Torbay has
approximately 11.8% forest cover made up of around 818000 trees at a density of 128 trees/ha; these trees represent an
estimated structural asset worth over £280million. In addition, Torbay’s urban forest provides the equivalent of £345811 in
ecosystem services annually. An estimated 98100 tonnes (approximately 15.4tonnes/ha) of carbon is stored in Torbay’s trees,
with an additional gross carbon sequestration rate of 4279 tonnes carbon per year, every year (approximately 671kg/ha/year).
This equates to £1474 508 in storage and £64316 in annual sequestration. Contributions to improving the air quality of
Torbay total over 50 tonnes of pollutants removed every year, which equates to an annual estimated value of £281495.
This paper explains the current limitations of the model, where research scope and methods can be improved and
which UK-specific data we were able to incorporate. It also presents a framework for applying the model in a wider UK
context. The study demonstrates that i-Tree Eco can be meaningfully applied to the UK, and there is therefore the
potential for similar studies in other urban areas.
Trees in the urban forest provide multiple ecosystem benefits (Nowak, 2006; Stenger et al.,
2009). Without measuring these ecosystem services no baseline can be established from
which to monitor trends or to identify where additional resources are required. With
increasing urbanisation there is a need to incorporate the role of the urban forest into long-
term planning and climate adaptation strategies in order to improve environmental quality
(Gill et al., 2007).
Many studies have assessed the environmental value of an ecosystem qualitatively, listing
the animals and plants found there and describing the network of systems – water, air,
nutrients – that provide the underlying function. Some studies have also valued these
services using contingent valuation (willingness to pay, willingness to accept), hedonic
pricing, or avoided cost methods. Yet, to incorporate the role of the urban forest in
environmental policies the impacts of trees need to be quantified. However, there have
been few quantitative studies undertaken ( Jim and Chen, 2009; de Groot et al., 2010) and
whilst there are systems that quantitatively measure the value of trees in the UK, none of
these take an ecosystem services approach.
Since the release of the Millennium Ecosystem Assessment (2005a) there has been increased
interest in defining and valuing our ecosystem services because, as a direct result of
undervaluation, over two thirds of our natural ecosystems have been degraded (Millennium
Ecosystem Assessment, 2005b). In order to develop viable strategies for conserving
services, it is important to estimate the monetary value so the importance can be
to the main stakeholders and beneficiaries (The Economics of Ecosystems and
benefit analysis, ecosystem
services, urban forest
Kenton Rogers,1
David Hansford,1
Tim Sunderland,2
Andrew Brunt3and
Neil Coish4
1Hi-line Consultancy, Exeter, UK
2Natural England, Sheffield, UK
3Forest Research, Alice Holt
Lodge, Surrey, UK
4Torbay Council, Devon, UK
Plenary session 1: Management of the urban forest 19
Biodiversity, 2009). Furthermore, the ecological state of a city
depends heavily on the state of its urban trees (Whitford et
al., 2001; Dobbs et al., 2011) and to estimate the structure,
function and value of the urban forest is an important first
step in the sustainable management of natural capital.
Study area
The study took place in the coastal borough of Torbay,
comprising the towns of Torquay, Paignton and Brixham.
The study area covers 63.75km² centred at 50° 27’ N and 3°
33’ W and lies in the southwest of England. Torbay has a
mild temperate climate due to its sheltered position and the
effect of the Gulf Stream, with mean annual precipitation of
1000 mm and a mean average maximum and minimum
temperature of 14oC and 7oC respectively (Met Office, 2010).
The population is circa 134000 (Torbay Council, 2010).
Materials and methods
The basic process used by the i-Tree Eco model (also known
as the Urban Forest Effects model or UFORE) is to calculate
the correct number of survey plots needed to give a
representative sample of an urban tree population. Survey
data from these plots is used to calculate the species and
age class structure, biomass and leaf area index (LAI) of the
urban forest. This data is then combined with local climate
and air pollution data to produce estimates of carbon
sequestration and storage, air pollution interception and
removal, the monetary value of these ecosystem services, and
the structural value of the trees. The model can also estimate
the predicted future benefits of the existing urban forest by
applying growth rate calculations to the current stock.
Field sampling
During the summer of 2010, 250 random 0.04ha plots were
distributed across the borough of Torbay. Plots were
allocated using randomised grid sampling. The borough
(study area) was divided into 250 equal grid cells with one
plot randomly located within each grid cell. The study area
was then sub-divided into smaller units of analysis (or strata)
after the plots had been distributed (post-stratification). This
approach better allows for future assessment to measure
changes through time and space but at the cost of increased
variance of the population estimates, because pre-stratification
can focus more plots in areas of higher variability (Nowak
et al., 2008a).
Out of the 250 plots, 241 were measured following field
methods outlined in the i-Tree Eco user manual v 3.1. (i-Tree,
2010). Of the remaining 9 plots, 2 were inaccessible and 7
were located on private property, where permission to
conduct the field measurements had been refused.
The 241 plots equate to 1 plot every 26.45 ha, which yields
a relative standard error (of tree population) of ±11%. Details
of how the number of plots influences the relative standard
error over area are given in Nowak et al. (2008a). Other
studies have frequently used 200, 0.04ha plots yielding
different variances (Nowak et al., 2008b). However, the
number of plots chosen for this size study area has been
determined to be sufficient to address the objectives of the
project. By way of comparison the Chicago study used 745
plots equating to 1 plot every 80.2ha, producing a standard
error of ±10% (Nowak et al., 2010).
Following the protocol specified in the i-Tree Eco user
manual v 3.1 (i-Tree, 2010), data was collected for each tree
on every plot. Tree measurements included species, number of
stems, diameter at breast height (dbh), total height, height to
base of live crown, crown width, percentage crown die-back,
crown light exposure and the position of the tree relative to
the plot centre. Other information on the plot included
percentage ground cover types, land use, percentage tree
cover and plantable space. Shrub data (species and leaf
volume) were also collected and their contribution included in
the calculations for pollution removal – but not for carbon
storage and sequestration. Full details of field data collection
procedures are given in Nowak et al. (2008a).
We used i-Tree Eco to calculate and describe the structure of
Torbay’s urban forest, including species composition, tree
density and condition, leaf area and biomass. This data was
combined with additional data, including local climate and
hourly pollution, and an estimated local leaf-on/leaf-off
date. These variables were then analysed to quantify the
ecosystem functions, including carbon sequestration and
storage, air pollution removal and structural value. Full
methodologies are included in Nowak and Crane (2000)
and Nowak et al. (2008a).
We did not carry out any analysis of tree shading and
evaporative cooling on building energy use and subsequent
avoided carbon emissions. This component of the i-Tree Eco
model is designed for US building types, energy use and
emissions factors, limiting its use in international
applications (i-Tree, 2010).
The model provides values in dollars. Pound values were first
converted to dollars with the submitted data, and returned
20 Trees, people and the built environment
dollar values were converted back into pounds using the
HM Revenue and Customs average for year spot rate to 31
March 2010 (£-$ = 1.517 and $-£ 0.659).
A number of UK-specific datasets were needed to run the
model for the Torbay study area.
Climate data
Weather data was obtained from the National Climatic Data
Centre (2010), which although based in the USA provides
datasets which are available for most major cities
worldwide. This study used hourly climatic data from the
Brixham weather station, which lies within the study area.
Albedo (solar radiation) coefficients are also required. These
do not vary much across the USA (Nowak et al., 2006) and
‘best fit’ values were used for Torbay based on the local
climatic and geographical data supplied. Work is currently
being undertaken in the USA to test how sensitive the model
is to these coefficients in order to assess how accurate these
values need to be; it is currently thought that they will not
affect final figures very much (Nowak, personal
communication, 8 February 2011).
Pollution data
We obtained hourly pollution data from Defra (2010a).
Archived pollution data is available online for pollution
monitoring stations across the UK. Monitoring stations
located in Torbay did not collect data on the complete set of
pollutants required by the i-Tree Eco model, therefore proxy
data was obtained from a monitoring station in Plymouth town
centre for the years 1997 onwards. This proxy dataset was also
incomplete due to the station being periodically in
active or
out of service. Therefore data for the various
pollutants over
a five-year period (2005–2009) was obtained.
This data was
then spliced where there were gaps in order to provide a
continuous hourly pollution dataset for O3, SO2, NO2, CO2,
and PM10 for one year.
Leaf-on, leaf-off dates
Mean average leaf-on/leaf-off dates were calculated using
datasets from the UK phenology records (Nature’s Calendar,
2010). The data from eight species were selected to
calculate an average (field maple (Acer campestre), sycamore
(Acer pseudoplatanus), birch (Betula pendula), hawthorn
(Crataegus monogyna), beech (Fagus sylvatica), ash (Fraxinus
excelsior), sessile oak (Quercus petraea) and English oak
(Quercus robur)) over a five-year period (2005–2009) from
data collected across the UK, to provide a leaf-on date.
However, because leaf-off is not in itself an event in the UK
phenology database, a further average was taken from the
‘first leaf fall’ and ‘bare tree’ events for the eight species
across the five years to provide an average date for the ‘leaf-
off’ event. The average dates calculated for these events
used in the study were; leaf-on, 19 April 2010 and leaf-off,
27 October 2010. As these are UK averages the estimate is
likely to be conservative when applied to Torbay, which is
widely understood to be subject to a milder microclimate.
Structural data
For transplantable trees the United Kingdom and Ireland
Regional Plant Appraisal Committee (UKI RPAC) – Guidance
note: 1 (Hollis, 2007) was used with the average installed
replacement cost (£500.00) and average transplantable size
(30–35cm) of replacement trees in Torbay to determine a
basic replacement price of £12.42/cm² (of cross sectional
area of tree). These averages were calculated by obtaining
the cost of supply of each replacement tree species and
associated planting and maintenance costs to derive the
installed replacement cost. Where no price existed for a
given tree species then the 16–18cm class price from the
UKI RPAC – Guidance note: 1 (Hollis, 2007) was used. This
installed replacement unit cost is multiplied by trunk area and
local species factor (0–1) to determine a tree’s basic value.
Local species factors for the USA are determined by the
Council of Tree and Landscape Appraisers (CLTA) regional
groups and published by the International Society of
Arboriculture. However, there is no published data for the
UK. To undertake a full appraisal of local species factors
would be a significant task (Hollis, 2007). Therefore, using
the list of recorded tree species from the field study,
knowledge of the locality and the species adaptability table
(6.1) in Hibberd (1989), the growth characteristics, pest and
disease susceptibility and environmental adaptability were
determined to broadly gauge the local species factor into
the following categories; low 0.33, medium 0.66 and high 1.
Carbon storage and sequestration
The UFORE model quantifies composition and biomass for
each tree using allometric equations from the literature.
Where no equation can be found for an individual species,
the average results from equations of the same genus are
used. If no genus equations are found then the model uses
average results from all broadleaf or conifer equations
(Nowak, 1994; Nowak et al., 2008a).
Where equations estimate total above-ground tree wood
biomass, the below-ground biomass was estimated using a
root-to-shoot ratio of 0.26 (Nowak et al., 2008a). Where
Plenary session 1: Management of the urban forest 21
equations calculate fresh weight biomass, species or
genus specific conversion factors were used to calculate the
dry weight.
Urban trees tend to have less above-ground biomass than
trees in forests. Therefore, biomass results for urban trees
were adjusted accordingly by reducing biomass estimates by
20%, although no adjustment is made for trees in more
natural stands (Nowak et al., 2008a). Estimates of annual
carbon storage are calculated by converting tree dry-weight
biomass by multiplying by 0.5 (Nowak et al., 2008a). Full
methodologies are included in Nowak and Crane (2002)
and Nowak et al. (2008a).
Gross carbon sequestration was estimated from average
diameter growth per year for individual trees, land use types,
diameter classes and dbh from field measurements (Nowak
et al., 2008a). Adjusting for tree condition, gross carbon
sequestration was calculated as the difference in the amount
of carbon storage between a measured tree’s actual and
predicted carbon storage in one year.
Net carbon sequestration includes released carbon due to
tree death and subsequent decomposition based on actual
land use categories, mortality estimates, tree size and
condition (Nowak et al., 2008a).
The model uses biomass formulas and standardised growth
rates derived from US data and therefore our estimates for
Torbay are sensitive to this. However, as the base growth
rates used are from northern US areas (Nowak et al., 2008a),
the growth and carbon sequestration rates are likely to be
conservative when applied to Torbay.
Since population carbon estimates are based on individual
trees, the model estimated the percentage of the measured
tree that will die and decompose as opposed to a percentage
of the tree population to die and decompose. These
individual estimates were aggregated to estimate
decomposition for the total population, based on field land
use and two types of decomposition rates, rapid and delayed
release (Nowak et al., 2008a). This assumes that urban trees
release carbon soon after removal, whereas trees in forest or
vacant areas are likely left standing for prolonged periods,
thus delaying release (Escobedo et al., 2010); again, this is
likely to result in a more conservative estimate of carbon
stored. Additional methods and assumptions on
standardised growth, decomposition rates and related
carbon emissions are presented in Nowak and Crane (2002).
The value of the carbon stored and sequestered annually is a
multiplication of the unit cost. The model uses the estimated
marginal social cost of carbon dioxide based on a stochastic
greenhouse damage model from a paper by Fankhauser
(1994). This estimates a social cost of carbon in the order of
$20.00 per ton carbon for emissions between 1991 and
2000 rising to $28.00 per ton carbon by 2021 (imperial). The
value used in the study was calculated for 2010 at $22.80
per tonne carbon (metric).
Air pollution filtration
Air pollution removal is modelled within UFORE as a
function of dry deposition and pollution concentration.
Estimates of hourly pollution removal and its value are based
on the local weather and solar radiation data, pollution data,
leaf area index, leaf-on, leaf-off dates and geographical
factors (Nowak et al., 2006).
Leaf area index (LAI) is calculated for trees and shrubs from
the field data. The UFORE model estimates leaf area using
regression equations (Nowak, 1994; Nowak and Crane, 2002;
Nowak, Crane and Stevens, 2006) based on the input
variables from the field data. Because trees can also emit
volatile organic compounds (VOC’s) – emissions that
contribute to the formation of O3and CO – biogenic
emissions from different tree species were accounted for in
the calculations (Nowak et al., 2008a).
The value attributed to the pollution removal by trees is
estimated within the model using the median externality
values for the USA for each pollutant. These values are given
in $ per metric tonne as O3and NO2= $9906 per metric
tonne, CO = $1407 per metric tonne, PM10 = $6614 per
metric tonne and SO2= $2425 per metric tonne (Nowak et
al., 2008a). These values are considered as the estimated
cost of pollution to society that is not accounted for in the
market place of the goods or services that produced the
pollution (Nowak et al., 2006).
Structural value
The structural value is based on methods from the Council
of Tree and Landscape Appraisers and is based on four
variables: trunk area (cross sectional area at dbh), species,
condition and location (local species factors). The field
measurements (species, cross sectional area at dbh) are used
to determine a basic value that is then multiplied by
condition and local species factors to determine the final
compensatory value (UFORE, 2010).
For trees larger than transplantable size the basic value (BV) was:
BV = RC+(BPx[TAa- TAr] x SF)
22 Trees, people and the built environment
where RC is the replacement cost at its largest transplantable
size, BP (basic price) is the local average cost per unit trunk
area (£/cm²), TAa is the trunk area of the tree being
appraised, TAr is the trunk area of the largest transplantable
tree and SF is the local species factor.
For trees larger than 76.2cm dbh, trunk area is adjusted
downwards based on the assumption that a large mature
tree will not increase in value as rapidly as its trunk area due to
factors such as anticipated maintenance and structural
safety (Council of Tree and Landscape Appraisers, 1992). The
adjustment is:
ATA = -0.335d² + 176d - 7020
where ATA = adjusted trunk area, and d= the trunk diameter
in inches.
Basic values for the trees were then multiplied by condition
factors based on crown die-back and local species factors
(UFORE, 2010). Data from all measured trees was used to
determine the total compensatory value (structural value) of
the tree population (Nowak et al., 2008a).
Results and discussion
Urban forest structure
There are approximately 818000 trees in Torbay, situated on
both private and public property. The results of the survey
found that the private/public ownership split for the plots is
71.1% private, 28.9% public ownership. This is higher than
the national average revealed in the results of Trees in Towns II
(Britt and Johnston, 2008), where two-thirds of all trees and
shrubs were found on private property (public ownership
indicates that the land falls under the duty assigned to Torbay
Borough Council to maintain at the public expense). Data for
land ownership under these headings is not included within
the parameters for i-Tree data collection. Instead, additional
data was collected at the time of survey by way of assigning a
percentage to each plot (rounded to the nearest 5%) for the
area in private/public ownership.
The most common tree species found in Torbay are
Leyland cypress (118306 trees, 14.5%), ash (94776 trees,
11.6%) and sycamore (81703 trees, 10%). Total tree leaf area
in Torbay is 51.7km2. (NB. whilst this is related to, it does not
substitute for canopy cover.) The most dominant tree
species in terms of total leaf area are ash (10.1km2, 19.5%),
sycamore (8.5km2, 16.4%) and beech (3km2, 5.8%) (results
are taken for trees only; results for shrubs are not included
within these values).
The most important species (calculated as the sum of
relative leaf area and relative composition) are those trees
which have attained a larger stature and therefore larger
stem diameters and total leaf areas (Table 1 shows the top
ten trees by importance value). The top ten trees account for
67.6% of the total leaf area. While being the most numerous
tree, Leyland cypress accounts for only 3.1% of the total leaf
area. The dominance of ash as the climax community large
canopy tree within Torbay’s woodlands accounts for its
status as the most important tree.
The recent Trees in Towns II survey (Britt and Johnston, 2008)
used aerial photography to report mean average canopy
cover for towns in England to be 8.2%. Mean canopy areas
per plot were calculated at 11.1% for the South West and
11.8% for the South East. The Torbay study estimated tree
canopy cover over the area of Torbay at 11.8% (a total of
752ha). For comparison, canopy cover for Chicago and
New York, USA, were estimated at 17% and 24% respectively
(Rodbell and Marshall, 2009). Shrub cover for Torbay was 6.4%.
Table 1 Species importance within Torbay.
Rank Species Percentage population Percentage leaf area Importance value
1Ash 11.6 19.5 31.1
2Sycamore 10.0 16.4 26.4
3Leyland cypress 14.5 3.1 17.5
4Hazel 7.4 4.9 12.4
5Beech 3.7 5.8 9.4
6Holm oak 4.4 4.9 9.3
7Elm 5.5 2.2 7.7
8Lawson cypress 2.5 3.7 6.2
9Hawthorn 5.4 0.8 6.2
10 English oak 2.2 3.7 6.0
Plenary session 1: Management of the urban forest 23
Of trees in Torbay 57.1% are less than 15.2cm diameter at
breast height. This distribution (although normal) is skewed
(Figure 1). Ideally one would expect a normal distribution
with most trees in the middle diameter classes. However, it
must be taken into account that because any stem over
2.5cm diameter was included in the study, many small
hedgerow trees were included within the analysis. This is
especially relevant for one of the most commonly used
amenity hedge species, Leyland cypress (with 65.8% of trees
within the population at less than 15.2cm stem diameter).
Large numbers of hedgerow Leyland cypress trees were
recorded with small stem-diameters and crown-volumes
(due to their repeated clipping as hedges). Also, within
woodland plots, many small trees in the understorey were
also included.
In terms of continental origin, Table 2 shows percentages for
each of the six continents from which the 102 species found
in Torbay originate. By far the most dominant continent of
origin is Europe. It is interesting to note that of the species of
European origin, 51.4% are native to the UK, which
represents 35.3% of all species found.
The structural value of Torbay’s trees amounts to
£280million. The CTLA value is a conservative value based
on a tree in average condition, which will overestimate the
value of some trees, and underestimate others. This
approach serves to give a credible value for all the trees in
Torbay. CTLA methodology does not apply a value to the
trees as an amenity, and this is not considered here. The value
of each tree applies to its replacement cost only, and is
partially theoretical, as it is not possible to buy and
transplant large trees in the event that they are lost. Through
depreciating the values for the trees by species (i.e. suitability
to the environment), condition (physiological and structural
defects, life expectancy) and location (as trees contribute to
the market value of property in an area, they can be assigned
a proportion of this value; larger trees are effectively ‘worth
more’), a realistic value for trees is obtained, which realises
the significance of the contribution of a tree to its
environment. See Hollis (2009) for a thorough evaluation
of the system.
Climate change, carbon storage and
Climate change is now recognised as one of the most serious
challenges facing us today (Wilby, 2007; Lindner et al., 2010)
and its potential impacts for trees and forests are well
documented (Freer-Smith et al., 2007). The UK climate change
scenarios (UKCIP, 2009) indicate average annual temperature
increases of between 1and 5oC by 2080. However, these
scenarios do not take urban surfaces into account (Gill et al.,
2007), which have the potential to further increase these
predicted temperatures due to the urban heat island effect.
Urban trees help mitigate climate change by sequestering
atmospheric carbon (from carbon dioxide) in tissue, by
altering energy use in buildings, thereby altering carbon
dioxide emissions from fossil fuel based power plants and
also by protecting soils, one of the largest terrestrial sinks of
carbon (Reichstein in Freer-Smith et al., 2007). They will also
be useful in adapting to climate change through evaporative
cooling of the urban environment (Gill et al., 2007; Escobedo
et al., 2010).
The model estimated that Torbay’s trees store 98100 tonnes
of carbon (15 tonnes of carbon per ha) and sequester a
further 4279 tonnes per year (0.7 tonnes of carbon per ha).
Net carbon sequestration is estimated at 3320 tonnes taking
into account tree mortality. As trees die and decay they
release much of the stored carbon back into the
atmosphere. This is illustrated most significantly in the net
amount for elm (Table 3), which despite a large population
have a negative net sequestration rate due to their short
lifespan; a consequence of Dutch elm disease.
Torbay’s baseline (2005/6) total emissions were estimated at
750000 tonnes of carbon (Torbay Council, 2008), over
seven times more than the total carbon stored in the
borough’s urban forest and equating to 5.6 tonnes of carbon
per capita. Based on these figures the urban forest can offset
the emissions from 592 residents, which accounts for less
than 0.5% of total emissions.
The direct impacts of trees on CO2seem at first glance to be
negligible. However, the potential for the urban forest to
reduce CO2emissions through energy reduction, and its role
in climate adaptation, lowering urban temperatures through
Origin Percentage
Europe 68.9
N. America 14.6
Asia 6.8
Australasia 5.8
S. America 2.9
Africa 1.0
UK (as % of European species) 51.4
UK (as % of total species) 35.3
Table 2 Origin of species within Torbay.
24 Trees, people and the built environment
evaporative cooling and protecting soil carbon, should not
be overlooked. Although these particular ecosystem
functions were not quantified as part of this study, Gill et al.,
(2007) reported that increasing green cover by 10% within
urban areas in Manchester could reduce surface
temperatures by 2.2 to 2.5oC.
Figure 1 Tree composition in Torbay by diameter class.
% Composition
<15.1 15.2 -
30.5 -
45.7 -
61.2 -
76.2 -
91.4 -
106.7 -
121.9 -
Diameter (cm)
Table 3 Carbon storage and sequestration of the ten most significant trees in Torbay.
of trees
Gross seq
Net seq
Leaf area
Carbon Net
Val SE Val SE Val SE Val SE Val SE Val SE Value (£) Value (£)
Leyland cypress 118 306 35 361 2430.77 662.22 268.75 74.93 255.68 71.12 1.581 0.433 370.55 101.43 36 536 3843
Ash 94 776 32 088 11 399.19 3771.22 506.6 145.48 470.61 134.75 10.091 2.976 1 073.56 316.56 171 337 7074
Sycamore 81703 23 197 18 142.32 7048.52 661.7 197.74 597.8 174.52 8.493 2.466 593.94 172.46 272 691 8985
Hazel 60 787 22 128 2344.55 963.41 186.59 67.86 160.9 64.64 2.549 0.899 177 62.41 35 240 2418
Elm 45 100 21 600 3466.27 1675.56 112.98 53.7 -289.69 263.14 1.147 0.559 78.09 38.07 52 100 -4354
Hawthorn 43 793 18142 800.52 299.41 87.54 31.14 84.47 29.69 0.432 0.151 54.4 19.04 12 032 1270
Holm oak 35949 12 999 9934.76 3845.19 425.14 160.98 291.65 158.86 2.54 0.974 233.13 89.34 149 326 4384
Beech 30 067 14 147 7385.11 3960.2 260.32 111.87 222.25 92.25 2.984 1.169 149.34 58.5 111 003 3341
Lawson cypress 20 262 5818 3945.47 2567.21 115.78 58.13 94.19 48.52 1.936 1.107 484.02 276.76 59 303 1416
English oak 18 302 7484 6713.92 3572.15 211.87 96.12 192.47 86.62 1.937 0.887 128.98 59.07 100 915 2893
Torbay has a large proportion of smaller (both in age and
ultimate size potential) trees and carbon sequestration from
small trees is minimal (Escobedo et al., 2010). However a
proportion of these trees will grow, thus offsetting the
decomposition from tree mortality.
The estimates of carbon stored in the urban forest are likely
to be conservative as soil carbon has not been factored into
the evaluation. Furthermore, the urban forest can also
reduce emissions indirectly, and if more trees able to
achieve a larger size are planted, additional carbon can be
stored in the urban forest. However, tree establishment and
maintenance operations will offset some of these gains.
Air pollution removal
Air pollution from transportation and industry is a major
public health issue in urban areas (Beckett et al., 1998;
Bolund and Hunhammar, 1999; Tiwary et al., 2009). Urban
trees can make significant contributions to improving urban air
quality (Freer-Smith et al., 2005) by removing air pollution
through dry deposition, a mechanism by which gaseous and
particulate pollutants are captured on plant surfaces and are
either absorbed into the plant through the stomata (Jim and
Chen, 2008), or introduced to the soil through leaf fall. Trees
are capable of higher rates of dry deposition than other land
types (McDonald et al., 2007) and also alter the urban
atmosphere by reducing levels of ozone, because although
some species can contribute to VOC emissions, the cooling
effect of the urban forest on air temperature reduces ozone
to greater effect (Nowak et al., 2000).
Torbay’s trees remove 50 tons of pollutants every year with
an estimated value of £281000 (Figure 2). Pollution removal
was greatest for ozone, O3, followed by PM10, NO2and SO2.
Recorded CO levels were negligible.
Figure 3 shows monthly removal, which varied, peaking in
May for O3and in October for other pollutants. The
monthly pattern of removal differed from observations in
Plenary session 1: Management of the urban forest 25
the USA in which peak removal rates tend to occur in the
summer months (Nowak, 1994). These differences could be
attributed to the poor summers of 2007–2009 from which
the climatic and pollution datasets were taken, as one would
typically expect pollution levels to build over the summer
months, peaking at the end of the summer.
Total pollution removal in Torbay is 0.002 tonnes per ha per
year. These values were lower than have been recorded by
other studies; 0.009 tonnes per ha per year in Tiwary et al.
(2009) for a site in London (PM10 only) and 0.023 tonnes
per ha per year in Jim and Chen (2008) for a site in
Guangzhou, China. However, the greater pollution
concentrations and canopy cover areas observed in these
studies will result in more pollutants being removed.
Greater tree cover, pollution concentrations and LAI are
the main factors influencing pollution filtration and
therefore increasing areas of tree planting has been
shown to make further improvements to air quality
(Escobedo and Nowak, 2009). Furthermore, because
filtering capacity is closely linked to leaf area (Nowak,
1994) it is generally trees with larger canopy potential that
provide the most benefits.
Available planting space in Torbay has been estimated from
the study at 8%. McDonald et al. (2007) reported in a
modelling study that by increasing tree cover by 13% in the
West Midlands, PM10 concentrations alone could be
reduced by up to 10%. Species selection is an important
consideration; for example, conifers are capable of capturing
more particulates but are not considered to be as tolerant as
broadleaves (Beckett et al., 1998). As different species can
capture different sizes of particulate (Freer-Smith et al., 2005)
a broad range of species should be considered for planting
in any air quality strategy. Donovan (2003), quoted in
McDonald et al. (2007), developed an Urban Air Tree
Quality Score as a decision support tool for this purpose.
Value (£)
Pollution removed (tonnes)
Pollution removed (tonnes) Value (£)
Pollution removed (tonnes)
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 2 Total pollution removed.
Figure 3 Monthly pollution removal.
26 Trees, people and the built environment
Uncertainties in the quantification have been
acknowledged, such as the application of US externality
values on the pollutants and the use of a local proxy site for
pollution data. While the USA uses abatement cost values
(based on what it would cost to clean the air by mechanical
means), in the UK pollution values are based on damage
costs, which were not suitable for local modelling without
further work and did not cover all the pollutants monitored
in the UK (Defra, 2010b). Furthermore, dry deposition rates
were modelled based on generic values due to lack of
empirical data and no account is made of wet deposition.
Tiwary et al. (2009) reported that although the UFORE
method has limitations based on these inherent
assumptions, a different methodology used by
Broadmeadow et al. (1998) in the UK gave results that would
suggest that the models being evaluated as part of that study
were reasonably reliable.
The UFORE model was originally developed using
geographically specific US growth rates. Tree species in the
UK have different growth rates, and therefore biomass and
leaf area estimates, and the subsequent provision of
ecosystem services will also differ. Applying i-Tree Eco to
British conditions could result in the over or under
estimation of the reported values. As the UFORE model
has been applied in other non-US cities, it would be
interesting to compare results. However, for the most accurate
use of the model, the algorithms should be adapted to suit
UK conditions.
The values presented in this study represent only a portion
of the total value of the urban forest of Torbay because only
a proportion of the total benefits have been evaluated. Trees
confer many other benefits. Benefits such as avoided energy
costs for cooling and heating, visual amenity, human health,
tourism, ecological benefits, and other provisioning and
regulating services such as timber and natural hazard
mitigation (de Groot et al., 2010) remain unquantified.
The importance of several of these benefits will increase as
the predicted effects of climate change (such as increased
summer droughts and winter rainfall) become more
apparent. Under these scenarios, a healthy and diverse
urban forest using appropriate species will be more resilient
to change.
Although there is scope to improve the approach used in
this study with UK-specific data, it still provides a useful
indicator of the monetary value of urban trees, and allows
for a better analysis of tree planting costs and benefits to be
undertaken. The findings should also raise awareness of the
wide range of ecosystem services delivered by trees in urban
areas, strengthening the case for increasing urban greening,
and promoting the sustainability of urban ecosystems.
We would like to thank the many residents of Torbay who
granted us access to their properties to collect the data, Tim
Jarret for assistance with the field data collection, and
Dr David Nowak of the US Forest Service and Adam Hollis
of LandMark trees for review of this paper. This work and
the assessment of the returned data was funded, in part, by
Natural England, Torbay Council and Hi-line Contractors
SW Ltd.
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Plenary session 1: Management of the urban forest 29
A framework for strategic urban forest
management planning and monitoring
With global climate change, ever-increasing urban populations and rapidly spreading invasive species and pests, the
challenges facing urban forests today are immense. To address these challenges and achieve true sustainability, urban
forest management programmes need to transition from a reactive maintenance approach to one of proactive management.
The clear solution is collaborative, long-term, strategic urban forest management planning. This paper outlines a three-
tiered planning framework comprising a high-level, 20-year strategic plan, with four five-year management plans, and 20
annual operating plans. The concept of active adaptive management is firmly embedded in this framework, providing
managers with the opportunity to review the successes and shortcomings of their management activities on a systematic
basis, and integrate new approaches or address new issues as required. The framework is further supported by a
comprehensive set of criteria and indicators for performance assessment. These 25 criteria and indicators support the
process of adaptive management by providing clear and consistent measures by which progress can be gauged, and are
positioned as tools for improving the development and implementation of urban forest management plans over time.
Finally, the flexibility of the framework and its applicability at different scales is highlighted with several case studies,
including the development of strategic urban forest management plans for municipalities and golf courses.
The benefits provided by healthy and well-managed urban forests are far-reaching and
extensively documented (e.g. Dwyer et al., 1992; McPherson, 1994; Simpson, 1998; Kuo, 2003;
Wolf, 2004; Donovan and Butry, 2010). There are, however, many challenges currently facing
trees in urban and peri-urban areas. Generous estimates suggest that the average lifespan of
a typical urban tree is 32 years and that many newly planted trees do not survive their first
year (Moll and Ebenreck, 1989). A number of factors contribute to such dismally short lifespans
and, as a result, few urban trees are ever able to reach their full genetic potential to provide
important social, economic and environmental services for urban residents.
Cities and their surrounding areas are complex and dynamic entities. A wide range of
decision makers, stakeholders and interest groups are active in setting the agenda in most
communities, and urban forest managers must compete with other interests for limited
resources. In spite of the additional challenges posed by invasive species, development
intensification, climate change and other stress factors, a solution to effective urban forestry
in this context lies in good planning that balances the need for immediate action with the
need for a long-term vision. Effective planning can support the development and
implementation of proactive, as opposed to reactive, management approaches in a strategic
and collaborative fashion. Proactive management leads to tangible results in the form of
increased operational efficiency, risk reduction, increased urban forest canopy and leaf area,
and, perhaps most importantly, the sustained provision of ecological, social and economic
benefits to urban residents and the greater environment.
The first part of this paper outlines the context for urban forest management planning and
presents an effective 20-year planning framework for use in the development of urban
forestry strategies. The second part builds upon the work of Clark et al. (1997) and
demonstrates how a comprehensive and practical set of monitoring criteria and indicators
tailored to assess urban forest sustainability can improve management planning and
adaptive management,
canopy cover, criteria and
indicators, municipal
planning, relative canopy
cover, sustainability, urban
Philip J.E. van Wassenaer,1
Alexander L. Satel,1
W. Andrew Kenney2and
Margot Ursic3
1Urban Forest Innovations,
Inc., Mississauga, Ontario,
2Faculty of Forestry, University
of Toronto, Ontario, Canada
3Beacon Environmental Ltd,
Guelph, Ontario, Canada
30 Trees, people and the built environment
implementation. Finally, the paper explores how these tools
have been applied in southern Ontario, Canada, to work
towards achieving true urban forest sustainability in
communities of various sizes.
The context for urban forest
management today
The challenges to growing and maintaining healthy urban
forests are numerous and, by necessity, must be addressed
on a long-term horizon. Urban foresters must remember
that they work on ‘tree time’. Trees are a long-term
investment, and successes and failures are rarely realised
overnight because trees can take years to respond to stress
factors or improvements designed to promote their health
and longevity.
From a basic biological perspective, cities are difficult places
to grow trees. Unlike in forests (where we all too often forget
that trees come from), urban soils are typically of poor
quality, limited in volume, and can be effectively sterile or
even contaminated. Often heavily modified, urban tree
rooting environments are typified by low biological activity,
poor nutrient availability, compacted pore space and a
number of other problems (Urban, 2008). Simply put, good
soil is in short supply. Furthermore, trees must compete for
space with various forms of built infrastructure, such as
roads, buildings and sewers. In many jurisdictions, these
grey infrastructure components take precedence over trees
and other forms of green infrastructure, which are seen as
additional niceties to be included in urban designs where
feasible and when budgets permit.
Compounding the difficulties associated with poor-quality
growing sites and inadequate soils is the reality of urban
intensification and development. In 2011, the world
population is expected to exceed seven billion, with over
half now residing in towns and cities (UNFPA, 2010). This
influx of urban citizens places increasing stresses on existing
trees and makes urban land a premium commodity. In
many areas, planning regulations require intensification in
urban centres and settlement areas in an attempt to curb
urban sprawl. Paradoxically, this leaves little room for trees in
the very places where they are most beneficial.
Finally, the additional stress factors presented by climate
change will continue to affect urban forests (2degreesC,
2007; Colombo, 2008; Galatowitsch et al., 2009). In highly
urbanised communities, climate change-related events such
as periods of extended drought, extreme winds, high
temperatures and shifting species distribution patterns for
both native and invasive species will further strain already
thin operating budgets.
The challenges outlined above, including poor urban soils,
intensification and climate change, are just three of many
factors weighing against urban forest sustainability. Others
include invasive species, pests and pathogens, limited
knowledge of proper tree care practices, poor public
perception of trees, and inadequate maintenance and
management practices, among others. No matter what the
threat, it is clear that attention needs to be given to planning
for the future health and enhancement of the urban forest
resource in any community, as was previously noted by van
Wassenaer et al. (2000).
Any efforts to proactively manage urban forests to provide
the greatest amount of benefits requires a targeted, strategic
approach that is collaborative in nature and considers the
wide range of stakeholders with interests in urban forest
sustainability. Providing a framework for such a planning
approach is one of the central objectives of this paper.
A strategic framework for urban
forest management planning
While the pace of daily life in urban areas is often
accelerated, trees in cities can be relatively slow to respond
to physical damage and environmental changes, whether
they are negative or positive. Similarly, municipal
governments are rarely, if ever, able to quickly summon the
financial and human resources necessary to make
meaningful changes to urban forest operations and
management. As a result, a long-term planning horizon is
needed in order to outline required action items, prioritize
implementation and accommodate long-term budget
planning. Even with the best laid plans, unexpected
occurrences such as long-term droughts, invasive pests, or
worsening economic circumstances may force significant
reprioritisation of short- and medium-term operations.
Planning on a longer time horizon can ensure that strategic
objectives are still met.
Planning horizon and temporal framework
A number of municipalities in southern Ontario, Canada,
have determined that a 20-year horizon is appropriate for
planning a sustainable and healthy urban forest, and have
developed plans using this framework. This timeframe
enables short- and medium-term financial and
organisational planning, while maintaining an established
overall strategic direction that will remain unchanged and
Plenary session 1: Management of the urban forest 31
thus enable the community’s vision for its urban forest to
become realised.
While a long-term planning horizon is necessary to achieve
urban forest sustainability, shorter-term objectives and day-
to-day operations must be supported by more readily
implementable directives. Therefore, an effective urban forest
management plan must make clear links between long-term
strategic directives, medium-term priorities, and day-to-day
operational activities such as tree pruning or establishment.
This can be achieved through a three-tiered temporal
framework (Figure 1) for urban forest management
planning, wherein a 20-year strategic plan is divided into
four five-year management plans, which are further
subdivided into annual operating plans.
Figure 1 Temporal framework for a strategic urban forest management
The highest level of the urban forest management plan sets
out the vision, goals and objectives to be achieved by the
end of the planning horizon. This 20-year strategic plan can
be developed as a separate document from lower-level
plans, and should provide connectivity to other relevant
strategic documents and policies in the community. A vision,
strategic objectives and guiding principles should be
developed in consultation with municipal staff, community
members and other stakeholders such as local land
developers, environmental groups and organisations, and
representatives of other levels of government (e.g. regional
councils). These goals and vision should guide the overall
direction of plan development, and must therefore be
developed early on in the process.
Effective urban forest management requires an end goal – a
reason to justify the expense and complexity associated with
the undertaking. While every community will develop its
own vision for what its urban forest should look like and
what benefits its residents will enjoy, a workable guiding
objective is presented below, stating that the goal of any
community’s urban forestry programme should be:
To optimise the leaf area of the entire urban forest by
establishing and maintaining a canopy of genetically
appropriate (adapted and diverse) trees (and shrubs) with
minimum risk to the public, and in a cost effective manner.
Nested within the 20-year strategic plan are four five-year
management plans. Each of these will be the first level of
operational planning and represents the link between
high-level strategic objectives and on-the-ground
management activities. This level of planning also presents
the opportunity to implement active adaptive management,
defined by the Millennium Ecosystem Assessment project
(2005) as:
A systematic process for continually improving management
policies and practices by learning from the outcomes of
previously employed policies and practices. In active adaptive
management, management is treated as a deliberate
experiment for the purpose of learning.
This concept recognises that urban forests are complex,
dynamic entities and that while managers may not always
be able to predict changes they must be prepared to
accommodate such changes while still working towards
broader goals for the management of the resources in their
care. Through active adaptive management, a problem is
first carefully assessed and a strategy or approach is
designed and implemented to address it. The results of the
approach are then monitored in a systematic manner and
any adjustments are made based on the experience gained
and new information that has become available. The
adjusted approach is implemented and the evaluation cycle
continues for as long as is necessary to accomplish the goals
or to accommodate changing environmental, social, or
policy directions. This is achieved through the review of
each five-year management plan near the end of its
planning horizon, and subsequent five-year management
plans are based upon the results of these reviews. Therefore,
the intention is not to attempt to develop all four plans at
once, but to develop them sequentially in response to
lessons learned and, if applicable, changing priorities. This is
represented graphically by the arrows connecting each five-
year management plan shown in Figure 1.
The final level of planning is the annual operating plan,
which directs day-to-day operations and can be used to
project budget requirements for all aspects of maintaining
the urban forest. Each annual plan may include detailed
plans for tree establishment, pruning, removals, inspections
and maintenance of the tree inventory. Such activities
should be guided by directions outlined in the strategic and
five-year plans. Initially, annual operating plans will need to
20-year Stategic Plan
Increasing Detail
Plan #1
Plan #2
Plan #3
Plan #4
Annual Operating Plans
32 Trees, people and the built environment
address priorities derived from a community’s tree inventory,
but, as these are addressed over time, more effort can be
focused on proactive management objectives. Annual
operating plans can be integrated with a community’s asset
management system and GIS information technology to
optimise resource allocation. For example, planting
locations can be mapped on a municipal GIS to inform all
related staff about the future location of street or park trees
to help plan future maintenance activities.
Key urban forest management
Several key themes and issues should be addressed as
components of any urban forest management plan, and
some must be addressed at all three (20-year, five-year and
annual) planning levels. The content and scope of each plan
component can vary depending on a variety of factors
specific to the community undertaking the planning process.
These factors may include the community’s urban forest
objectives; its historical, current and anticipated land use
cover; the degree to which it has already begun to
undertake urban forest management; available resources;
the level of stakeholder and community interest; and the
willingness of the community and its residents to invest in
the local urban forest.
Figure 2, below, represents the basic structure of a typical
urban forest management plan developed using the
framework outlined in this paper. The top row (the overall
plan) is divided into five key components, which are further
subdivided into different topic areas, or planning
components. As stated, these will vary and should be
tailored to each municipal context.
As noted above, some of these components (shaded in
Figure 2) are addressed at each planning level. To illustrate
how these components can be addressed at each level, let
us consider the example of tree establishment. On a
long-term horizon (20-year strategic plan), the plan can set
long-term objectives such as increasing species diversity,
developing improved tree planting standards, or increasing
tree canopy cover through tree planting. At the medium-
term (five-year management plan) level, the plan can
commit to implementing pilot projects to test new tree
Figure 2 Typical components of a strategic urban forest management plan.
Strategic Urban Forest Management Plan
Goals and
Public Education/
Communication Budget
History and
Mapping /
Shaded areas are found
in all plan levels
Plant Health Care/
Integrated Pest
Tree Risk
Plenary session 1: Management of the urban forest 33
species or planting methods, or might identify particular
locations for targeted planting to provide specific benefits
(e.g. more trees in urban heat island areas). At the annual
operating plan level, operations staff might prepare planting
lists and locations for next year’s plantings to ensure
adequate budget and staff allocations that address the mid
and long-term objectives.
Conversely, other components (not shaded in Figure 2) may
or may not need to be addressed at each planning level. For
example, there may not be a need to plan for coordination
with higher level management policies during day-to-day
operations, and these would therefore not be considered in
the development of an annual operating plan.
In order to effectively develop and support
recommendations designed to improve urban forest
management, each plan component must contain four
elements to inform the recommendations. The first element
is a review of current management practices and policies in
the community, with regard for the particular subject area in
question. The second is a review of relevant ‘best
management practices’ from scientific and technical
literature and precedents from other jurisdictions. The third
component should compare the current status to best
practices, and identify gaps and opportunities for
improvement. Finally, the fourth component should review
and consider input and ideas from the various internal and
external stakeholders, typically garnered through a multi-
part consultative process. This information provides the
background and rationale for recommendations and
resource requirements proposed in the management plan.
The key sections of a typical urban forest management plan
are outlined in more detail, below.
Urban forest/tree inventory
As is the case with any renewable resource, an inventory is
an essential tool for the development of management
strategies. It will identify details of the structure of the urban
forest, which are necessary for the planning of management
activities to achieve specific goals. These details may include
species composition, the mixture of native and non-native
species, age structure, tree condition, location, size,
management history and habitat. Inventories may also
reveal other valuable assets such as the presence of rare or
endangered species that may otherwise be overlooked. A
wide range of inventory options are available, from basic
street tree assessments to broader urban forest resource
analysis studies (e.g. i-Tree Eco), which can provide a better
understanding of urban forest structure and function in both
the public and private realm. The type of inventory used
may also vary depending upon the extent of urban forest
management in a given area. For example, intensively
managed zones such as streets may have a higher level of
inventory detail (e.g. individual tree assessment) than
extensively managed zones such as natural areas (e.g. forest
stand inventory or ecological classification).
Communities with well-developed inventories may develop
much of the management direction based upon the results
of such studies in this section of the plan. Communities with
limited or no inventories may direct the plan towards
collecting such data in order to inform future management.
A key component of the tree inventory section should also
be an inventory maintenance plan, outlining how the
inventory will be updated and used to its fullest capacity on
an ongoing basis.
Tree establishment
At the level of the strategic plan, tree-planting priorities
should reflect overall objectives with respect to tree cover,
species distribution, tree replacement policies, stock
specifications, habitat requirements and other considerations.
At the management plan level, planting plans can be drawn
up once an accurate assessment of the plantable spots is
determined from the inventory or from other means of
spatial analysis. Innovative approaches to providing suitable
tree habitat should also be identified and recommendations
to implement them should be developed.
Tree maintenance
At the level of the strategic plan, the plan should establish
overall goals for tree maintenance such as pruning, and
define the minimum standards to be applied. Objectives to
enable a transition from reactive to proactive management,
including grid pruning, regular inspection, etc., should be
developed. In the medium-term management plan, the plan
should identify the areas in which tree maintenance will take
place over the five-year term.
Tree protection
This section should review current practices and threats
related to tree protection and the municipal development
approval process (if applicable) with respect to trees and
tree protection. This section may also discuss existing,
proposed or potential tree protection by-laws as well as
tree-related guidelines for protection during the
construction process.
34 Trees, people and the built environment
Plant health care and integrated pest
The urban environment is hostile to the long-term health of
trees and shrubs. Environmental stresses both above and
below ground weaken natural defence systems and leave
plants prone to insect infestations and diseases. Plant Health
Care (PHC) and Integrated Pest Management (IPM) planning
should be an integral part of any strategic plan. PHC is a
proactive approach to tree management that strives to
increase the health and vigour of trees such that their
natural defence mechanisms will protect them. IPM includes
similar aspects, with a focus on reducing pesticide use and
managing and monitoring pest populations. Some aspects
of PHC and IPM are:
Proper tree selection: the right tree in the right place;
Early pruning of young trees to establish strong
structure for long-term stability;
Fertilisation and watering according to the soil
conditions and the species requirements;
Structural support systems;
Utilising an array of cultural practices and biological
controls to reduce the use of fungicides, pesticides
and herbicides;
Pest vulnerability analysis;
Regular monitoring and reporting;
Active adaptive management.
Tree risk management
Liability is a major concern for urban forest managers. At the
strategic level, the plan should commit to developing a tree
risk management strategy if one is not already in place,
tailored to available resources and tolerance for risk. At the
five-year management plan and annual operating levels, the
plan should identify risk trees and outline implementation of
mitigation practices.
Outreach and public engagement
Effective communication is a vital part of urban forest
management. In most jurisdictions, the urban forest is an
‘unknown’ entity that both the public and administrators
take for granted rather than recognise as an important
municipal and community asset. In many communities
most of the urban forest is privately owned. Therefore, an
educational communications and outreach programme for
the community should be developed and implemented in
order for urban forest management to be effective. This
component should also outline existing and potential
partnerships and funding sources.
At the strategic level, items that must be considered in
management and operational plans will be ascertained. The
initial budget available to the urban forest management
process will help to focus or prioritise the issues that can be
addressed. Sources of funding, as well as opportunities for
resource sharing, should also be identified. It is important to
note that while recommendations should be realistic from a
budgetary standpoint, current available resources should
not limit or guide the direction of the plan, or prevent the
development of progressive initiatives and
This section of the plan should include mechanisms for
monitoring the implementation of the plan’s recommendations
and assessing successes and shortcomings. It is recommended
that a criteria and indicators based approach to monitoring,
as outlined in the following section of this paper, be used at
the end of every management plan (i.e. five-year) cycle. This
section should also include the baseline criteria and
indicators-based analysis to provide a benchmark of the
state of the urban forest prior to the development and
implementation of the plan.
In keeping with the proposed plan framework, it is
suggested that recommendations to be implemented within
the first five years be supported with accurate budget
forecasts, clear priority rankings, delineation of
responsibilities, and other supporting information such as
potential partnerships, funding sources, etc.
Recommendations for implementation in the remaining
years of the strategic horizon can be supported by a
priority ranking or a time range (e.g. 2015–2019), or can
be slotted into one of the future five-year management
plans (e.g. within 3rd planning cycle).
Integrating criteria and indicators
into strategic planning
A progressive urban forest management plan should include
recommendations that improve the effectiveness and
efficiency of a community’s urban forestry programme,
moving it from reactive maintenance to proactive
management. However, the concept of active adaptive
management embedded in such a plan necessitates regular
monitoring to ensure that progress is being made towards
Plenary session 1: Management of the urban forest 35
urban forest sustainability. A means of defining sustainability
is also required. For these reasons, the framework of criteria
and indicators of urban forest sustainability, developed by
Clark et al. (1997) and refined and updated by Kenney et al.
(2011), is well suited for integration into the development
and implementation of an urban forest management plan
for any community.
The publications referenced above have discussed criteria
and indicators in detail, and they will not be greatly
expanded upon in this paper. In summary, this approach to
planning includes 25 distinct criteria under three general
topics (Vegetation Resource, Community Framework and
Resource Management Approach). A community’s current
standing relative to each criterion is assessed by means of
four indicators, ranging from low through moderate, good or
optimal. Each indicator refers to a key objective; moving
along the scale from low to optimal for each criterion places
the community closer to achieving a sustainable urban
forest. Table 1 shows three example criteria and their related
indicators and key objectives.
A major strength of the criteria and indicators approach is
that it enables an in-depth and comprehensive assessment
of the current status and progress of an urban forest
management programme. It also challenges the all-too-
prevalent notion that overly simplistic metrics such as
canopy cover percentage or the number of trees planted
per year are, in and of themselves, good indicators of urban
forest sustainability. Moreover, a criteria and indicators
assessment illustrates the strengths of a community’s urban
forest management programme and, more importantly,
clearly highlights opportunities for improvement. This in
turn enables managers to more effectively allocate limited
resources with the objective of moving towards optimal
performance levels and sustainability.
Criteria and indicators are most useful at two stages of the
management planning process. Firstly, they can be used
to undertake a baseline assessment of the current status of
a community’s urban forest and forestry operations.
Secondly, they are an invaluable tool for tracking the
successes and shortcomings of each of the five-year
management plans discussed in the previous section, in
order to inform goal setting and prioritisation for each
subsequent planning horizon.
As a method for undertaking a baseline assessment, criteria
and indicators are typically reviewed at the outset of the
management planning process by a community’s head urban
forester, or preferably by an inter-departmental committee
including staff such as engineers, planners, communications
personnel and information technologists. Outside the
municipal realm, criteria and indicators can be reviewed by
the various stakeholders who are in a position to inform and
improve the indicators. Completing the level of assessment
required to determine the appropriate indicator for each
criterion may take some time and effort, but is an effective
way to set the priorities for the strategic management plan.
Once the baseline performance assessment is completed, the
Table 1 Three example criteria for urban forest sustainability with associated indicators and key objectives.
Criteria Performance indicators Key objectives
Low Moderate Good Optimal
Relative canopy
The exiting canopy
cover equals 0–25%
of the potential.
The existing canopy
cover equals 25–30%
of the potential.
The existing canopy
cover equals 50–75%
of the potential.
The existing canopy
cover equals
75–100% of the
Achieve climate and
region appropriate
degree of tree cover,
community wide.
General awareness
of trees as a
community resource
Trees seen as a
problem, a drain on
Trees seen as
important to the
Trees acknowledged
as providing
social and economic
Urban forest
recognised as vital to
the community’s
social and economic
well being.
The general public
understanding the
role of the urban
Tree habitat
Trees planted without
consideration of
site conditions.
Tree species are
considered in
planting site
guidelines are in
place for the
improvement and
the selection of
suitable species
All trees planted in
sites with adequate
soil quality and
quantity, and
growing space to
achieve their genetic
All publicly owned
trees are planted in
habiats which will
maximise current
and future benefits
provided to the site.
36 Trees, people and the built environment
planning effort may focus on moving the lowest assessed
criteria towards the optimal range. Alternately, managers can
choose to prioritise management to address the key
objectives that are most closely in line with broader
community strategic objectives. Finally, the assessment may
serve as an information-gathering exercise; simply going
through a collaborative assessment process will provide
managers with invaluable insight into the state of the urban
forest and the perspectives of other stakeholders.
Criteria and indicators are also a key component of the active
adaptive management cycle. Near the end of each five-year
management plan’s scope, urban forest managers can use
the criteria and indicators to evaluate the strategic plan by
tracking in which direction the indicators for each criterion
have transitioned on the scale, if at all. Then, by comparing
where recommendations and resource allocations were
initially focused relative to successes and shortcomings,
alternative strategies can be developed as required.
Practical applications of the
strategic planning framework
To date, the strategic management planning framework and
criteria and indicators have been adopted by several
municipalities in southern Ontario, Canada, as part of the
process of developing each community’s urban forest
management plan. Each community’s experience has been
unique, and the differences in each case highlight the flexibility
of the conceptual and temporal framework presented here.
Two distinct examples of the application of the strategic
planning framework are the Town of Ajax and the City of
Burlington. Located to the east and west, respectively, of the
most populous city in Canada – Toronto – both municipalities
have dedicated and skilled urban forest managers, but differ
in terms of the resources available for urban forestry, with Ajax
having the smaller urban forestry programme. Both
municipalities undertook the plan development process in
2010, albeit with markedly different approaches.
Ajax’s focus was strongly geared towards developing a
medium-term plan to improve on-the-ground operations
within the first five years, with fewer long-term strategic
objectives or recommendations. To this end, much of the
up-front consultation, such as visioning sessions and goal-
setting, was undertaken by municipal staff internally and
with key stakeholders well in advance of developing the
plan. In Ajax, the plan development had the benefit of being
informed by a recently completed urban forest study that
collected and analysed data on overall urban forest cover,
structure and species composition. This study developed its
recommendations in the context of the urban forests
sustainability criteria and indicators (Kenney et al., 2011) and
highlighted gaps in areas such as tree inventory, canopy
cover and leaf area assessment. Criteria and indicators were
then recommended for use as part of the urban forest
monitoring programme, to be implemented towards the
end of the first five-year management plan to inform the
subsequent plan.
The City of Burlington adhered more rigorously to a three-
level strategic planning framework, with a focus on both
short- and medium-term operational improvement as well
as more long-term strategic objectives. Consultations were
held throughout the planning process, with internal and
external stakeholders being given an opportunity to
participate extensively in the visioning process, development
of strategic priorities and review of recommendations. There
was also a strong desire to maintain consistency with the
direction of the City’s overall strategic plan, which is updated
every four years. Unlike in Ajax, a preliminary criteria and
indicators assessment was undertaken at the outset of the
project, and helped inform the direction of the plan by
highlighting key gaps and issues to be addressed. As in Ajax,
criteria and indicators also form the main component of
the active adaptive management strategy to measure the
success of plan recommendations in promoting urban
forest sustainability.
Overall, the experiences of developing urban forest
management plans for the two communities discussed
above, as well as the final products, were quite different.
Both municipalities tailored the framework requirements to
better suit their needs, illustrating the flexibility of the
strategic model. Whereas one community focused more on
short- to medium-term operational improvements, and the
other on long-term strategic objectives, in neither plan were
any key urban forestry issues overlooked or given less than
the necessary level of attention or detail. This is due in part
to a strategic framework that clearly identifies the important
items for all urban forest managers to consider, and outlines
the appropriate planning horizons to enable effective
management actions to be implemented.
Although this paper focuses on the use of the planning and
monitoring framework in the municipal realm, it can also be
applied in other urban forest management contexts. The
same plan framework has been successfully tailored by
other stewards of the urban forest, which, although they
manage fewer trees, contend with many similar issues.
These have included large landholders such as golf and
country clubs. Issues such as cyclical maintenance, tree
Plenary session 1: Management of the urban forest 37
establishment, protection and risk management, invasive
species, and even community stewardship and public
awareness, are equally relevant and pressing for such
institutions as they are for municipalities, albeit on a smaller
scale. Planning horizons may or may not be as long as for
municipalities; some courses have elected to shorten their
long-term plans to ten years, while others have maintained
a 20-year frame of reference.
In the context of golf course tree management, a number of
criteria may not be useful, applicable or practical. For
instance, assessing the relative canopy cover on golf course
grounds has little utility since landscaping needs typically
take precedence on such lands and obtaining full canopy
cover is not practical. Many other criteria, however, remain as
important as they do for municipalities. These include tree
species diversity, cooperation with local governments and
community buy-in into tree management, among others.
Adoption of this strategic framework and monitoring
approach by smaller institutions and landowners further
highlights the model’s flexibility. Similarly, the framework has
been implemented by at least one municipality to
neighbourhood scale planning, with city staff and resident
representatives working jointly on a steering committee to
develop and implement plan recommendations. This pilot
project is still in its infancy and the success of this
application is yet to be determined, but it holds promise,
and the process itself is a good opportunity for
neighbourhood residents to become more engaged in their
part of the urban forest. The same community is looking for
ways to tailor the criteria and indicators approach to
undertaking a gap analysis for management of a significant
natural area. It is anticipated that many of the current criteria
will need to be replaced, while some will be equally
applicable as they are to urban forest management.
Concluding remarks
In this paper, we have presented a temporal and contextual
framework for strategic urban forest management
planning and reviewed how a comprehensive monitoring
framework can be integrated into the plan development
and review process.
The three-tiered framework is well suited to addressing the
challenges faced by urban forests through planning for at
least three reasons. Firstly, it enables real linkages between
long-term, high-level strategic objectives and daily
on-the-ground management and maintenance activities,
by way of intermediate management plans. Secondly, it is
flexible enough to enable a community, or others involved
in planning, to tailor it to suit their needs, while ensuring
that important issues are not overlooked. Thirdly, with
built-in mechanisms to ensure adaptive management by
way of management plan review, progress towards
achieving urban forest sustainability is, if not ensured, then
greatly enhanced. With the integration of criteria and
indicators, this planning approach effectively addresses
urban forest management and sustainability issues on a
long-term horizon.
The challenges to urban forests are clear and undeniable. It
is our hope that more communities, institutions and
landowners recognise the value of a strategic and
collaborative approach to urban forest planning so that
future generations might enjoy all of the important benefits
that trees provide us with today.
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Results of a long-term project using controlled
mycorrhization with specific fungal strains on
different urban trees
Several research projects have been undertaken in the past years to identify the effects of mycorrhiza, which include
increased water and nutrient uptake, and protection against drought, salinity, heavy metals and pathogens (Augè, 2001) on
widely planted shade trees. However, most experiments were carried out under conditions different to those commonly
found in the urban environment. The aim of this work was to investigate the effect of different strains of mycorrhizae-forming
fungi specifically selected for the urban environment in different situations (i.e. urban and historical parks, parking lots,
boulevards) usually found in cities all over the world. The project began in 2006 and was carried out on several of the most
widely planted shade tree species of different ages ranging from newly planted to mature trees located in our historical
parks. Trees were inoculated with specific mycorrhyzal inoculi according to species and environmental conditions.
Different growing conditions were tested ranging from trees growing in a parking lot, to trees growing in historical or
peri-urban parks. Results obtained to date have been variable according to species and environmental conditions. Some of
the test species (i.e. Celtis australis) responded quickly to mycorrhizal fungi that were extremely effective in increasing plant
growth and leaf gas exchange. Other species (i.e. Tilia spp.) showed a different response according to plant age and
planting site. Other species (i.e. Fraxinus excelsior) had a slow response to mycorrhyzal inoculation. In general there has
been a positive (sometimes very strong) response to mycorrhizal inoculation and further data will be harvested in 2011.
Mycorrhizae-forming fungi are ecologically significant because they form relationships in and
on the roots of a host plant in a mutualistic association. The host plant provides the fungus
with soluble carbon sources, while the fungus provides the plant with several benefits including
enhanced nutrient, especially phosphorus, uptake (Yao et al., 2001; Habte, 2006); protection
against drought through increased water use efficiency and enhanced root exploration of the
available soil volume (Espeleta et al., 1999; Augè, 2001; Kaya et al., 2003); and reduction in
disease incidence (Thygesen et al., 2004), pathogen development (Cordier et al., 1996) and
disease severity (Matsubara et al., 2001). It has been reported that mycorrhiza protect the host
plant from heavy metals (Smith and Read, 1997; Joner et al., 2000) and salinity by protecting
cell membrane integrity through higher root accumulation of P and Ca2+ and by increasing the
efficiency of sodium-excluding mechanisms in infected roots (Mancuso and Rinaldelli, 1996;
Rinaldelli and Mancuso, 1996). However, the urban environment is markedly different from
natural and forest environments where mycorrhizal fungi have evolved and adapted and,
consequently, the ecological distribution of fungi is probably altered in an urban environment.
Recent work analysed mycorrhizal colonization patterns of Tilia grown in the urban, nursery
and forest environment (Timonen and Kauppinen, 2008). They showed that healthy street and
forest trees had higher number of mycorrhiza morphotypes than unhealthy urban trees.
Surprisingly, none of the mycorrhizal fungi found in the nursery were found in the urban
environment, suggesting that the nursery genotypes are either not adapted to street conditions
or they are outcompeted as transplanted trees establish a more mature mycorrhizosphere
(Timonen and Kauppinen, 2008). Since drought, use of de-icing salts, lack of nutrients and
attack from pathogens are among the main causes of failure of urban trees (Fini and Ferrini,
2007), the inoculation of urban trees with selected, native, competitive and effective
mycorrhiza may enhance tree growth and survival in the urban environment. However, studies
tree physiology, growth,
transplanting, photosynthesis
Francesco Ferrini and
Alessio Fini
Department of Plant,
Soil and Environmental
Sciences, University of
Florence, Italy
Parallel session 1a: Tree planting and establishment 39
40 Trees, people and the built environment
regarding mycorrhizal inoculation of urban trees in Europe
are few. The aim of this project was to evaluate the effect of
inoculation with selected native mycorrhizal fungi on trees
growing in a street environment, in a parking lot and in an
historical and peri-urban park. The results are a part of a long-
term research project started in 2006 with an initial
inoculation and that will conclude in 2011.
Material and methods
Selection, propagation and distribution of
the mycorrhizal fungi
Selection, multiplication and distribution of the specific
mycorrhizal inocula were as described in Fini et al. (2011).
Briefly, five to seven healthy mature trees growing in the
urban and peri-urban environment were selected and fine
roots were sampled by digging holes around the tree. Holes
were deep and wide as required to harvest a sufficient
amount of fine, absorbing root. Trees were selected on the
basis of the following criteria: 1) health; 2) age; 3) same
species as the trees to be inoculated; 4) similar
environmental conditions to those of the site where the
inoculum had to be distributed. Each sample weighed
approximately 500grams of roots + soil. Root samples were
analysed at the MycoMax laboratory (MykoMax Gmbh,
Wuppertal, Germany). Mycorrhizal species were isolated
and multiplied in a greenhouse in non-sterile conditions
following the procedure developed by MycoMax in
agreement with German FLL standards for mycorrhiza
inoculation. Criteria for the selection of mycorrhizal fungi
were: 1) frequency of mycorrhizal root tips (ecto-) or
intensity of root colonization (VAM); 2) structure and vitality
of the Hartig net (ecto-) or arbuscules (VAM); 3)
phosphatase activity (VAM). After at least eight months of
culture on living and viable roots, roots containing fungal
mycelium were harvested and mixed with montmorillonite
clay and a hydro-gel to maximize durability. The fungal
inoculum was distributed within one month of its
production. Three holes exposing the absorbing roots of the
tree to be inoculated were dug for each 10cm of stem
diameter (measured at 1.3m trunk height) of the tree to
inoculate. 125ml of product were placed in each hole to
ensure contact between fungal mycelium and absorbing
tree roots. Holes were quickly re-filled with a shovel.
Container-grown trees in nursery production
A total of 80 two-year-old hedge maples (Acer campestre L.),
80 littleleaf lindens (Tilia cordata Mill.) and 80 pedunculate
oaks (Quercus robur L.) were potted in 3 litre containers using
a peat:pumice (3:1) substrate amended with 3kgm-3
dolomite to neutralize pH. A reduced dose (1kgm-3) of a
controlled release fertilizer (Ficote®, 15–3, 5–10, 8–9 months
formulation, Scotts, Marysville, OH) was used in this
experiment to avoid an excessive soil chemical fertility
which may decrease mycorrhizal colonization. Container
capacity, wilting point and effective water holding capacity
of the substrate was determined using the gravimetric
method described by Sammons and Struve (2008). 40 plants
per species were inoculated with specific mycorrhizal fungi
(ECM in oak, VAM in maple and both ECM and VAM in
linden). Inoculation was done on March 2008 using 25ml of
inoculum per plant. Plants were either irrigated daily in order
to restore container capacity, well watered (WW) or irrigated
daily to 30% of container water capacity, water shortage (WS).
The experimental was a randomized block design with 6
blocks and 5 plants per species and treatment in each block.
Trees from the nursery to the landscape:
plant material and growing conditions
A total of 48 plants (14–16cm circumference) were selected
in Lappen Nurseries (Nettetal, Germany) in winter 2007. In
April 2007, 24 plants were inoculated (+IN) with specific ecto-
and endomycorrhizal fungi selected in Milan urban area and
the remaining 24 plants were not (-IN). In May 2008, all plants
were root pruned. Then, plants were grown in the nursery until
early spring 2010, when they were moved to Milan. At
transplanting, half of the plants were inoculated with the same
fungi as in 2007 (+IT) and the remaining half were not (-IT).
Therefore, four treatments were compared: 1)+IN+IT: plants
inoculated both in the nursery and at transplant; 2) +IN-IT:
plants inoculated in the nursery but not at transplant; 3) –IN+IT:
plants inoculated only at transplant; 4)-IN-IT: control plants
(never inoculated). Plants were arranged with a factorial
randomized block design with 8 blocks and 4 plants per block.
Young trees in urban parks
In November 2005, 62 pedunculate oaks (Quercus robur;
10–12cm circumference) were planted in two rows in an
urban park in San Donato Milanese (Milan, Italy). Distance
between plants was 8 m within the row and 8m between the
rows. 24 trees were inoculated with selected specific ecto-
mycorrhizal fungi and 24 were not inoculated. Inoculation
was performed in November 2006, approximately one year
after planting. Trees were arranged in a randomized block
design with 3 blocks and 8 plants per treatment within each
block. 14 remaining oaks were used to separate, on the row,
inoculated and control plants, to reduce the risk of unwanted
contamination on non-inoculated plants.
Parallel session 1a: Tree planting and establishment 41
Trees in parking lots: plant material and
growing conditions
In November 2005, 24 European hackberry (Celtis australis;
14–16cm circumference) were planted in a parking lot in San
Donato Milanese (Milan, Italy). Trees were planted in a
planting hole with an unpaved surface of about 0.5m2,
surrounded by asphalt and concrete. Trees were arranged in
a randomized complete block design with 6 blocks and 2
plants per treatment within each block. 12 trees were
inoculated with species-specific, native strains of
ectomycorrhizal fungi, and 12 trees were not inoculated and
acted as controls. Inoculation was undertaken in November
2006, approximately one year after planting.
Street trees: plant material and growing
In spring 2004, 20 European ashes (Fraxinus excelsior
‘Westhof’s Glorie’; 20–25cm circumference) were planted
along a street characterized by high traffic and pollution,
located in Florence (Italy). The size of the planting hole was
about 1m2. Trees were planted in a randomized complete
block design with 5 blocks. 10 trees were inoculated with
species-specific strains of endomycorrhizal fungi and 10
trees acted as control. Inoculation was in April 2006.
Trees in a historical park: plant material
and growing conditions
In autumn 2006, 14 mature European linden (Tilia x europaea;
170–220cm circumference) and 14 mature horse chestnut
(Aesculus hippocastanum; 120–160cm circumference) were
selected in a historical park located in the city-centre of
Milan. 14 additional newly planted Tilia x europaea (18–20cm
circumference) and 14 Aesculus hippocastanum (20–25cm
circumference) were selected in the same location. Trees were
planted in a heavily compacted soil. The experimental set-up
was a complete randomized design using a single tree per
replicate and 7 replicates. 7 mature and 7 young trees of each
species were inoculated with selected native and species-
specific strains of both ecto- and endomycorrhiza (linden) or
with endo-mycorrhizal fungi (horse chestnut). Inoculation was
in November 2006.
Measurements of tree growth and vitality
One year after inoculation, a sample of fine root + soil was
harvested from inoculated and control trees. Samples were
harvested from one (historical park, street trees) or two
(nursery) plants per treatment and replication. Roots were
carefully separated from the soil and cut into 1cm long
pieces. Frequency of ectomycorrhizal roots was measured on
200 root tips as the ratio of mycorrhizal root tips to total root
tips (Newton and Pigott, 1991). To evaluate VAM colonization,
roots were stained using 0.05% Trypan blue in lactoglycerol
(Koske and Gemma, 1989). Percentage of root colonization
was measured by counting cross-hair intersections using a
stereomicroscope (McGonigle et al., 1990).
Biomass of container-grown plants was determined after
two years from inoculation (2009). To measure biomass,
plants were cut at the root flare, roots were cleaned with a
flush of compressed air and leaves were excised from stems.
Roots, stems and leaves were then oven-dried at 70°C for 72
hours and weighted separately to determine dry weight.
Biomass of field-grown trees was estimated measuring shoot
and diameter growth. According to the different experiments,
the following parameters were measured during the entire
duration: Shoot growth was on 20 shoots per treatment per
replicate. Stem diameter was measured at 1.3m trunk height.
Leaf gas exchange was generally measured on three fully
expanded leaves per treatment and block/replicate with an
infrared gas analyser (Ciras-2, PP-System, Hertfordshire, UK).
Measures were taken at saturating (1300mol m-2 s-1) light
intensity, ambient temperature and 360 ppm CO2. Water use
efficiency was calculated as the A to E ratio (Fini et al., 2009).
Chlorophyll fluorescence was measured with a portable
Plant Efficiency Analyzer (Hansatech Instruments Ltd, King’s
Lynn, UK) on the same leaves as gas exchange. Fluorescence
values were obtained after adapting leaves to darkness for
30 min by attaching light-exclusion clips to the leaf surface
of whole trees. Upon the application of a saturating flash of
actinic light (3000mol m−2 s−1 for 1 sec), fluorescence raises
from the ground state value (Fo) to its maximum value, Fm.
This allows the determination of the maximal quantum yield
of PSII (Fv/Fm).
Chlorophyll content was measured two times during the
growing season in 2007 (only on Fraxinus excelsior) and 2008
with a SPAD-meter (Konica Minolta Holding Inc., Tokyo,
Japan). Nine measurements per treatment per replicate were
undertaken. Readings were taken in the medial section of
the lamina, taking care not to include leaf veins in the
measurement chamber.
All data were analysed with one- or two-way ANOVA using
the SPSS statistical package for Windows (SPSS Inc., Chicaco,
IL, USA). Differences between means were determined using
Duncan’s Multiple Range Test.
42 Trees, people and the built environment
Results and discussion
Root colonization in inoculated and
control plants
Inoculation with selected mycorrhiza increased root
colonization in container-grown maples, lindens and oaks
(Table 1). Even if control trees were not inoculated, some
mycorrhiza were also found on their roots. Morphotyping of
control plant roots classified these mycorrhiza as ‘nursery
mycorrhiza’ (Fini et al., 2011). It is common to find ‘nursery
mycorrhiza’ on nursery stock, and in any case these fungal
species have been reported to be unable to thrive and
provide benefits to the host tree in urban conditions
(Timonen and Kauppinen, 2008). Similarly, inoculation of
oak trees in an urban park increased the frequency of
mycorrhizal root tips. This indicates the ability of selected
fungal strains to compete with native microorganisms and
efficiently form a symbiotic relationship, even when the
native mycorrhizal population is well developed (control
had 76% colonized root tips; Table 1). The endomycorrhizal
inoculum developed for ash was found to be effective in
street environments, even in those characterized by a well-
developed native mycorrhiza population. This is important
because native mycorrhizal populations are likely to provide
lower benefits to plants than selected fungal strains. If fungi
in the inoculum are quickly outcompeted by native
microorganisms or their infection is slowed down by
adverse environmental conditions, colonization of the host
plant is reduced and little or no benefit can be expected
from inoculation. Possibly this was the case for the newly
planted lindens and horse chestnuts in a historical park
where poor soil conditions such as heavy soil compaction
and lower carbon availability for the mycorrhizal fungus due
to lower carbon assimilation (thus lower availability of C to
support fungal growth and activity) of newly planted trees
resulted in a low inoculum efficacy (Nadian et al., 1997).
When the same ECM (linden) and VAM (horse chestnut)
inocula were used on mature, established trees, root
colonization was increased (Table 1).
Site Species Treatment % colonization (ECM) % colonization (VAM)
(in container)
Acer campestre
Inoculated - 53%
Control - 24%
P - **
Tilia x europaea
Inoculated 81% 17%
Control 59% 10%
P ** *
Quercus robur
Inoculated 80% -
Control 41% -
P ** -
Urban park Quercus robur
Inoculated 85% -
Control 76% -
P ** -
Street trees Fraxinus excelsior
Inoculated - 81%
Control - 71%
P - *
Historical park
Aesculus hippocastanum
(newly planted)
Inoculated - 59%
Control - 51%
P - n.s.
Aesculus hippocastanum
(mature trees)
Inoculated - 76%
Control - 63%
P - **
Tilia x europaea (newly
Inoculated 45% 37%
Control 44% 28%
P n.s. n.s.
Tilia x europaea (mature trees)
Inoculated 49% 39%
Control 36% 32%
P * n.s.
Data were collected one year after inoculation. * and ** indicate significant differences between treatments within the same species and planting site at
P<0.05 and P<0.01
Table 1 Percentage of colonization by ectomycorrhizal and endomycorrhizal fungi in inoculated and non-inoculated tree species planted in the nursery or
in different urban sites.
Parallel session 1a: Tree planting and establishment 43
Container-grown trees in nursery production
Inoculation with specific mycorrhiza did not enhance
biomass accumulation of maple, linden and oak saplings
growing in containers (Table 2). Plants growing in water-
stressed conditions had lower leaf, stem and root (except for
oak) dry weights than well-watered plants of the same
species, regardless of whether inoculated or not (Table 2).
Similar results were found by other authors on several
landscape trees (Gilman, 2001; Wiseman and Wells, 2009).
Induction of greater stress tolerance and therefore the
possibility to grow nursery crops with lower resource input
has been reported as the major benefit of mycorrhizal
technology in plant production systems (Davies, 2000). In
2009, water-stressed inoculated plants of maple and linden
showed higher carbon assimilation and similar transpiration
rates and therefore higher water use efficiency than water-
stressed control plants (Table 3). Water-stressed inoculated
oak had higher transpiration and similar carbon assimilation
and water use efficiency than controls. In well-watered
conditions, differences between inoculated and control
maple and linden were not significant (except for
assimilation in maple). Data indicated that in optimal
conditions the benefits of mycorrhiza are not always
conclusive. The inoculation-induced increase in
photosynthetic rate and water use efficiency may become
clearer under stress conditions and this may play a major
role in determining plant survival when plants are moved
from the optimal growing conditions of a nursery to the
suboptimal or stressful conditions of an urban environment.
Trees from the nursery to the landscape
Inoculation had no effect on stem diameter growth during
the nursery period (2007–2009) and in the first year after
planting into the landscape (2009–2010: Table 4). In 2007,
inoculation had no effect on shoot growth. In May 2008,
roots were pruned in the nursery according to best
management practices and this resulted in some degree of
stress to linden trees, as shown by a large decrease in shoot
growth in 2008 compared to 2007. When above-ground
growth was limited by root pruning, inoculation with
Data are the average of two samplings done in 2009. Different letters within the same column indicate significant differences between treatments at P<0.01.
* and ** indicate significant differences between treatments within the same species at P<0.05 and P<0.01.
2009 Acer Tilia Quercus
Leaf DW Stem DW Root DW Leaf DW Stem DW Root DW Leaf DW Stem DW Root DW
Effect of inoculation
Mycorr. 35.1 92.9 120.3 19.9 64.8 75.6 30.0 84.3 75.2
Control 32.3 90.2 116.4 19.3 63.3 70.7 28.0 83.0 88.7
P n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.
Effect of water stress
WW 38.1 111.5 142.4 22.9 77.6 89.8 34.2 111.4 88.1
WS 29.3 71.6 94.3 16.3 60.5 56.6 24.0 55.6 75.9
P * ** ** ** ** ** ** ** n.s.
Inoculation x water stress
P n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.
Table 2 Effects of inoculation, water stress and their interaction on leaf, stem and root dry weights (DW, g) in inoculated and control Acer,Tilia and Quercus
growing in containers in well-watered (WW) or water shortage (WS) conditions.
Table 3
Effects of inoculation, water stress and their interaction on carbon assimilation (A, µmol m-2 m-1), transpiration (E, mmol m-2 m-1)
and water use
efficiency (WUE, µmol CO2/mmol H2O) in inoculated and control Acer,Tilia and Quercus growing in containers in well-watered (WW) or water shortage
(WS) conditions.
2009 Acer Tilia Quercus
Myco. WW 9.24 a 3.07 a 3.01 b 7.12 a 3.55 a 2.01 ab 10.90 a 3.99 a 2.74
Contr. WW 7.64 b 2.87 a 2.66 b 6.15 a 3.17 a 1.94 ab 11.43 a 4.11 a 2.78
Mico. WS 4.27 c 1.05 b 4.08 a 3.38 b 1.33 b 2.57 a 8.08 ab 2.57 b 3.15
Contr. WS 1.60 d 0.62 c 2.58 b 1.11 c 0.75 b 1.50 b 5.09 b 1.73 c 2.95
P ** ** ** ** ** ** ** ** n.s.
44 Trees, people and the built environment
selected mycorrhiza resulted in significantly longer shoots
than with untreated plants (Table 4). One year after root
pruning (2009), shoot growth recovered to levels similar to
2007 and no significant differences between treatments
were recorded. Lindens were transplanted into the urban
environment in spring 2010. Transplant stress occurred in
the following growing season and greatly reduced shoot
growth (Table 4). Again, when stress occurred, an
inoculation-induced increase in shoot growth was found. In
particular, shoot growth was higher in plants inoculated in
the nursery and both in the nursery and at planting when
compared to control and plants inoculated only at planting
(Table 4).
Carbon assimilation was not affected by inoculation
with specific mycorrhiza during the nursery phase, even
after a root pruning treatment (Figure 1). After planting
in the landscape, plants inoculated both in the nursery and
at planting showed higher carbon assimilation than non-
inoculated control plants. Inoculating plants both in the
nursery and at transplanting possibly contributed to a
greater root colonization by mycorrhizal fungi, which
resulted in higher photosynthetic rates. Transpiration,
stomatal conductance and water use efficiency were little
affected by mycorrhizal treatment, during both the nursery
period and after transplanting (data not shown). Therefore,
we can speculate that trees inoculated both in the nursery
and at planting had a higher photosynthesis on a plant-scale
basis (higher A) and this may have contributed to greater
shoot growth. Previous research in this area has shown that
whole-plant photosynthetic rate under resource-unlimited
conditions is proportional to shoot growth and leaf area
(de Palma et al., 2004).
Young trees in an urban park
Stem diameter growth of newly planted pedunculate oak
(Quercus robur) was not affected by inoculation with selected
specific ectomycorrhiza throughout the experiment (Table
5). Shoot growth was increased by inoculation in the first
growing season after inoculation, although shoot growth
was very low due to transplant stress (Table 5). In 2008 and
2009 shoot growth was significantly greater in inoculated
oaks when compared to control, which indicates a
beneficial influence of mycorrhizal inoculation regarding the
establishment of oak trees. Even after establishment,
differences between treatments were confirmed and
inoculated plants showed higher shoot growth in both 2008
and 2009 compared to control plants (Table 5). SPAD values
were higher in inoculated plants in both years. Recent
papers on some woody species showed that SPAD readings
are highly correlated to leaf chlorophyll content (measured
using traditional destructive methods) (R2>0.82), leaf
carotenoids (R2>0.82) and leaf N-content (R2>0.53) (Luh et
al., 2002; Percival et al., 2008). Therefore, higher SPAD
readings in treated leaves may indicate a higher nutritional
status of inoculated oaks than control ones when planted
into an urban park. After September 2008, carbon
Inoculation Ø (cm) Shoot growth (cm)
Nursery Transplant 07/08 08/09 09/10 2007 2008 2009 2010
+IN-IT0.58 0.74 0.20 51.89 9.78 a 45.75 8.21a
+IT0.33 7.81 a
-IN-IT0.47 0.71 0.30 56.08 6.56 b 42.55 6.28 b
+IT0.35 5.84 b
P n.s. n.s. n.s. n.s. ** n.s. **
Table 4 Effect of inoculation in the nursery phase and/or at planting with specific mycorrhiza on linden trees growing in the nursery (2007–2009) and after
transplant in the landscape (2010). In 2008 trees were root pruned to prepare them for transplant.
Different letters within the same column indicate significant differences between treatments at P<0.01.
root pruning
Carbon assimilation in linden
A (µMOL m-2 s-1)
2008 2009 2010
Figure 1 Carbon assimilation (A, µmol m-2 s-1) in linden inoculated in the
nursery (+IN-IT), in the nursery and at transplanting (+IN+IT), not inoculated
(-IN-IT) and inoculated only at transplant (-IN+IT). Different letters within the
same sampling date indicate significant differences at P<0.05.
Parallel session 1a: Tree planting and establishment 45
assimilation was generally higher in inoculated oaks, even if
significant differences were found only on 18 May 2009
(Figure 2, left). Also, when significant differences were found,
inoculated plants had higher WUE than non-inoculated
ones (Figure 2, right). Higher WUE in plants inoculated with
selected fungal species were also found in other work and
were attributed to stomatal and nutritional effects induced
by inoculation (Guehl and Garbaye, 1990; Guehl et al., 1990;
Dunabeitia et al., 2004). Taking into consideration that WUE
is one of the main growth determining factors in potentially
harsh sites such as a urban environments, results obtained in
the third growing season after inoculation suggest that
ectomycorrhizal colonization may increase long-term
tolerance to water stress. Fv/Fm was not affected by
inoculation in 2008, and Fv/Fm was higher than 0.80 in
both treatments, a value indicative of healthy plants
(Percival, 2005; Figure 3). In 2009, inoculated plants had
higher Fv/Fm than non-inoculated plants. The higher
maximum yield of PSII (Fv/Fm values) measured in 2009 in
inoculated plants may explain the higher gas exchange
values found in treated oaks in 2009.
Carbon assimilation in oak Water use efficiency in oak
** **
A (µMOL m-2 s-1)
Mycorrhizal Control Mycorrhizal Control
Quercus robur
Ø (cm) Shoot growth (cm) Chlorophyll content (SPAD)
06/07 07/08 08/09 2007 2008 2009 June 2008 Sept. 2008
Mycorrhiza 0.70 1.30 1.43 13.52 68.22 71.4 43.2 43.6
Control 0.52 1.27 1.27 4.13 41.38 48.8 39.1 39.8
P n.s. n.s. n.s. ** ** ** * **
Table 5 Effects of inoculation with selected ectomycorrhiza on diameter and shoot growth and on chlorophyll content of Quercus robur planted in an
urban park.
* and ** indicate significant differences between mycorrhizal and control trees of the same species at P<0.05 and P<0.01.
Street trees and trees growing in a parking lot
Inoculation with local strains of species-specific mycorrhizal
fungi increased stem diameter growth in young, newly
planted European hackberry, growing in a parking lot
Figure 2 Carbon assimilation (A, µmol m-2 s-1, left) and water use efficiency (WUE, mol CO2/mmol H2O, right) in inoculated and non-inoculated
pedunculate oaks planted in an urban park. * and ** indicate significant differences between mycorrhizal and control trees within the same sampling date at
P<0,05 and P<0,01.
Fv/Fm in oak
Mycorrhizal Control
Figure 3 Maximal quantum yield of photosystem II (Fv/Fm) in inoculated
and non-inoculated pedunculate oaks planted in an urban park. * indicates
significant differences between mycorrhizal and control trees within the
same sampling date at P<0.05.
46 Trees, people and the built environment
(Table 6). Significant differences in stem diameter annual
growth between inoculated and non-inoculated plants were
found both in the first and the second year after inoculation,
but not in the third. Inoculation with German, species-
specific endomycorrhiza for Fraxinus excelsior failed to
increase diameter growth in ash trees growing along a road
(Table 6). Effect of mycorrhiza on shoot growth was highly
significant in 2007 and 2008 in ash and in 2007, 2008 and
2009 in European hackberry (Table 6). Mycorrhizal
inoculated ashes had 48% and 42% longer shoots than
control trees in 2007 and 2008, respectively. Shoots of
mycorrhizal inoculated hackberries were 55%, 98% and 80%
longer than those of non-inoculated control trees in 2007,
2008 and 2009, respectively.
Mycorrhizal inoculation increased carbon assimilation and
water use efficiency of hackberry in all sampling dates,
except in September 2008 (Figure 4). Five months after
inoculation (September 2006), no difference in carbon
assimilation and water use efficiency was found between
mycorrhizal inoculated and non-inoculated control ashes
(Figure 5). In 2007, mycorrhizal inoculated ashes had both
higher assimilation and water use efficiency than non-
inoculated plants, with significant differences confirmed in
2008 (Figure 5). Therefore, possibly, the inoculation-induced
increase in WUE allowed mycorrhizal trees to fix more
carbon dioxide per unit of transpired water, thus giving
greater carbohydrate availability for growth and defence.
The maximal quantum yield of photosystem II (Fv/Fm) is a
widely used index for measuring plant vitality and early
diagnostic measure of stress (Willits and Peet, 2001). Fv/Fm
measurement of healthy, unstressed plants is associated with
values ranging from 0.75 to 0.85 (Percival, 2005). Both
control and inoculated hackberries consistently showed
higher Fv/Fm values than 0.75, which indicated a high
adaptability of this species to difficult planting sites such as a
parking lot. Inoculated plants had significantly higher
Fv/Fm values than control plants in July 2008 and June
2009 (Figure 6). This indicated that the phytochemistry of
photosystem II was improved by mycorrhizal inoculation,
which can result from lower oxidative damage within
chloroplasts and/or from a better nutritional status of the
leaves. Chlorophyll content was higher in mycorrhizal
inoculated hackberries compared to control plants both at
the middle and at the end of the growing season (Table 6).
The higher SPAD-value measured in mycorrhizal inoculated
hackberries reflects a higher nutritional status of plants
compared to non-inoculated controls, when grown in a
stressful environment such as a parking lot (Luh et al., 2002;
Percival et al., 2008). No difference in leaf chlorophyll
content and Fv/Fm (data not shown) were found between
treatments in Fraxinus (Table 6).
Trees in a historical park
In 2006–2007, stem diameter growth was unaffected by
mycorrhizal inoculation on both young and mature linden
and horse chestnut (Table 7). Mature trees of both species
had greater stem diameter growth than newly planted trees.
In 2007–2008 mycorrhizal inoculation increased stem
diameter growth in mature lindens, but had no significant
effect on young trees. In the second year mycorrhizal
inoculated mature lindens had 318% higher diameter growth
than untreated control. No difference among treatments was
found in horse chestnut. In 2008–2009, stem diameter
growth of linden trees was similar among treatments, while it
was significantly higher in young horse chestnut than in
mature ones (Table 7). In 2008, shoot growth was significantly
increased by inoculation in mature lindens and horse
chestnuts, which had 20% and 55% longer shoots than
control trees, respectively (Table 7). No significant difference
was found for shoot growth in newly planted linden and
Ø (cm) Shoot growth (cm) Chlorophyll content (SPAD)
Celtis australis 06/07 07/08 08/09 2007 2008 2009 June 2008 Sept. 2008
Mycorrhiza 0.57 1.26 0.45 23.86 30.33 36.55 45.37 48.77
Control 0.30 1.07 0.37 15.40 15.25 20.25 39.06 35.68
P ** * n.s. ** ** ** ** **
Fraxinus excelsior 06/07 07/08 08/09 2007 2008 2009 2007 2008
Mycorrhiza N.D. 0.71 N.D. 7.05 10.12 N.D. 29.04 30.10
Control N.D. 0.88 N.D. 4.76 7.11 N.D. 30.03 30.40
P - n.s. - ** ** - n.s. n.s.
Table 6 Effects of inoculation with selected mycorrhiza on diameter and shoot growth and on chlorophyll content of Celtis australis and Fraxinus excelsior
planted in a parking lot and along a street, respectively.
* and ** indicate significant differences between mycorrhizal and control trees of the same species at P<0.05 and P<0.01. N.D. = not determined.
Parallel session 1a: Tree planting and establishment 47
horse chestnut trees. In 2009, shoot growth of linden was
higher in inoculated mature trees than in mature untreated
trees which, in turn, had higher shoot growth than both
inoculated and control young lindens. In horse chestnut,
shoot growth was increased by mycorrhizal inoculation in
both mature and young trees. As for diameter, shoot growth
was higher in young horse chestnut trees than mature ones.
Chlorophyll content was affected by mycorrhizal inoculation
in mature lindens and young horse chestnuts (Table 7), but was
unaffected by mycorrhizal treatments in newly planted linden.
Inoculation affected carbon assimilation (A, µmol m-2s-1)
of linden and horse chestnut (Figure 7). When significant
differences were found, inoculated plants always had higher A
when compared to control plants of the same age.
Carbon assimilation in hackberry Water use efficiency in hackberry
** **
** **
A (µMOL m-2 s-1)
Mycorrhizal Control Mycorrhizal Control
Figure 4 Carbon assimilation (A, µmol m-2 s-1, left) and water use efficiency (WUE, µmol CO2/mmol H2O, right) in inoculated and non-inoculated
hackberry trees planted in a parking lot. * and ** indicate significant differences between mycorrhizal and control trees within the same sampling date at
P<0.05 and P<0.01.
Carbon assimilation in ash Water use efficiency in ash
n.s. n.s.
** **
A (µMOL m-2 s-1)
average 06 average 07 average 08 average 06 average 07 average 08
Mycorrhizal Control Mycorrhizal Control
Figure 5 Carbon assimilation (A, µmol m-2 s-1, left) and water use efficiency (WUE, µmol CO2/mmol H2O, right) in inoculated and non-inoculated ash
trees planted as street trees. * and ** indicate significant differences between mycorrhizal and control trees of the same species at P<0.05 and P<0.01.
Figure 6 Maximal quantum yield of photosystem II (Fv/Fm) in inoculated
and non-inoculated hackberry planted in a parking lot. * and ** indicate
significant differences between mycorrhizal and control trees of the same
species at P<0.05 and P<0.01.
Fv/Fm in hackberry
Mycorrhizal Control
48 Trees, people and the built environment
Results obtained to date showed that the work of selecting,
multiplying and inoculating woody species with site- and
species-specific native mycorrhizal fungi can result in greater
growth (especially of field-planted trees, as no growth
increment was found in container-grown trees) and improved
physiology, as can be seen from leaf gas exchange and
chlorophyll fluorescence measurements. Time of response
was also affected by tree species. For example, Celtis australis
responded very quickly to mycorrhizal treatment, showing
significant differences for shoot growth and chlorophyll
content in the first growing season after inoculation, whereas
Fraxinus required at least two growing seasons before the
effect of mycorrhizal inoculation became significant. Tree
age also affected success of mycorrhizal inoculum. We
tested the same product on newly planted and mature Tilia
and Aesculus growing in a poor, heavily compacted soil and
found that symbiosis was more successful on mature trees,
compared to newly planted ones. There is evidence that soil
compaction limits root growth and activity (Fini and Ferrini,
2007) and reduces mycorrhiza formation (Nadian et al.,
1997; Entry et al., 2002). It is possible that roots of young,
newly planted trees were more affected by compaction than
those of large, established ones. High mortality of fine
absorbing roots especially on young linden may explain the
Ø 06/07 (cm) Ø 07/08 (cm) Ø 08/09 (cm) Shoot growth
2008 (cm)
Shoot growth
2009 (cm)
Chl. content
2008 (SPAD)
Mature mycorrhizal 2.7 a 1.4 a 0.8 14.5 a 21.5 a 52.4 a
Mature control 1.7 a 0.3 b 1.3 12.1 b 14.8 b 47.6 b
Young mycorrhizal 0.6 b 0.2 b 0.6 9.7 c 8.6 c 42.0 c
Young control 0.8 b 0.2 b 1.2 12.6 b 7.7 c 39.8 c
P (inoculation) n.s. n.s. n.s. n.s. ** *
P (age) ** ** n.s. ** ** **
P (IxA) n.s. * n.s. * * *
Mature mycorrhizal 1.8 a 0.6 0.4 b 8.8 c 9.5 c N.D.
Mature control 1.1 ab 0.7 0.4 b 5.7 d 6.1 d N.D.
Young mycorrhizal 0.6 b 0.3 0.7 ab 13.7 a 15.4 a 43.4 a
Young control 0.9 ab 0.5 1.1 a 12.1 b 10.9 b 40.3 b
P (inoculation) n.s. n.s. n.s. ** ** *
P (age) * n.s. * ** ** -
P (IxA) n.s. n.s. n.s. n.s. n.s. -
Table 7 Effects of inoculation with selected mycorrhiza, tree age and their interaction on diameter and shoot growth and chlorophyll content of Tilia and
Aesculus planted in an historical garden in the centre of Milan.
Different letters within the same column and species indicate significant differences between treatments at P<0.05 (*) and P<0.01 (**).
Carbon assimilation in linden Carbon assimilation in horse chestnut
A (µMOL m-2 s-1)
A (µMOL m-2 s-1)
11 June 08 21 May 09 17 June 09 27 June 10 21 May 09 17 June 09 27 June 10
Mycorr mature
Control mature
Mycorr young
Control young
Mycorr mature
Control mature
Mycorr young
Control young
Figure 7 Effects of selected mycorrhiza on carbon assimilation of young and mature Tilia (left) and Aesculus (right) planted in an historical garden in the
centre of Milan. * and ** indicate significant differences between treatments within the same sampling date at P<0.05 and P<0.01.
Parallel session 1a: Tree planting and establishment 49
reduced effect of mycorrhizal inoculation. The process of
selection of native, specific mycorrhizal strains must be
implemented by selecting new strains and fungal species for
areas which have already been studied and identifying new
fungal species/strains in new geographic areas.
The authors would like to thank Floricoltura San Donato-
Grandi Trapianti Italiani (S. Donato Milanese, Milan, Italy) for
funding this experiment. A special acknowledgement to Dr.
Jurgen Kutscheidt (MicoMax GmbH, Wuppertal, Germany)
for his kind assistance during mycorrhiza selection and
inoculum preparation.
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mycorrhizal inoculation affects root development of Acer
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Mobilization of sparingly soluble inorganic phosphates
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fungus. Plant Soil 230, 279–285.
Parallel session 1a: Tree planting and establishment 51
Fundamentals of tree establishment: a review
‘The best time to plant a tree was twenty years ago.
The second best time is now.’
Mortality of landscape trees regularly reaches 30% in the first year after planting. This review aims to highlight the
fundamental factors and procedures critical to tree establishment. If these are fully considered and acted upon,
significant reductions in transplant losses can be expected. The principal elements essential for successful tree
establishment have been identified as tree ecophysiology; rooting environment; plant quality and planting and post-
planting. These are presented in a model which helps describes the multiplicity of factors involved in successful
establishment and, importantly, their interrelated nature. An understanding of how transplant survival can be markedly
influenced by these factors is paramount and failure to consider any one element may lead to tree mortality. Attention is
also given to practices which have been demonstrated to greatly enhance tree vitality during the establishment phase.
The challenge of tree establishment
Trees planted into urban landscapes such as streets, recreational areas and car parks provide
important benefits to urban populations. These include absorption of pollutants, reduction
of traffic noise, windbreaks and shelter, as well as reduction of radiation and solar heat gain
through shading and evapotranspiration (NUFU, 2005; Hiemstra et al., 2008; Forest
Research, 2010). Trees also provide shape, scale, form and seasonal changes to the
landscape. However, as early as the 1980s failure rates for amenity tree planting were
commonly recorded as 30%, but failure rates of 70% were reached with disturbing regularity
during the first growing season (Gilbertson and Bradshaw, 1985, 1990). Further research in
the late 1990s and 2008 highlighted similar failure rates (Johnston and Rushton, 1999; Britt
and Johnston, 2008). In view of the resource life-history an amenity tree has in terms of
irrigation, fertilisers (if applied), transport costs, planting materials, labour, etc., in addition to
the actual loss of the tree, the persistence of these failure rates can no longer be accepted.
Such significant losses also challenge us to consider why, over a 30 year period, mortality
rates of 30–50% are still commonplace during the first year after planting.
A number of reasons exist. While it is appreciated by professionals involved in urban tree
management that trees are planted into suboptimal conditions for growth, the extent and
diversity of stresses urban environments impose is frequently under-estimated. Table 1
identifies abiotic stresses which may affect urban trees.
Transplant survival is influenced by the range of factors outlined in Figure 1. Tree ecophysiology
considers the genetic potential of trees to establish in a given environment and species
characteristics which may reduce the impact of a particular stress. High plant quality is an
essential foundation for any planting project. Planting and post-planting practices are
fundamental to establishment success. The rooting environment is critical in ensuring future
resource availability and anchorage. Failure to give full consideration to any one of these
factors increases the likelihood of a high mortality rate in a tree planting scheme.
tree establishment, tree
Andrew D. Hirons1and
Glynn C. Percival2
1Myerscough College,
Lancashire, UK
2R.A. Bartlett Tree Research
Laboratory, University of
Reading, UK
52 Trees, people and the built environment
Transplant stress
The common observation of slow growth, tree decline
and/or death following transplanting is characterised as
transplant stress. The marked reduction in root:shoot
ratio due to the lifting process in the nursery results in a
severe limitation to resource capture. Newly transplanted
trees are, therefore, incapable of meeting the water and
nutrient demands of the canopy. Consequently, the
efficient return to a pre-transplant root:shoot ratio is
essential for survival and establishment of transplanted trees
(Davies et al., 2002).
Table 1 Potential abiotic or non-living stresses affecting urban trees.
Abiotic stresses
High irradiance (photoinhibition, photooxidation) Herbicides, pesticides, fungicides
Heat (increased temperature) Air pollutants (SO2, NO, NO2, NOx)
Low temperature (chilling, frost) Ozone (O3) and photochemical smog
Drought (desiccation problems) Formation of highly reactive oxygen species (1O2, radicals, O2- and OH, H2O2)
Natural mineral deficiency Photooxidants (peroxyacylnitrates)
Waterlogging (root deoxygenation) Acid rain, acid fog and acid morning dew
Competition for light, water, nutrients Acid pH of soil and water
Excess de-icing salts (Na, Cl) Over supply of nitrogen (dry and wet NO3 deposits)
Heavy metals Increased UV-radiation
Increased CO2 levels (global climate change)
Figure 1 A model of the key factors involved in successful tree establishment
Parallel session 1a: Tree planting and establishment 53
Tree ecophysiology
Each tree species has an inherent capacity for growth. This
relates to a complex array of morphological, anatomical and
physiological attributes. Most obviously, these influence
tolerance to climate (and microclimate), but a number of
characteristics have been observed to promote tolerance to
Local climate
The significance of climatic factors on tree performance is
broadly appreciated by those involved in tree management.
When, however, it is necessary to make decisions on tree
selection for a given site it is soon apparent that robust data
on climatic suitability is poorly developed or non-existent.
Inherently poor climatic fit in terms of growing season
temperature and solar radiation can markedly influence the
performance of many species that are of continental
European-Asian or North American distribution, which
perform satisfactorily in South East England but struggle
within a UK northern climate (Percival and Hitchmough,
1995). Problems can be exacerbated within an urban
landscape where several microclimates (a local atmospheric
zone where the climate differs from the surrounding area)
may exist within very short distances. Microclimates exist,
for example, near bodies of water which may cool the local
atmosphere, or in heavily urban areas where brick, concrete
and asphalt absorb the sun’s energy and radiate that heat to
the ambient air, resulting in an urban heat island. South-
facing slopes are exposed to more direct sunlight than
opposite slopes and are, therefore, warmer for longer. Tall
buildings create their own microclimate, both by
overshadowing large areas and by channelling strong winds
to ground level. Local climate knowledge is important as the
biological events of trees (flowering, seed set, bud burst, etc)
are controlled by environmental triggers. Disruption to these
triggers can be manifest for example by cherries under
artificial street lights flowering in winter due to a disrupted
photoperiod (Harris et al., 2004). Consideration of the
precise environmental conditions in which the tree will be
located is an essential criterion for tree selection.
Tree tolerance
Tolerance to transplanting has been shown to vary widely
between different genera with Populus, Salix and Alnus
widely regarded as transplant tolerant while Fagus, Juglans
and Aesculus are transplant sensitive (Watson and Himelick,
1997). Reasons for these differences are complex and have
never been fully elucidated, although some of the salient
factors have been identified.
Soil moisture and temperature are most influential in
determining the periodicity of root growth but in reality
multiple factors are involved (Eissenstat and Yanai, 2002).
Ease of transplanting has been linked with root
morphology and the rate of root regeneration. For
example, root regeneration rates of green ash began at 9
(root tip elongation) and 17 (formation of adventitious
roots) days after planting, while in red oak such responses
were not recorded until days 24 and 49 (Arnold, 1987).
Species with fibrous root systems that have significantly
more profusely branched root systems are suggested to be
easier to transplant than species with coarse root systems
(Struve, 1990). Although variation between species will
exist, at least six or more lateral roots should be present
when planting as lower numbers of lateral roots are
associated with a decrease in survival rates (Struve, 1990).
Likewise, trees that possess physiological adaptations to
waterlogging such as the formation of aerenchyma
(intercellular gas-filled spaces) in the root cortex, the
development of adventitious roots and enlarged lenticels,
anaerobic carbohydrate catabolism and oxidisation of the
rhizosphere tend to have higher survival and
establishment rates than species which do not possess
these characteristics. Trees with specific anatomical