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Better Forests, Better Cities


Abstract and Figures

Better Forests, Better Cities evaluates how forests both inside and outside city boundaries benefit cities and their residents, and what actions cities can take to conserve, restore and sustainably manage those forests. This report is the first of its kind comprehensive resource on the connection between cities and forests, synthesizing hundreds of research papers and reports to show how all forest types can deliver a diverse suite of benefits to cities.
Better Forests,
Better Cities
Authors: Sarah Jane Wilson, Edie Juno, John-Rob Pool, Sabin Ray, Mack Phillips, Scott
Francisco, and Sophie McCallum
Contributors: Craig Hanson, Kathleen Wolf, Katherine Lord, Paige Langer, Terra Virsilas,
Caledonia Rose Wilson, Lisa Beyer, James Anderson, Lizzie Marsters, Todd Gartner, and
Suzanne Ozment
Wilson, S.J., E. Juno, J.R. Pool, S. Ray, M. Phillips, S. Francisco, and S. McCallum. 2022. “Better
Forests, Better Cities.” Report. Washington, DC: World Resources Institute. Available online at
November, 2022
is the Director of Nature-Based Projects at
Pilot Projects.
is a Forestry Specialist at National Wildlife
is the Manager for Knowledge and
Partnerships for the UrbanShift Program in
WRI’s Ross Center for Sustainable Cities.
is an Associate at Ecosystem Integrity Fund.
is a Design Strategy Associate and
Researcher at Pilot Projects.
is the Founder and Director of Pilot Projects.
is a Research Assistant at Pilot Projects.
The authors would like to thank Norway’s International Climate and Forest Initiative (NICFI) for its
financial support to Cities4Forests, without which this report would not have been possible, and the
Cities4Forests cofounding partners—Pilot Projects and REVOLVE—for their collaboration and support
in creating the Cities4Forests initiative.
The authors are grateful to the following groups of people who provided constructive comments
that strengthened this report through the review process and contributed in various other ways.
Contributors are listed alphabetically by last name.
Advisors and Thematic Experts
Robin Abell (Conservation International), Elleni Ashebir (WRI), Patricia Balvanera (Universidad
Nacional Autónoma de México), Robin Chazdon (Forestoration International), Theodore Eisenman
(University of Massachusetts, Amherst), Natalie Elwell (WRI), Aarin Gross (Conservation International),
Craig Hanson (WRI), Nancy Harris (WRI), Nick Hewitt (Lancaster University), Viniece Jennings (Agnes
Scott College), Paige Langer (WRI), Dexter Locke (U.S. Forest Service), David Nowak (U.S. Forest
Service), Suzanne Ozment (WRI), David Rojas-Rueda (Colorado State University), Jessica Seddon
(WRI), Ayushi Trivedi (WRI), Sara Walker (WRI), and Kathleen Wolf (University of Washington).
Internal Reviewers
Chip Barber, Lisa Beyer, Beatriz Cardenas, David Gibbs, Suzanne Ozment, Alex Rudee, Tim
Searchinger, Frances Seymour, Gregory Ta, Teresa Tattersfield, Ayushi Trivedi, Laura Vary, Leandro
Vigna, and Laura Villegas-Ortiz (all WRI during the internal review process).
External Reviewers
Ann Bartuska (Resources for the Future), Harriet Bulkeley (Durham University and Naturvation), Bobby
Cochran (Willamette Partnership), Gillian Dick (Glasgow City Council), Emily Lombardo (NICFI), Lydia
Scott (Morton Arboretum and Chicago Region Trees Initiative), Xueman Wang (World Bank), and
Bianca Wernecke (South African Medical Research Council).
Operational, Communications, Research and Design Support
Sadof Alexander, James Anderson, Caroline Black, Shannon Collins, Sod-Erdene Davaadorj, Bill
Dugan, Rosie Ettenheim, Christopher Gillespie, Yichen Hao (American University), Renee Pineda,
Maria Santarelli, Lauri Scherer (LSF Editorial), Auston Smith, Emilia Suarez, Gregory Ta, and Romain
Warnault (all WRI, except where noted).
ii | WRI.ORG
3 Foreword
5 Executive Summary
Health and Well-Being
30 Background
31 Goal 1: Reducing Extreme Heat
34 Goal 2: Enhancing Urban Air Quality
38 Goal 3: Promoting Physical and Mental
Health in City Residents
41 Goal 4: Creating Walkable, Safe Streets
43 Goal 5: Supporting Community Connections
45 Goal 6: Reducing Urban Environmental
46 Goal 7: Ensuring Provision of Food, Medicine,
and Raw Materials
48 Goal 8: Enhancing Economic Well-Being
49 Concluding Thoughts
52 Background
54 Water Challenge 1: Too Dirty
56 Water Challenge 2: Too Much
62 Water Challenge 3: Too Little
64 Water Challenge 4: Too Erratic
69 Concluding Thoughts
72 Background
77 Forests inside Cities and Climate Change:
Urban Cooling and Carbon Sequestration
84 Climate Change and Forests outside Cities,
Near and Far
91 Concluding Thoughts
94 Background
95 Nine Things Cities Need to Know About
107 Concluding Thoughts
Recommendations for Policy and Action
110 Introduction
114 Actions for City Practice and Policy
129 Concluding Thoughts
130 Appendices
130 Appendix A: Research Methodology
136 Abbreviations
137 Endnotes
141 References
Better Forests, Better Cities | 1
Cities across the world are reeling from the direct
impacts of deforestation and forest degradation. In
neighborhoods with low tree cover, higher mortality
rates from extreme heat are on the rise, disproportion-
ately aecting poor and marginalized communities.
Meanwhile, deforestation and other disturbances to
the world’s forests emit an average of 8.1 billion metric
tonnes of carbon dioxide every year, contributing to
devastating climate impacts in cities across the world.
At the same time, cities are the ones shaping forests
around the world. Although cities cover only a small
proportion of the earth’s surface area, their carbon
footprints are large and extend far beyond their limits.
Ongoing consumption of globally traded agricul-
tural commodities – the vast majority of which are
consumed by urban residents – is the leading driver
of tropical deforestation. For example, 80 percent
of permanent deforestation in South-East Asia is
the result of land conversion to grow commodities
such as oil palm.
e fates of cities and forests are deeply intertwined—
which also oers us an opportunity to change course.
Cities can redene the role of nature within and
outside of their boundaries. By taking action to protect,
restore and sustainably manage forests at all scales, cit-
ies can tackle the global climate and biodiversity crises
while promoting the well-being of their residents. e
public policies and procurement practices of cities have
enormous potential to support the rejuvenation of the
world’s forests—with huge benets in return.
Until now, there has been a lack of clarity on how
forests directly benet cities and their residents. is
report synthesizes hundreds of research articles to char-
acterize the wealth of benets that forests oer to cities
in terms of human well-being, water security, climate
mitigation, and biodiversity. e report evaluates the
full range of benets that cities receive from forests,
whether they are inside, nearby, or far away from their
boundaries, highlighting innovative examples from
cities who are already leading the way. For example,
the Green Cadaster in Skopje, North Macedonia oers
a comprehensive map and catalog of every single tree
and shrub in all public green zones within the city,
making it easier for city ocials to manage and track
the benets of green spaces. In Quito, Ecuador, a
multi-stakeholder water fund known as FONAG has
protected and restored over 40,000 hectares of forests
with the support of over 400 local families. ese are
shining innovations and examples that decision makers
across the world can learn from and seek to replicate in
their own cities.
e report also echoes the Cities4Forests Call to Action
on Forests and Climate that more than 50 mayors issued
in 2021, calling for accelerated national and subnational
action to support forests. As the rst of its kind, this
report provides city leaders with a new authoritative
source to develop and expedite their eorts to protect
forests worldwide.
As centers of political, economic, and cultural clout,
city leaders have an increasingly important role to play.
Mayoral voices are not yet tapped to their full potential
in support of the world’s forests. is needs to change,
and soon. After all, the battle on climate change will be
won or lost in cities. We cannot aord to lose any more
time. By 2050, an estimated 70 percent of the world’s
population will live in cities. City leaders must act
decisively in order to secure a green future for their
growing populations and reap the benets of a world
with healthy forests.
President & CEO
World Resources Institute
Better Forests, Better Cities | 3
Executive Summary
This report evaluates how and when forests –
inside, near and far away from cities – contribute
to health and well-being, water security, climate
change mitigation, and biodiversity conservation
benefits for cities and their residents. As city leaders
face demands from growing urban populations,
coupled with the impacts of a rapidly changing
climate, they should consider the role that forests
can play in addressing these challenges and
meeting the needs of city residents.
Better Forests, Better Cities | 5
A growing body of scientific evidence shows
that conserving, restoring, and sustainably
managing forests can provide robust, low-
cost infrastructure solutions to help cities and
their leaders meet the myriad demands of
growing urban populations, such as increased
clean and reliable fresh water, safe and
healthy environments, and protection from
natural disasters.
Cities around the world are responding to
this evidence, increasingly using forests
inside, near, and far away from cities to
address their challenges and meet the aspira-
tions of residents.
Forests are particularly eective at providing
cities and their residents with four benefits:
human health and well-being, a clean and
reliable water supply, climate regulation, and
biodiversity conservation.
This report evaluates the evidence base to
show how and where these benefits are deliv-
ered and what immediate actions cities can
take to better conserve, restore, and sustain-
ably manage forests for the desired benefits.
It presents a review of hundreds of synthesis
papers, original research papers, and key
reports and collectively shows how dierent
forest types at dierent levels can deliver a
diverse suite of benefits to cities.
In the coming decade, city mayors and managers will face
unprecedented demands from growing urban populations.
Rapid urbanization and environmental changes are putting
new pressures on burgeoning cities. City leaders are charged
with providing urban residents with a safe place to live and
work, environments that promote good health, clean and
reliable freshwater, and protection from natural disasters.
ey will need to step up their climate action and meet other
sustainability commitments—all of which are rising on polit-
ical and media agendas. At the same time, city leaders will
need to juggle these demands amidst dynamic conditions,
often with tight nancial resources.
Nature-based solutions1 (NBS)—such as trees and for-
ests—can help cities meet many of these needs. A growing
body of scientic evidence shows that conserving, restor-
ing, and sustainably managing forests can provide robust,
low-cost infrastructure solutions to complement other
traditionally built infrastructure. Cities around the world
are responding to this evidence, increasingly using forests to
address their challenges and meet the aspirations of residents.
Forests inside, near, and far away from cities (Figure ES-1)
can help cities both meet their needs and contribute to
commitments to act on global challenges:
Inner forests include street trees, trees and forests on
private property, patches of native woodland, forested
ravines and corridors, and so forth, found within city
boundaries. Inner forests can improve air quality, reduce
the heat island eect (leading to lower energy use and
energy bills), reduce stormwater runo and urban ood-
ing, provide access to nature and respite from the built
environment, and support human health and wildlife.
Nearby forests are trees, woodlands, and forests in the
watersheds surrounding cities. ey contribute to cleaner
air in cities, support stable supplies of clean drinking
water, reduce ooding, provide wildlife habitat, and oer
space for recreation.
Faraway forests are substantial, intact, and remote forests
that are most often located far outside a city’s boundary.
ese forests—particularly those in the tropics—seques-
ter large amounts of carbon, generate reliable rains for
cities and the world’s agriculture belts, provide a wealth
of products used by cities every day (including medicines,
food, and building materials), and host the majority of
the world’s land-based biodiversity.
FIGURE ES | Inner, Nearby, and Faraway Forest Benefits
Note: Forests at three levels provide benefits to cities and contribute to the achievement of the UN Sustainable Development Goals.
Source: Cities4Forests n.d.a.
Clean air
Drinking water
Reduced flooding
Reduced soil erosion
Carbon storage
Rainfall generation
Clean air
Shade from sun
Urban wildlife
Higher property values
Forests are particularly eective at providing cities and
their residents with four benets: human health and
well-being, a clean and reliable water supply, climate reg-
ulation, and biodiversity conservation. Better Forests, Better
Cities evaluates the evidence base to show how and where
these benets are delivered by forests and, in unique circum-
stances, when and where they are not. is report presents
a review of hundreds of synthesis papers, original research
papers, and key reports. Collectively, this research shows how
dierent forest types at dierent levels can deliver a diverse
suite of benets to cities.
Health and Well-Being
Cities aord their inhabitants many benets, but they also
create conditions that can have negative impacts on health
and well-being (Kuddus et al. 2020). Forests and trees,
particularly in the inner forest, can improve the health and
well-being of urban residents by these actions:
Reducing extreme heat. e urban heat island eect—in
which urban areas experience higher temperatures than
their rural surroundings—presents a number of risks to
human health. ese include increased risk of heat-re-
lated deaths, increased concentrations of urban smog
and ground-level ozone, spikes in energy and water
Better Forests, Better Cities | 7
demand, and power outages (Heaviside et al. 2017).
Urban trees and forests can mitigate the urban heat
island eect by providing shade and cooling the air via
evapotranspiration. ese processes reduce both the risk
of heat-related illness or death and increase the livability
of cities (Bowler et al. 2010a; Mohajerani et al. 2017;
Wolf et al. 2020).
Enhancing urban air quality. Ambient air pollution
threatens the well-being of most urban residents. Nine
out of 10 people breathe polluted air worldwide, leading
to about 4.2 million deaths globally. Low- and mid-
dle-income countries are disproportionately aected
(WHO 2016). Reducing emissions from the source is
key, but carefully planned and managed inner forests can
further improve air quality by removing and dispersing
air pollutants (Nowak et al. 2014; Kumar et al. 2019;
Hewitt et al. 2020).
Promoting mental and physical health. Living in cities can
take a toll on mental and physical health. Pollutants,
being sedentary, and living close to other people can
increase the prevalence of many kinds of diseases (Bai et
al. 2012; Ventriglio et al. 2021). Forests and trees reduce
noise, pollution, and other stressful conditions, and they
provide opportunities for rest, relaxation, and recreation
in nature (Hartig et al. 2014; Kuo 2015; Bratman et al.
2019; Wolf et al. 2020). Preventing deforestation and
degradation of biodiverse forests outside cities may also
reduce the spillover of infectious diseases, including novel
viruses, from animal hosts to humans (Alimi et al. 2021).
Creating safe, walkable streets. Cities around the world are
working to increase biking and walking as ways to travel.
Trees along streets and urban green spaces encourage
active transport, providing shade, reducing localized air
pollution, and making streets and pathways more beauti-
ful and pleasurable (Wolf et al. 2020).
Supporting community connections. Forests and other
green spaces can build cohesion among urban residents
by providing places for communities to gather, enhanc-
ing a “sense of place,” and creating space for spirituality
and reection (Wolf et al. 2014; Jennings et al. 2016).
Inner and nearby forests are desirable locations for social
gatherings, recreation, tourism, and spiritual practice and
contemplation (Kuo 2015; O’Brien et al. 2017; Irvine and
Herrett 2018; Ngulani and Shackleton 2019).
Reducing inequity and empowering residents. Social and
economic inequality is a challenge facing most cit-
ies. Lower levels of urban tree canopy cover have been
associated with relatively low-income and marginalized
populations in some cities (Schwarz et al. 2015; Jennings
et al. 2016; Gerrish and Watkins 2018; Watkins and Ger-
rish 2018).Unequal tree distribution can translate into
unequal distribution of the important human health and
well-being benets trees provide (Jennings and Johnson
Gaither 2015; Braubach et al. 2017). Engaging commu-
nities to plan and integrate more trees and natural areas
into neighborhoods with marginalized and low-income
residents can help to address systemic inequalities in
urban areas (Wolch et al. 2014; Kondo et al. 2015; Jelks
et al. 2021). Meaningful community engagement and
leadership is essential to realize these benets.
Providing food, medicine, and raw materials. Although city
residents rely heavily on imported goods sold in formal
markets (e.g., stores), inner and nearby forests can help
improve food access, especially for lower-income or mar-
ginalized groups in cities. ese forests can provide food,
medicines, and raw materials for subsistence or can pro-
vide income (Pramova et al. 2012; Shackleton et al. 2015).
Enhancing economic well-being. Inner forests can provide
multiple economic benets to cities and city residents
(Nesbitt et al. 2017). Trees can increase property values
for residents and associated property tax revenues for
municipal governments (Roy et al. 2012). ey can serve
as a form of “green infrastructure” that can lower the costs
of stormwater management, reduce ooding risks, lower
energy costs, and provide other cost-saving measures.
Forests and trees at all three levels can be a cost-eective
way to help improve and stabilize city water resources.
Many cities struggle to provide ample clean water (water
is “too dirty”), address ooding and erosion (there is “too
much” water), plan for droughts (there is “too little” water),
and deal with new levels of inconsistency in once-reliable
rain patterns (water is “too erratic”).
Too dirty: Many cities nd it dicult to provide residents
with a reliable supply of clean drinking water. Contam-
inated drinking water causes severe health issues in many
regions, and water treatment facilities can be costly to
establish and maintain. Forests in watersheds can prevent
soil erosion and lter sediment and pollutants (Kuehler et
al. 2017), keeping surface waters and aquifers cleaner and
reducing costs to cities. For example, recent analysis nds
that upstream forest protection and restoration can reduce
costs for water utilities in the world’s 534 largest cities collec-
tively by US$890 million per year (McDonald and Shemie
2014). Mature native forests provide these benets more
reliably than plantations.
Too much: By 2030, riverine ooding will impact around
130 million people and $535 billion in urban property, and
coastal ooding will impact another 15 million people and
$177 billion in urban property.2 Forests—especially nearby
forests—can prevent or reduce the severity of ooding.
Forests intercept and store rainwater, reducing stormwa-
ter runo. ey improve the ability of soil to hold water,
increasing both inltration (entry) and percolation (down-
ward movement) of rainwater (Berland et al. 2017; Kuehler
et al. 2017). ey increase the amount of water returned to
the atmosphere by evapotranspiration. And they can store
excess runo, holding and slowing the release of water much
like a sponge. Forested watersheds (near cities) regulate
water ows and help prevent ooding and landslides. Trees
and other vegetation in bioretention areas, green roofs,
and bioswales can also complement traditional, engineered
water infrastructure solutions for stormwater management
in urban areas.
Too little: Water scarcity can be caused by drought, ground-
water depletion, or reduced river ows. Many cities around
the world—especially in arid regions—face seasonal
or year-round issues with water supply. e “Day Zero”
drought-induced water crisis in Cape Town of 2017–18 drew
worldwide attention to the risks of too little water: thou-
sands of people lost their jobs, food security decreased, and a
political crisis ensued. Preventing deforestation and restoring
forests can help sustain water availability (Brauman et al.
2007; Filoso et al. 2017; van Dijk and Keenan 2007; Zhang
et al. 2017) by increasing the inltration capacity of soils,
which promotes groundwater recharge, although benets
may lag in reforested areas and water yields may decline
initially in the years immediately following restoration or
reforestation (Filoso et al. 2017). Forests also aect rainfall
patterns at regional and even global levels. By capturing and
recycling precipitation, evapotranspiration sends water into
the atmosphere, creating “ying rivers” that transport water
to fall as rain in downwind regions far from the forest.
Too erratic: Urban residents are vulnerable to increas-
ingly erratic weather patterns, including longer and more
intense droughts and heavy rainfall, linked to climate
change. Variability and unpredictability in precipitation
and water supply create additional challenges for municipal
leaders, such as providing a reliable water supply to residents
or preparing for unpredictable water highs and lows. Because
of their role in the global water cycle, forests can help reduce
this variability. Forests, especially large tracts of intact forests
and rain forests, recharge atmospheric water supplies and
thereby inuence rainfall patterns hundreds to thousands of
miles away. Forests also can reduce local water variability by
enabling a slow release of water over time. Conserving and
Better Forests, Better Cities | 9
restoring forests are important strategies for stabilizing pre-
cipitation levels and groundwater availability in a changing
climate (Melo et al. 2021).
e eects of climate change—including heat waves,
ooding, rising sea levels, and droughts—threaten both
the well-being of urban residents and the costs of operat-
ing a city. Not surprisingly, urban residents’ concerns about
climate change are growing rapidly. Forests are good for
both climate change adaptation and mitigation, and some
of the adaptation benets (for example, ooding reduction)
have previously been mentioned. is section focuses on how
forests can mitigate climate change. Cities around the world
are committing to bold action to reduce their greenhouse
gas (GHG) emissions and tackle climate change. C40 Cities
Climate Leadership Group (an international network of
megacities that have committed to take action on climate
change), ICLEI—Local Governments for Sustainability, and
the Carbon Neutral Cities Alliance are all examples of city
networks committed to reducing GHG emissions. e rst
important step is to reduce GHG emissions from sources
within cities and from city consumption, but forests can help
cities go further.
Forests and trees in cities can reduce energy-related GHG
emissions by modulating temperature. Inner forests reduce
extreme heat in summer and shade buildings (Mullaney et al.
2015; Ko 2018). ese trees can help residents and businesses
adapt to rising temperatures while simultaneously reducing
emissions generated by cooling and heating buildings with
fossil fuels. In the United States alone, urban forests reduce
electricity use by 38.8 million megawatt-hours at a savings of
$4.7 billion annually, with reductions in heating use esti-
mated at 246 million British thermal units at a savings of
$3.1 billion annually, and avoided emissions valued at $3.9
billion annually (Nowak et al. 2017).
Inner forests provide modest opportunities to sequester
and store carbon in wood and soils (Nowak et al. 2002; Roy
et al. 2012; Nowak and Greeneld 2018b). However, total
carbon storage is limited by the cost and availability of space
in cities, and both total storage and sequestration rates in
urban forests vary with climatic and other biophysical factors
(Nowak et al. 2013; Dobbs et al. 2014; Chen 2015). Cities
with favorable growing seasons, ample water supplies for
vegetation, and robust urban forest management programs
tend to store more carbon. Although inner forests do store
carbon (and provide many cobenets), planting trees and
expanding the urban tree canopy will never be a sucient
way for cities to meaningfully compensate for their energy
and transportation emissions. e number of trees that can
t within an urban area (and thus their stored carbon) is
very small relative to a city’s annual carbon emissions (Pataki
et al. 2011). Urban forests can only sequester a tiny frac-
tion—often less than 1 percent—of overall city emissions.
Urban forests can also be carbon neutral or carbon positive
in some cases, meaning that they may emit as much or more
carbon as they sequester. roughout China, for example,
the annual carbon sequestration of urban vegetation in 35
of its largest cities could oset only 0.33 percent of these
cities’ total annual emissions (Chen 2015). Importantly and
in all instances, urban forests will always sequester more
carbon than they would if the forests were converted to
other land uses.
Protecting and restoring faraway forests is critical to
reduce emissions and mitigate global climate change.
Often underappreciated by city climate action planners,
faraway forests provide large-scale carbon sequestration
for climate change mitigation. Forests, especially tropical
forests, are large reservoirs of carbon that are released if the
forest is cleared. But if forests are conserved, those stores
are protected, and standing or restored forests continue to
sequester even more carbon. Cities can play a big role in
realizing this carbon opportunity and can help meet their
own carbon reduction or neutrality commitments in the
process. For instance, cities can lower their forest-carbon
footprint by ensuring that the commodities they purchase
for city infrastructure and operations—such as timber, paper,
and food—come from deforestation-free supply chains or
by reducing food loss and waste or shifting the diets of their
residents towards more plant-based foods. Cities can partner
with selected faraway forests that have a social or economic
link to the city, oering programs that support the conser-
vation and/or restoration of that faraway forest. Moreover,
cities can nancially support reductions in tropical forest-re-
lated emissions by participating in jurisdictional REDD+
(reducing emissions through deforestation and degradation,
plus the sustainable management of forest and the conser-
vation and enhancement of forest carbon stocks) programs
veried by a credible standard.
10 | WRI.ORG
Biodiversity—global and local—provides many direct and
indirect benets to cities, and cities can play a key role in
protecting biodiversity at regional and global levels. e
biodiversity of plants, animals, fungi, and other life forms is
declining rapidly because of human activities, both in and
outside of cities (Tilman et al. 2017; Mazor et al. 2018).
Maintaining—or even increasing—biodiversity in inner for-
ests is increasingly appearing on municipal agendas (Brende
and Duque 2021). Yet municipal policies and practices can
support forest biodiversity in nearby and faraway forests too.
Supporting forest-based biodiversity is important to cities for
a number of reasons, including providing direct benets and
supporting many of the benets in the other three sections
of this report.
Biodiverse forests often provide more—and more reliable—
goods and services (Fischer et al. 2006; Flynn et al. 2011;
Cardinale et al. 2012; Oliver et al. 2015). To provide
the myriad benets of trees to urban residents, forests
must be able to persist and recover from changes in the
environment, including storms, droughts, and a changing
climate. High levels of biodiversity can serve as biological
“insurance”—when an ecosystem has many species fulll-
ing similar roles, it can continue to function even if some
of those organisms are lost or if a disease (e.g., Dutch elm
disease; chestnut blight) wipes out an entire species from
an area (Yachi and Loreau 1999; Brandon 2014).
Biodiverse forests store more carbon, more reliably. Undis-
turbed native forests sequester more carbon and store it
for longer than degraded forest or monoculture planta-
tions (Holl and Brancalion 2020; Watson et al. 2020).
Biodiverse forests have higher resilience to uctuations in
climate, pest outbreaks, and diseases than tree monocul-
tures. is higher resilience makes them a more reliable
carbon sink (Turner et al. 2009; Brandon 2014; Sed-
don et al. 2019).
Biodiverse forests protect watersheds. Native, biodiverse
forests in watersheds are more eective than planted
monocultures at supplying water resources to downstream
cities (Alvarez-Garreton et al. 2019; Bonnesoeur et al.
2019; Yu et al. 2019). is is due to the structure, impact
on soils, and greater resilience of native forests creating
better conditions for storing and ltering water.
Biodiversity provides blueprints for new medicines. Bio-
diversity within forests has provided compounds and
genetic material for making antibiotics, anticancer
agents, anti-inammatory compounds, and analgesics
used around the world (Chivian and Bernstein 2010;
Sen and Samanta 2014). In developing countries, 70–95
percent of the population, including those living in
cities, rely on traditional remedies such as herbal med-
icines derived from forests for primary care (Robinson
and Zhang 2011).
Biodiverse forests support urban food supplies (Krishnan et
al. 2020). irty-ve percent of food produced globally
comes from 800 plants that rely on pollination by insects
and other animals (Klein et al. 2007). Forests provide
critical habitat for many of these pollinators (Öckinger
and Smith 2007; Nicholls and Altieri 2013; Bailey et al.
2014; Hipólito et al. 2019).
Protecting biodiverse forests can reduce risks of zoonotic and
vector-borne diseases. Deforestation, forest degradation,
and the associated wildlife trade has been linked with the
spread of diseases that jump from animals to humans—
which cause immense health and economic damages
(Wolfe et al. 2007; Karesh et al. 2012; Jones et al. 2013;
Borremans et al. 2019). Examples include the Ebola
virus, yellow fever, malaria, Zika virus, and coronaviruses
(Guerra et al. 2006; Wilcox and Ellis 2006; Karjalainen
et al. 2010; Monath and Vasconcelos 2015; Olivero et
al. 2017). Evidence suggests that conserving tropical
Better Forests, Better Cities | 11
forests and sustaining their high levels of biodiversity can
decrease transmission of some infectious diseases (Evans
et al. 2020; UNEP 2020).
Access to biodiverse nature in cities can provide more reliable
and richer benets to residents, including an important list
of mental and restorative health benets (Fuller et al. 2007;
Lai et al. 2018; Wood et al. 2018; Marselle et al. 2019;
Ngheim et al. 2021). Urban trees and forests are one of
the main ways urban residents experience nature (Pre-
gitzer et al. 2019). Biodiversity in the urban forest also
contributes to the distinctive character of cities around
the world (Hausmann et al. 2016).
Inner forests can house high biodiversity. Urban forests
can be highly biodiverse and can serve as corridors for
some species. But they also tend to have more invasive
species, “generalist species, and fewer endemics (species
with very limited ranges) than rural forests in the same
habitat type (Concepción et al. 2015; Ducatez et al. 2018;
Borges et al. 2021). Managing urban forests for biodiver-
sity can provide access to nature within cities and create
more resilient urban forests, essential for delivering other
forest benets.
Tropical forests hold most—up to 90 percent—of the plan-
et’s terrestrial biodiversity and thus are essential to urban
well-being (Wilson 1988; Reid and Miller 1989; WRI
et al. 1992). Tropical forests continue to be lost at
alarming rates.
Cities around the world are responsible for the lion’s share
of deforestation via their consumption. is also puts
them in a strong position to improve their own biodiversity
impacts through local policies that reduce negative impacts
on tropical forests.
Right Trees, Right Place
Forests can provide the many benets described in this
report. But sometimes, the wrong trees in the wrong places
can result in unintended and negative consequences. For
example, monocultures of trees along city streets are vulnera-
ble to pest and disease outbreaks (such as Dutch elm disease
and the emerald ash borer). Our research found that some
tree species emit volatile organic compounds and bioparticles
(such as pollen) that can worsen urban air quality. In some
situations, trees planted in urban street “canyons” formed by
tall buildings can trap polluted air near the ground, prevent-
ing air currents from dispersing the pollution. Invasive tree
species planted in cities can reduce native biodiversity and
can even damage neighboring forests if they spread beyond
city limits. Some species of tree also fail to thrive in harsh
urban environments where air pollution, wind, and harsh
temperatures can damage trees. In nearby and faraway for-
ests, monoculture tree plantations can decrease biodiversity
and sometimes even reduce carbon stores, especially if they
replace native forests. In certain circumstances, upstream
tree planting or forest restoration will decrease downstream
water availability. For example, in Quito, Ecuador, mil-
lions of eucalyptus trees planted throughout the city and in
nearby watersheds now diminish urban biodiversity, create
forest re risks, and can lead to soil erosion (compared to
native tree species). us, ensuring the “right trees” are in
the “right place” is critical for receiving the full benets of
forests at all levels.
Recommendations for Policy
and Action
What can city leaders do to realize the myriad benets
forests provide to their cities and residents? Our analysis
has identied actions cities can take, and our synthesis of
the literature and interviews categorized these under ve
thematic categories:
1. Measurement and monitoring
2. Planning
3. Partnerships
4. Finance
5. Markets
e following are a suite of “no regrets” measures that
allow a city to take immediate action to capture the poten-
tial of inner, nearby, and faraway forests to help meet their
goals (Figure ES-2). While not exhaustive, they provide
directions towards tangible actions. Underpinning these
measures are a set of guiding principles that apply to all
recommendations (Box ES-1). Suggested policy actions are
divided by level—inner, nearby, and faraway forests—and the
thematic category that each action addresses.
12 | WRI.ORG
FIGURE ES | Forest-Positive Actions across Five City Action Categories and Three Forest Levels
1. Measurement 1. Map, inventory, and monitor your city’s
urban forest
2. Quantify the benefits of urban trees
3. Align forest monitoring metrics with
city goals
4. Articulate clear forest-related goals
1. Map peri-urban and watershed forests
and identify where forests are being lost
2. Quantify the benefits of trees in areas
around the city
1. Conduct an analysis of city-wide
consumption linked to tropical
2. Identif y and track local attitudes
and initiatives towards promoting
deforestation-free commodities
3. Articulate clear goals to guide action
2. Planning 5. Develop an urban forest
management plan
6. Designate land specifically for
natural areas
7. Create connectivity
3. Support the development of “nearby
forest” management plans
4. Articulate clear forest-related goals
4. Calculate and develop an action plan to
reduce the consumption of forest-risk
commodities and city-driven carbon
dioxide emissions associated with
3. Partnerships 8. Seek out organizations conducting
innovative work on inner forests
9. Cultivate interagency and cross-
jurisdictional collaboration
5. Articulate and amplify shared goals 5. Establish a “partner forest
6. Establish relationships with
organizations involved in forest
conservation, restoration, and
sustainable management to help
implement faraway forest programs
7. Call on subnational and national
governments as well as businesses
and financiers to conserve, restore, and
better manage tropical forests
8. Incentivize the use of responsibly
sourced forest-risk products
4. Finance 10. Explore diverse, long-term financing
6. Clarify that forest protection and
management are eligible infrastructure
7. Make the economic and business case
for action on forests
8. Establish upstream-downstream
partnerships to finance watershed
9. Compensate for urban emissions by
funding tropical forest conservation
10. Match conservation and restoration
eorts in the city with conservation in
faraway forests
5. Markets 11. Develop wood waste reuse programs 9. Implement a robust procurement policy
for local, sustainably sourced wood
10. Explore the role of carbon markets
to finance forest conservation or
11. Establish ecotourism ventures to
conserve and sustainably manage
forests threatened by competing land-
use pressures
12. Initiate tropical forest-positive
procurement policies and campaigns
Source: Authors.
Better Forests, Better Cities | 13
BOX ES | Guiding Principles
Conserve first, restore second. Conserving native forests
is a more eective and cost-eective way of sequestering
carbon, conserving biodiversity, and maintaining water
resources than planting new forests.
Protect large, old trees. Old trees support biodiversi-
ty and provide benefits that cannot be replaced by
planting new trees.
Define forests as essential infrastructure. Forests are often
seen as a luxury or amenity, but given the benefits they
provide, they should be viewed in policy and practice as
essential infrastructure for cities alongside traditional built
or “gray” infrastructure.
Create a clear vision for the role of forests. Forests and
trees can serve multiple city goals and also imply trade-
os. It is important to collaboratively develop a vision for
the role that forests can play in reaching success.
Give voice to communities. Empower and engage
community members, including a diversity of voices
to ensure benefits are equitably distributed and suit
residents’ needs.
Emphasize equity. For low-income and marginalized
populations, the benefits of forests and trees may hold
disproportionate value.
Collaborate across jurisdictions and city agencies. Col-
laboration across agencies, sectors, and jurisdictions
(including both other municipalities and regional and
national governments) is crucial for capturing synergies in
data, expertise, and resources.
Use forests to complement measures to reduce greenhouse
gas emissions. As a climate change mitigation strategy,
forest conservation and restoration should complement
city eorts to reduce urban emissions from energy gen-
eration, industry, and transportation. Reducing emissions
will help keep forests healthy—a double win for climate
change mitigation.
Prioritize biodiverse, native forests. Biodiverse forests and
native tree species, as opposed to monoculture planta-
tions or non-native species, are more resilient to stress
and provide a broader suite of benefits.
Use the “right tree, right place” approach. The species and
placement of forest planting and regrowth should be
aligned with the specific goals, adapted to local condi-
tions, and resilient to a changing climate.
Recommendations for Inner
Forests: Urban Trees, Parks,
Green Infrastructure, and
Natural Areas
e following options can help city leaders advance the
quantity and quality of inner forests—and thus the benets
those forests provide to urban residents. Since most inner
forests fall within a city’s jurisdiction, cities often have full
authority to pursue these recommendations.
Measurement and Monitoring: Inner Forests
Map, inventory, and monitor your city’s urban forest.
Develop an urban tree cover baseline and land cover map
as a rst step towards planning and monitoring urban
forests. Include an inventory of large, old, and cultur-
ally relevant trees. Evaluate key urban environmental
challenges that could be improved through better forest
management, such as heat islands, urban ooding, and
inequities in access to green space (WRI Mexico 2016;
Singapore-ETH Centre n.d.).
Quantify the benets of urban trees, especially iconic and
mature ones. Such an analysis is critical for informing
policies and investments in urban trees and can garner
political and resident support. For example, following
14 | WRI.ORG
its success in the United States, i-Tree Eco3—an online
tool developed by the U.S. Forest Service to quantify and
value ecosystem services provided by trees—was recently
adapted, translated, and launched for Mexican cities,
allowing cities across Mexico to quantify the extent and
composition of urban forests and calculate ecosystem
services and monetary values.
Align forest monitoring metrics with city goals. Although
canopy cover is often measured to assess urban for-
ests, this single metric does not provide comprehensive
information on all forest benets. Use other metrics
that improve forest function, such as forest types, spe-
cies diversity, carbon density, proximity to residents, and
distribution (Pregitzer et al. 2019).
Articulate clear goals. ese are a few examples:
Increase forest canopy by X percent. e appropriate can-
opy cover targets will depend on what is appropriate
for local conditions (e.g., climate, natural tree canopy
cover outside the city) and should be used with addi-
tional targets—such as species diversity or a mix of
stand ages—to ensure forest diversity and health.
Ensure every resident has green space within a half mile
of home. is addresses the increasing appetite of
cities to achieve equitable access to green space for
their residents.
Reduce heat island or stormwater threats by X percent.
In the face of climate change, cities are increasingly
looking to establish targets that address climate risks,
such as ooding, drought, and heat
Planning: Inner Forests
Develop an urban forest management plan. e plan should
be scientically informed, inclusively developed, and
climate resilient. e plan should inform and be informed
by other citywide plans, such as transportation, housing,
land use, parks, and economic development.
Designate land specically for natural areas. ese include
parks, vacant lots, and along roadways. For example, the
Miyawaki method—in which diverse plantings of native
trees and shrubs are used to create “microforests”—has
been used to improve local access to nature and increase
urban biodiversity in many cities around the world (Nargi
2019). Be explicit about the use of these natural areas to
promote community gathering and better access to nature
for all residents.
Create connectivity. Corridors of tree-covered green space
can facilitate the spread of pollinators, support wildlife,
alleviate stress, increase foot and bike commuting, and
reduce exposure to pollution for residents. Successful
examples of green corridor projects include the Medellín
Green Corridors (UNEP 2019) and the Barcelona Green
Corridor Network (O’Sullivan 2017).
Better Forests, Better Cities | 15
Partnerships: Inner Forests
Seek out organizations conducting innovative work on inner
forests. For example, the Natural Areas Conservancy in
New York City is a formalized partnership that focuses
on maintaining and improving the city’s vast natural areas
network, integrating the city’s needs with the conserva-
tion benets these areas provide.4
Cultivate interagency and cross-jurisdictional collaboration.
Managing forests for multiple benets spans dierent city
agencies, including health, water, land use, transportation,
economic development, climate, air pollution/quality, and
parks/recreation. e Joint Benets Authority5, which is
being pioneered in San Francisco, is an example of a new
mechanism that allows multiple departments within a
city to jointly plan, implement, and nance projects to
increase the quantity and quality of inner forests.
Finance: Inner Forests
Explore diverse, long-term nancing mechanisms to manage,
protect, and expand urban forests. Innovative nancing
tools include the following:
Green bonds and climate bonds, which fund pro-
jects that have positive environmental and/or
climate impacts through the use of proceeds or
asset-linked bonds
Pay for performance environmental impact bonds
(also known as pay for success bonds and social
benet bonds), which allow private investors to fund
specic interventions and earn a return based on per-
formance (i.e., paying for results rather than services)
Community-based public-private partnerships
between local governments and private entities, which
align the interests of public, private, and community
stakeholders around common goals
Tree-planting funds from taxes and stormwater fees
Tree banks, which collect funds when trees are
removed and their replacement value cannot be
achieved and support replacements in other places
throughout the city
Mitigation fees, which require that development activ-
ities mitigate their impacts by planting trees on sites
where disturbance occurs or pay the equivalent fees
into the city’s tree canopy conservation account
Integration of forests into compliance plans for envi-
ronmental requirements
Incentives for city residents to support trees and for-
ests through tax reductions
Markets: Inner Forests
Develop wood waste reuse programs. Rather than dispos-
ing of wood from urban trees in landlls, municipalities
can develop wood waste reuse programs. Dead trees can
become timber for local industry and construction and a
variety of other energy-saving products. ese programs
help defer costs, create employment, store carbon, and
foster integrative thinking and charismatic sustainable
policies centered on trees in cities.
Recommendations for Nearby
Forests: Watershed and
Recreation Areas around Cities
e following options can help city leaders advance the
quantity and quality of their nearby forests—and thus the
benets those forests provide to urban residents. Since
most nearby forests fall outside city agency jurisdiction, part-
nership, and collaboration with other government agencies
(e.g., state, provincial, federal), landowners, and managers
will be necessary for implementation.
16 | WRI.ORG
Measurement and Monitoring: Nearby Forests
Map peri-urban and watershed forests and identify where
forests are being lost around the city. Understanding where
forests are, where loss is occurring, where risk of loss from
re or land-use change are high, and where restoration
opportunities exist is essential for planning engagement
with nearby forests.
Quantify the benets of trees in areas around the city. is
can help garner support from residents and partners to
support watershed management for city water supply.
Planning: Nearby Forests
Support the development of “nearby forest” management
plans with measurable goals and success metrics. A city
could provide resources, such as funding, administrative
support, and sta participation, and promote collaborative
planning between government jurisdictions.
Articulate clear goals. ese are a few examples:
Restore X hectares by 2030.
Remove invasive species from key watersheds.
Partnerships: Nearby Forests
Articulate and amplify shared goals. Forming collaboratives
between city agencies, other government agencies, and
landowners can be an eective way to do this. For exam-
ple, the city of Denver collaborates with the National
Forest System and state agencies in the Forests to Faucets
initiative6, which has the shared aim of reducing wildre
risks and improving watershed services across Colorado’s
Front Range (CSU n.d.).
Finance: Nearby Forests
Clarify that forest protection and management are eligible
infrastructure expenses. Many existing funds for infra-
structure have not clearly stated their ability or priority
for funding NBS, such as forests. Explicitly making NBS
eligible for funds can open new funding sources for forest
protection and management.
Make the economic and business case. A “Green-Gray
Assessment”7 (Gray et al. 2019) assesses the costs and
benets of using green infrastructure (i.e., forests and
trees) or green and gray infrastructure versus relying
solely on traditional gray infrastructure for securing stable
and clean water supplies.
Establish upstream-downstream partnerships to nance
watershed management. Identifying the downstream
beneciaries (e.g., water utility, beverage company) of
forest watershed services is a key rst step to securing
performance-based arrangements with the upstream
land managers. Types of nancing mechanisms being
pioneered by cities include green bonds, forest resilience
bonds, water funds, and water utility rate surcharges.
Markets: Nearby Forests
Implement a robust procurement policy for local, sustainably
sourced wood. Sourcing wood from sustainably certied
managed forests within a city’s “woodshed” can help keep
forests from being converted to other land uses.
Explore the role of carbon markets to nance forest conser-
vation or restoration. King County in the U.S. state of
Washington established the Forest Carbon Program8; it
provides the opportunity for local companies to compen-
sate a portion of their own carbon emissions and support
healthy forests within the county (King County 2020).
Recommendations for Faraway
Forests: Intact and Remote
Forests, Especially in the Tropics
City leaders can advance the quantity and quality of far-
away forests—and thus the benets those forests provide
to urban residents. Because faraway forests fall outside a
city agency’s jurisdiction, partnership and collaboration with
other governments and stakeholders will be necessary for
implementing the following actions. Given the critical role of
tropical forests in mitigating climate change and the current
threats they face, cities should allocate special attention to
conserving and restoring tropical forests.
Measurement and Monitoring: Faraway Forests
Conduct an analysis of city consumption linked to tropical
deforestation. Tools such as the Forest Footprint9 can esti-
mate a city’s impact on tropical deforestation driven by
urban consumption of commodities (e.g., beef, soybeans,
timber) associated with tropical deforestation (Cities-
4Forests n.d.b).
Better Forests, Better Cities | 17
Identify and track local attitudes and initiatives towards
promoting deforestation-free commodities. is can help
gauge levels of political support a city may have in
taking steps to drive deforestation-free commodity pro-
curement policies.
Articulate clear goals to guide action. is is an example:
X percent of tropical wood and forest-risk commodi-
ties will be sustainably procured by X date.
Planning: Faraway Forests
Calculate and develop an action plan to reduce the consump-
tion of forest-risk commodities and city-driven carbon dioxide
emissions associated with deforestation. e Forest Footprint
tool can help cities to identify the size of their forest
impact and the key commodities driving deforestation,
which can help them plan their mitigative actions.
Partnerships: Faraway Forests
Establish a “partner forest.” A partner forest10 is a faraway
(usually tropical) forest connected to a city through a
meaningful and mutually benecial exchange. e city
supports the partner forest by directing its purchas-
ing power towards a product or service that the forest
provides (e.g., shade-grown coee, climate benets, eco-
tourism). e goal of a partner forest program is to visibly
support a tropical forest that provides direct benets to
the city and raise awareness of those benets among city
residents (Cities4Forests n.d.c).
Establish relationships with organizations involved in forest
conservation, restoration, and sustainable management to
help implement faraway forest programs. Instead of trying
to develop in-house expertise, cities can partner with one
or more nonprot organizations with on-the-ground
experience in the forests of interest to help scope, design,
and implement a faraway forest program.
Call on subnational and national governments as well as
businesses and nanciers to conserve, restore, and better man-
age tropical forests. Being home to the majority of voters
in many countries, cities can ex their political muscle by
being vocal with state and national government leaders
about the importance of faraway forests for city resident
well-being. If faraway forests are to remain, the voice of
cities needs to be heard.
Incentivize the use of responsibly sourced forest-risk products.
For example, the UK city of Chester, led by the Chester
Zoo and the local member of Parliament, worked to
encourage local businesses to use and sell products with
palm oil certied by the Roundtable on Sustainable Palm
Oil. Chester was recently certied as the rst sustainable
palm oil city worldwide (Chester Zoo 2019).
Finance: Faraway Forests
Compensate for urban emissions by funding tropical forest
conservation. Cities will have diculty reaching carbon
neutrality by cutting their direct emissions alone. Financ-
ing tropical forest conservation and restoration, certied
by credible jurisdictional REDD+ programs, may oer
ways to compensate for remaining urban emissions. A
“climate co-op” could be created where cities purchase
high-quality forest carbon credits via the voluntary car-
bon market to nance long-term forest conservation with
associated carbon benets.
Match conservation and restoration eorts in the city with
conservation in faraway forests. For example, for every
tree planted within the city, a city could support paral-
lel restoration eorts in a tropical forest. e London
Eneld Council woodland restoration project is develop-
ing such a partnership on restoration with the city of Port
Moresby (Papua New Guinea).
Markets: Faraway Forests
Establish ecotourism ventures to conserve and sustainably
manage forests threatened by competing land-use pressures.
Cities can support the implementation of community
owned and operated sustainable tourism programs by
promoting these amongst their residents to develop a
steady clientele pipeline, thereby bolstering the eorts of
regional governments to boost local economies while also
conserving faraway forests (Fitzgerald n.d.).
Initiate tropical forest-positive procurement policies and
campaigns. Cities can implement policies that discour-
age purchasing commodities implicated in deforestation
and provide incentives for purchasing better-sourced
commodities (or alternatives with lower tropical for-
est impacts). Tropical timber, coee, chocolate, soy,
and beef are commodities that are especially amenable
to this approach.
18 | WRI.ORG
Home to more than half of the world’s population, cities
are growing in their size, power, and impact on the natural
environment. ey face pressing challenges to provide their
residents with essential services, including healthy, livable
neighborhoods, clean and reliable water, action on climate
change, and access to nature and biodiversity. Cities can use
trees and forests to help meet these challenges.
Within cities, trees and forests—inner forests—can reduce
extreme temperatures, reduce stormwater runo, promote
mental health, and provide shared spaces for recreation
and relaxation. Forests around cities—nearby forests—can
improve water resources, provide many forest goods, and
oer access to nature. And faraway forests around the world
are key to mitigating climate change, conserving biodiversity,
and maintaining global rainfall patterns. Cities have many
options available to support forests at all three levels and
make the best use of the benets they provide. Forests can
also help cities reduce operating costs and pay long-term
dividends that often increase over time. e best time to plant
a tree was fty years ago. e second-best time is today.
BOX ES | The Importance of
Communications and Resident Engagement
To achieve forest-related goals, city leaders will need
to communicate with city residents to raise awareness,
generate a shared vision, and mobilize political support
and individual action. These are some of the key fea-
tures of an eective communications program:
Educate residents about the value of inner, nearby,
and faraway forests.
Engage youth through classroom education
and field trips.
Cultivate trusted messengers.
Articulate clear city goals with respect to inner, near-
by, and faraway forests.
Use storytelling and highly visible demonstration
projects to garner local support and make forest
benefits “real,” such as how the city of Glasgow is
doing through the Every Tree Tells a Story program.
For all of these measures—for inner, nearby, and faraway
forests—healthy communications and engagement with city
residents will be important (Box ES-2).
Better Forests, Better Cities | 19
Forests around the world are under severe threat.
Despite this, the evidence base that shows how
and where forests provide benefits to cities and
their residents is growing. As centers of untapped
political, economic, financial and social power,
cities can play a role in protecting, restoring
and sustainably managing the world’s forests,
to ensure the long-term sustainability of the
benefits they provide.
Better Forests, Better Cities | 21
City leaders around the world are working hard to meet
the needs of ever-growing urban populations. By 2050, an
estimated 70 percent of the world’s population will live
in cities (UNSD n.d.). City leaders strive to provide their
residents with a safe place to live and work and with access
to resources and environments that promote good health.
ey seek to improve and sustain clean, reliable water
supplies and provide protection from natural disasters.
And cities are increasingly stepping up to take action on
climate change mitigation and to meet other sustainability
commitments. International agreements to combat climate
change and conserve biodiversity, city-level commitments to
reduce greenhouse gas (GHG) emissions, and the need for
companies based in cities to reduce their carbon footprints
all put pressure on cities to nd cost-eective solutions to
environmental challenges. At the same time, they juggle
these demands in dynamic environments, often with tight
nancial resources.
At their disposal is a nature-based solution that can help
cities meet many of these aspirations: forests and trees.
Cities around the world are turning to nature-based solu-
tions11 (NBS) to address their challenges and meet their
goals. Forests, in particular, are increasingly recognized as a
cost-eective way to deliver multiple benets. is report
synthesizes the literature on how forests can deliver four key
benets12 for cities and their residents:
Health and well-being by creating habitable, healthy, and
favorable living conditions for city residents
Water by securing access to clean and reliable water
supplies, both within cities and in the key agricultural
regions that feed them
Climate by contributing to climate change mitigation and
its eects on millions of urban residents
Biodiversity by protecting essential global biodiversity,
which supports many of the systems people rely upon,
such as pollinating crops, providing medicines, regulating
climate, and underpinning many spiritual values
Forests—both within and beyond city boundaries—provide
benets to cities and their residents. Our framework (Fig-
ure 1) divides forests into three levels—inner, nearby, and
faraway—to show what the benets are, how they dier
depending on the location of the forest, and how cities can
support forests to harness the greatest benets. is frame-
work was conceived by the founders of the Cities4Forests13
initiative, based on multiple projects and engagements with
both forest and city landscapes.
Inner forests include trees and forests growing along
streets, in city parks, on private property, as remnant
patches of native forests or woodland, and in urban
coastal areas within cities. ese inner forests can improve
air quality, oset heat islands (leading to lower energy use
and bills), reduce stormwater runo and urban ooding,
and support human health and wildlife.
Nearby forests include forests, woodlands, and trees found
in watersheds surrounding cities. ey enhance urban air
quality, regulate temperature, provide stable supplies of
clean drinking water, reduce ooding, and oer opportu-
nities for relaxation and recreation.
Faraway forests are intact forests located beyond a city’s
watershed. ese forests, particularly those in the tropics,
sequester large amounts of carbon, generate rain for cities
and the world’s farm belts, provide a wealth of useful
products, and host the majority of the world’s land-
based biodiversity.
FIGURE  | Inner, Nearby, and Faraway Forest Benefits
Note: Inner, nearby, and faraway forests provide multiple benefits to cities, many of which are aligned with the UN Sustainable Development Goals.
Source: Cities4Forests n.d.a.
Clean air
Drinking water
Reduced flooding
Reduced soil erosion
Carbon storage
Rainfall generation
Clean air
Shade from sun
Urban wildlife
Higher property values
Better Forests, Better Cities | 23
FIGURE  | Tropical Deforestation Rates over Time
Source: WRI n.d.a.
2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Primary forest loss (hectares)
Global deforestation is not evenly distributed; tropical
forests are being cleared at much higher rates than those in
temperate and boreal regions. Commercial production of
globally traded agricultural commodities—such as soy, beef,
and palm oil—is the leading driver of tropical deforestation
(Curtis et al. 2018). In response to consumer demand from
people—including city residents—thousands of miles away,
tropical deforestation rates continue to rise. For the past
several decades deforestation rates have been much lower in
temperate and boreal regions, although in temperate areas
many of these forests were cleared in the past (Currie and
Across the globe, forests are under threat. During the decade
2010–19, global forest cover declined by an average of 4.7
million hectares (ha; 11.8 million acres) per year (FAO
2020).14 In 2019 alone, the world lost an area of tropical
primary forest the size of a football pitch every six seconds
(Weisse and Goldman 2020). In the world’s largest intact
forests, deforestation and forest degradation are driven
largely by agriculture, res, and logging (Figure 2). Inside
cities, pollutants, high temperatures, compacted soils, pests,
and diseases create challenging conditions for trees to grow
and survive. Urban tree cover has been decreasing at an
average rate of 0.04 percent per year (Nowak and Green-
eld 2020). In the United States alone, this loss represents
about 36 million trees per year, equating to an estimated
nancial loss of US$96 million per year in benets (Nowak
and Greeneld 2018a). Urbanization often encroaches on
woodlands surrounding cities, and an estimated 9 in 10 cities
have lost signicant amounts of natural land cover in their
source watersheds to agriculture and development (McDon-
ald et al. 2016).
24 | WRI.ORG
Bergen 2008). Forest loss in temperate and boreal regions is
now mainly due to re and harvesting for wood and paper
products, and forests sometimes regrow after clearing (Figure
2; Curtis et al. 2018).
Cities suer the costs. e impacts of deforestation and for-
est degradation always extend far beyond the cleared region.
Cities experience poorer air quality, ooding, landslides, and
more extreme weather events, to name a few, as a result of
deforestation near and far. Deforestation decreases the ben-
ets that cities receive from forests (Figure 1) with negative
impacts on the physical, economic, and mental well-being of
urban residents. In a world struggling to combat COVID-
19, deforestation and forest degradation have been linked to
increased incidence of vector-borne diseases such as malaria
(Karjalainen et al. 2010) and the emergence of infectious
zoonotic diseases such as coronaviruses (Afelt et al. 2018).
Cities play a major role in forest loss and degradation.
Consumption in urban areas is directly responsible for
about 75 percent of global carbon dioxide (CO2) emissions
and two-thirds of global energy use (Seto et al. 2014). And
although cities cover only a small proportion of the earth’s
surface, their footprints are large: as major “net importers,”
cities depend heavily upon resource extraction and produc-
tion beyond their boundaries (Weinzettel et al. 2013). A
midsize city in North America, for example, is responsible
for thousands of hectares of tropical deforestation per year
via the goods it consumes.15
Cities—as places where people increasingly live and work—
can make major contributions to addressing these issues.
e public policies and procurement practices of cities—as
well as the values, votes, and consumption patterns of resi-
dents—have enormous potential to support the conservation,
restoration, and sustainable management of forests. Many
cities already support forests, for example, through eorts
to expand urban tree cover and parks. Some cities provide
incentives to protect watersheds for their water supplies, and
there is growing investment in forests as “green infrastruc-
ture.” Some cities are implementing procurement guidelines
to reduce tropical deforestation driven by city consumption.
But the potential to do more is immense. e Intergovern-
mental Panel on Climate Change’s (IPCC) Special Report
on Climate Change (Jia et al. 2019) estimates that 11 percent
of global carbon emissions come from land-use change—
especially tropical deforestation. If tropical deforestation
were a country, it would rank third only behind China and
the United States in GHG emissions (Gibbs et al. 2018).
Conserving, sustainably managing, and restoring forests and
other ecosystems could reduce global GHG emissions by up
to 30 percent and provide 23 percent of the cost-eective
mitigation measures needed to prevent global temperatures
from rising 2°C (Griscom et al. 2017; Wolosin and Harris
2018). Much more can be done to promote forests as a cli-
mate change solution (Seymour and Busch 2016).
is report addresses two key questions: How and under
what conditions do forests support cities? And what can
cities do to support forests? To answer the rst question,
we rigorously research and explore four ways forests benet
cities16 and describe each in its own section (Table 1).
TABLE  | The Four Sections of This Report
Thriving, vibrant cities provide their
residents with ample opportunities
for social interaction, with food
and water security, with enhanced
economic opportunities, and with
comfortable, safe living conditions.
Forests in and around cities
can promote recreation, mental
restoration, and spirituality. They
also help to mitigate hazardous
urban environmental conditions
related to extreme temperatures
and exposure to air pollutants.
Finally, they can supplement food
supplies and provide livelihoods
for many–including vulnerable and
marginalized populations.
Faraway forests also play a critical
role in providing the templates
for new pharmaceuticals. And by
protecting tropical forests and
other biodiversity hotspots from
degradation, we may be able to
avoid novel infectious diseases.
Forests interact with climate and the
hydrological cycle at local, regional,
and global scales.a
Inner forests can support cities as
they strive to provide clean, readily
available water to their residents and
can also reduce burdens on urban
infrastructure and prevent flooding.
Nearby forests improve urban water
quality by shielding rivers from
high temperatures, pollutants, and
erosion that can negatively aect the
natural balance of the ecosystem.
Faraway forests—especially tropical
forests—influence precipitation
patterns in cities and agricultural
regions hundreds of miles away
as they cycle water into the
Climate change poses a special
threat to cities. Cities experience
higher temperatures than rural
areas, and many cities lie on coasts.
As concentrated centers of people,
culture, and economic activity, urban
areas are extremely vulnerable to
natural disasters.
Forests can help to mitigate climate
change and promote adaptation.b
Photosynthesis—nature’s own
carbon capture and sequestration
solution—makes forests a highly
cost-eective climate change
mitigation option.
Investing in forests inside cities
can lower city emissions via
urban cooling. Conserving and
restoring forests outside cities via
deforestation-free consumption and
sustainable forest management can
protect some of the most important
carbon sinks on the planet: large,
contiguous forests—especially
tropical forests.
Biodiversity, the variation of life on
Earth, supports ecosystem function,c
fosters connection to place,
stimulates tourism,d and harbors
potential blueprints for nutritional
and medicinal products key to
human health.e It also provides
endless opportunities for discovery
and wonder.
Inner and nearby forests can serve
as habitat, climate refugia, and
corridors for key flora and fauna,
including pollinators, edible plants,
and iconic birds and mammals.
Forests at all levels can play a key
role in preserving biodiversity for
future generations and promoting
ecosystem functioning, but
conserving and sustaining intact
tropical forest is vital. Tropical
forests contain most of the planet s
biodiversity on land. Yet to date,
only 18% of the world’s forests and
27% of tropical forests are currently
Note: References for the summaries in this table are given in the relevant sections of the report.
Sources: a. van Noordwijk et al. 2014; b. Tye et al. 2022; c. Cardinale et al. 2012; d. Hausmann et al. 2016; e. Karjalainen et al. 2010; f. FAO and UNEP 2020b.
26 | WRI.ORG
City representatives can use each section separately or all
together, depending on their focus and objectives. For exam-
ple, a city employee concerned with improving air quality
could read Section 2 to learn more about how trees and for-
ests can help. An ocial interested in making trees and green
space a focus of the city agenda might read all four sections.
Sections 2–5 have a similar overall structure, with a Back-
ground” section about the topic and its relevance for cities as
well as a section that describes what forests do and—when
data allows—quanties their benets. is latter section is
formatted dierently between sections to suit the specic
subject. e “Caveats and Considerations” section highlights
the nuances that are important for realizing the benets,
including when forests will not produce benets, and what
types of forests are most appropriate. is section helps urban
decision-makers avoid unintended consequences and get the
most from their investment in trees and forests. Collectively,
this information can be used by city governments, city man-
agers, other agencies, and groups such as nongovernmental
organizations (NGOs) and community-based organizations
that work with cities to rethink how they should engage with
and use forests to achieve specic end goals.
e question “What can cities do to support forests in
return?” is addressed in Section 6, which outlines what city
leaders, managers, and other city stakeholders can do. e
section includes recommendations on resident engagement
and awareness, communications campaigns, public policies
and procurement, nance, and more.
is report synthesizes the latest research on how inner,
nearby, and faraway forests benet cities and urban residents.
It summarizes research ndings gathered through several
literature reviews, including several “reviews of reviews”
(surveys of published review papers; e.g., van den Bosch and
Sang [2017]), supplemented by reviews of primary literature
and expert-recommended texts. Reviews covered four main
topics: health and well-being, water, climate, and biodiver-
sity. ere is a geographical bias of the “reviews of reviews”
methodology, which was limited to published documents
written in English. We partially correct for this by including
ndings from relevant empirical papers and by including
case studies from under-represented areas identied through
reference lists and expert recommendations. Sections of the
report, such as Section 6, are also partly based on conver-
sations with city representatives, interviews with topical
experts, and experiences from years of projects that World
Resources Institute (WRI), Pilot Projects, and Cities4Forests
have worked on.
ere is no universally accepted denition of forest (Chaz-
don, Brancalion, et al. 2016), and dierent elds of study use
the term in dierent ways. We include research on both trees
and forests as well as on natural and planted forests, and we
make distinctions where appropriate. We also include work
on urban nature, green infrastructure, and green spaces more
broadly when forest-specic studies are lacking. A more
detailed methodology can be found in Appendix A.
Better Forests, Better Cities | 27
Health and Well-Being
Integrating trees and forests into the urban
landscape makes cities more vibrant and livable,
and can generate a diverse suite of health
benefits, from cooler temperatures to improved
mental health to space for social interaction and
community building. Outside cities, forests hold the
blueprints to medicines, help provide cleaner water,
and provide spaces to relax and recreate.
Better Forests, Better Cities | 29
Living in cities provides numerous benets, including access
to economic and educational opportunities, shorter com-
mutes, public services, and intercultural exchange. But urban
living can have negative impacts on the mental and physical
health of residents. Exposure to air pollution, chronic stress-
ors such as noise pollution, sedentary lifestyles, increased risk
of communicable disease in crowded conditions, and extreme
temperatures in the built environment can erode overall
health and quality of life—and can sometimes be deadly (Bai
et al. 2012; Kuddus et al. 2020). Climate change threatens to
amplify impacts through higher temperatures, aberrant rain-
fall, and, for coastal cities, rising sea levels (Revi et al. 2014).
As they strive to create healthier cities, city leaders can
embrace sustainably managed inner or “urban” forests as an
NBS. When trees are integrated into the urban landscape in
socially and ecologically appropriate ways, cities can become
more livable and vibrant spaces. Evidence suggests that,
unlike many issue-specic municipal investments, the “urban
forest” can generate a diverse suite of benets at the same
time—from cooler temperatures and air quality improvement
to improved health and space for social interaction.
How forests relate to human
In 1984, a pioneering study found that patients whose
windows looked out upon a group of trees healed from
surgery faster and needed fewer painkillers than those whose
windows had no view of nature (Ulrich 1984). Since then,
a rapidly expanding and compelling body of evidence—
spanning disciplines as diverse as epidemiology, psychology,
forestry, and geography—suggests that forests and nature
play an important role in human health. Evidence also
indicates that forests in and around cities may contribute to
social and economic well-being because benets accrue to
individuals using forests to support their livelihoods, to prop-
erty owners whose parcels increase in value, and to entire
regions as forests support tourism or other industries.
Why context matters
Cities are immensely diverse in climate, culture, politics,
language, and environmental contexts. As such, considering
the local cultural, political, climatic, environmental, and soci-
oeconomic contexts is important for successfully integrating
trees and forests into city planning. e benets that forests
in cities provide will vary from city to city around the world.
Inner forests (and related green infrastructure) may not
always provide the intended benets, and they can some-
times present unintended risks (Hartig et al. 2014; Lõhmus
and Balbus 2015). By understanding these risks and deliber-
ately incorporating how to address them into the planning
and decision-making processes, the potential for unintended
negative outcomes can be minimized and the many positive
benets realized (Lõhmus and Balbus 2015; Wolf 2017).
And by empowering communities to guide urban greening
initiatives and stewardship of the inner forest, these living
elements of urban infrastructure can help to diminish—
rather than exacerbate—inequities among groups.
About This Section
Forests near and far support human health and well-be-
ing—socially, economically, and ecologically. In the following
sections, we summarize the ways that inner and nearby
forests aect the quality of life of city residents. Local forests
provide many direct and indirect health benets. is section
thus focuses on inner forests that provide unique opportuni-
ties for leaders seeking to create healthy and habitable cities.
Nearby and faraway forests also provide health benets via
water access and treatment, climate change mitigation, and
biodiversity, which are addressed in the respective sections.
To explore these benets, we synthesized statements and
goals shared by many city leaders around the world into eight
specic goals related to health and well-being:
1. Reducing extreme heat
2. Enhancing urban air quality
3. Promoting physical and mental health in city residents
4. Creating walkable, safe streets
5. Supporting community connections
6. Reducing urban environmental inequity
7. Ensuring provision of food, medicine, and raw materials
8. Enhancing economic well-being
30 | WRI.ORG
Cities suer from the heat island eect. Most cities are
dominated by buildings and pavement, with relatively little
vegetation and green space, which contributes to “urban heat
islands”—elevated temperatures in urban areas compared to
their rural surroundings. Urban areas can be 2°C–4°C—and
as much as 15°C—warmer than adjacent areas (Taha 1997;
Heaviside et al. 2017; Mohajerani et al. 2017). e heat
island eect presents a number of risks to human health and
well-being, including the following:
Increased risk of heat-related mortality and morbidity,
especially during heat waves; high temperatures can cause
heat stroke, dehydration, exacerbate existing diseases, and
even cause death (Luber and McGeehin 2008); stiing
heat may also interfere with worker productivity (Zander
et al. 2015) and with learning and educational achieve-
ment (Park et al. 2020)
Potential for negative eects on mental health, although
more research is needed (ompson et al. 2018)
Spikes in energy demand (Li et al. 2019)
Power outages due to high energy demand at midday,
which can further aect resident safety, impair economic
activity, and burden health and emergency services
(WMO and WHO 2015)
Degradation of environmental quality, such as increased
concentrations of urban smog (Akbari et al. 2001),
increased ground-level ozone (Luber and McGeehin
2008; Jacob and Winner 2009), and decreased water
quality (Phelan et al. 2015; Heaviside et al. 2017)
Climate change will exacerbate these risks. Since the advent
of the Industrial Revolution, the average temperature near
Earth’s surface has increased about 1°C (1.8°F; IPCC 2018).
e urban heat island eect magnies the eects of climate
change for cities, leading to higher temperatures than rural
areas and more extreme heat waves (Estrada et al. 2017).
Already, extreme heat in cities has been responsible for thou-
sands of excess mortalities in recent decades (Heaviside et
al. 2017). e 2003 European heat wave, for example, killed
more than 70,000 people (Robine et al. 2008). ousands
in India and hundreds in Pakistan died as temperatures sur-
passed 45°C in 2015 (Masood et al. 2015; Sarath Chandran
et al. 2017). In 2021, a lingering heat wave shattered records
in western North America, spiking heat-related illnesses and
killing hundreds, an event the World Weather Attribution
initiative described as “virtually impossible without human-
caused climate change” (WWA 2021). Records suggest July
2021 was the hottest month on record (NOAA 2021).
Some urban residents are more susceptible to these risks
than others. In general, lower-income and marginalized
communities are disproportionately exposed to the delete-
rious eects of heat islands (UN DESA 2020). Children,
people above the age of 50, and those with preexisting health
conditions are particularly vulnerable to heat-related illnesses
(Kovats and Hajat 2008). Densely settled and lower-income
communities often lack access to places to cool down, such
as shaded green spaces and open areas (Harlan et al. 2006;
Luber and McGeehin 2008). In addition, many low-income
households lack insulation, air conditioning, and access to
resources necessary to cope with extreme temperatures (Har-
lan et al. 2006; Ko 2018).
What roles can trees and
forests play?
Trees in urban areas can mitigate the urban heat island eect,
especially locally, by the following actions (Figure 3):
Shading surfaces and people. Tree canopies intercept and
reect up to 90 percent of incoming solar radiation.
Shade makes heat more tolerable and can protect people
from excessive sun exposure during travel, work, or leisure
(Nowak and Dwyer 2007). Trees that shade buildings can
reduce surface temperatures in a wide variety of contexts
(Wang et al. 2014); for example, surface temperatures
were reduced by 11°C–25°C in Sacramento, Califor-
nia (Akbari et al. 1997); by 5°C–7°C in Akure, Nigeria
(Morakinyo et al. 2013); and by 9°C in Melbourne,
Australia (Berry et al. 2013). In Bangalore, India, streets
Better Forests, Better Cities | 31
with trees had local ambient air temperatures that were
5.6°C lower than streets without trees, and their surface
temperatures were 27.5°C lower (Vailshery et al. 2013).
Cooling the air via evapotranspiration. During the day,
trees may create lower air temperatures by releasing water
into the air as they photosynthesize (Bowler et al. 2010a;
Säumel et al. 2016). As water vapor is released, it takes
with it some of the ambient heat. Large trees with ample
access to water may evaporate more than 100 liters of
water in a single day, which dissipates about 70 kilo-
watt-hours of solar energy that would otherwise remain
stored in the urban environment (Fath 2018).
Eects of cooling are most pronounced locally. A 2010
global meta-analysis found that parks were, on average,
0.94°C cooler during the day than surrounding urban areas,
with greater benets in larger parks and in those con-
taining trees (Bowler et al. 2010a). A more recent review
suggests large urban parks and green spaces (more than 10
ha)—especially those with mature trees—can reduce air and
surface temperatures by 1°C–2°C (Aram et al. 2019). Some
evidence shows that areas adjacent to green spaces also are
cooler (Mohajerani et al. 2017), from a few hundred meters
(Tyrväinen et al. 2005; Aram et al. 2019) to perhaps as much
as a kilometer (Bowler et al. 2010a).
e urban forest can both reduce the risk of heat-related
illness or death and increase perceived comfort for residents
(Taha 1997; Tyrväinen et al. 2005; Salmond et al. 2016;
Gunawardena et al. 2017; Wolf et al. 2020):
Researchers estimate that in 97 U.S. cities alone, urban
tree cover helps to avoid 245–346 premature deaths and
50,000 hospitalizations annually (McDonald et al. 2020).
In Toronto, residents in neighborhoods with the lowest
tree canopy cover (less than 5 percent) made 5 times as
many heat-related emergency calls as residents in neigh-
borhoods with more than 5 percent canopy cover and
nearly 15 times as many emergency calls as residents in
neighborhoods with more than 70 percent canopy cover
(Graham et al. 2016).
FIGURE  | Localized Cooling Benefits from Trees through Shading and Evapotranspiration
Notes: Pavement and concrete in cities absorb energy from the sun and then radiate that energy out, heating the air in cities more than in the surrounding countryside. Urban
trees provide shade, preventing pavement and concrete from heating up, and also cool the air by transpiring water. Trees can cool neighborhoods by up to 4 degrees Fahrenheit
(McDonald et al. 2016).
Source: Authors. Adapted from McDonald et al. (2016).
Heat is generated and
emitted by buildings and
paved surfaces where
there is little vegetation
Vegetated Areas
around cities stay
Urban Trees
provide shade
which cools
localized areas
Pavement and concrete in cities absorb energy from the sun and then radiate that energy out, heating the air in cities
more than in the surrounding countryside. Urban trees provide shade, preventing pavement and concrete from
heating up, and also cool the air by transpiring water. Trees can cool neighborhoods by up to 4 degrees Fahrenheit.
Caveats and considerations
Urban forests may complement or be incorporated into other
interventions to reduce the urban heat island eect. ese
interventions include permeable pavements and green roofs
(Mohajerani et al. 2017).
Cooling by urban trees will be greatest in local areas,
and forests may not provide net benets in some situa-
tions, such as these:
Trees and shrubs very close to buildings may prevent
nighttime radiative cooling of buildings (Bowler et al.
2010a; Wang et al. 2014; Ko 2018).
Tall trees can reduce wind speeds (Mohajerani et al.
2017). is can be a benet in cold climates but can be
a disadvantage in warm or humid climates (Ko 2018).
At night, tree canopies can also reduce airow and thus
retain heat (Bowler et al. 2010a; Salmond et al. 2016).
e microclimate-altering eects of urban trees and other
vegetation are more pronounced in cities in warm and dry
climates (Taha 1997).
Because evapotranspiration increases humidity (Sal-
mond et al. 2016), high levels of evapotranspiration
may reduce comfort for urban dwellers in hot, humid
climates, even as evapotranspiration lowers near-ground
temperatures. However, the increase in humidity may be
small compared to the reductions in temperature (Vailsh-
ery et al. 2013).
Better Forests, Better Cities | 33
Some species provide more cooling benets than others.
Leaf area index, evapotranspiration rate, crown diameter,
and the albedo of dierent tree species aect the cooling
benets they provide (Jim and Chen 2009; Bowler et al.
2010a; Smithers et al. 2018). Fast-growing, long-lived,
and drought-tolerant native species of trees with relatively
reective surfaces are most likely to deliver cooling benets
(Smithers et al. 2018).
Air pollution threatens the well-being of most urban dwell-
ers. An estimated 9 out of 10 people breathe polluted air
worldwide (WHO 2016). Responsible for approximately
4.2 million deaths globally in 2016 (WHO 2016), exposure
to ambient air pollution is considered one of the greatest
risk factors for global public health (Burnett et al. 2018).
Exposure to air pollution disproportionately aects residents
of low- and middle-income countries. It disproportionately
aects lower-income and racial or ethnic minority residents,
as documented in North America (Landrigan et al. 2018;
Tessum et al. 2019; Nicolaou and Checkley 2021).
Air pollution needs to be addressed at the source (Baldauf
and Nowak 2014; EPA 2019) because more pollutants are
emitted than can reasonably be contained with mitigation
measures.But eliminating air pollution is an intractable
challenge to even the most well-resourced governments—
especially pollution from nonpoint sources such as vehicles
and woodsmoke/biomass burning from residences. Trees
and other green infrastructure can help to remove these
pollutants locally and/or be used to create barriers between
pollutant sources and the people or organisms exposed
(Baldauf and Nowak 2014; Hewitt et al. 2020; Wernecke
and Pool 2022).
What roles can forests play?
Typically, urban forests reduce air pollution by around 1
percent at the city scale (Litschke and Kuttler 2008; Baldauf
and Nowak 2014; Salmond et al. 2016; Sicard et al. 2018;
Xing and Brimblecombe 2020). But even a modest reduction
in pollution can be very valuable to cities. For example, in
2010, forests in the continental United States (both inside
and outside of cities) removed an estimated 17.4 million tons
of air pollutants such as particulate matter (PM), contrib-
uting to health benets—including 850 avoided premature
mortalities—worth an estimated $6.8 billion (Nowak et al.
2014). However, reducing pollutants further may require a
large expansion in tree canopy cover (Litschke and Kuttler
2008; Nieuwenhuijsen et al. 2017). Models of the eects of
urban trees on local air quality (i.e., site scale) suggest larger
reductions are possible with proper planning and species
selection (Pugh et al. 2012; Janhäll 2015; Abhijith et al.
2017; Barwise and Kumar 2020).
Forests and other vegetation can have positive or negative
eects because they interact with urban air pollutants in sev-
eral dierent ways. Urban trees alter pollutant concentrations
by trapping pollutants or by redirecting airow:
Removing particles from the air (deposition) by either
taking in gaseous pollutants or having particles settle on
their surfaces (Beckett et al. 1998). Trees remove pollut-
ants at faster rates than other types of vegetation (Fowler
et al. 2004). Dense but porous vegetation serves as an
ideal surface for deposition, superior to the comparatively
smooth surfaces of buildings and roads.
Dispersing pollutants (dilution) in the urban environment
by altering airow patterns and slowing wind (Abhijith
et al. 2017). Dilution of highly polluted air with clean air
from surrounding areas enhances urban air quality. Trees
can help or hinder dilution: they may act as an obstacle,
slowing wind speeds and reducing the exchange between
clean and polluted air, suppressing pollutant disper-
sion (Säumel et al. 2016; Abhijith et al. 2017; Xing and
Brimblecombe 2020) or as a source of turbulence that
increases the exchange, based on characteristics of the
built environment and on meteorological conditions.
But trees can also emit two types of particles that
aect air quality:
Biogenic volatile organic compounds (bVOCs) can act as
precursors to pollutants such as ozone and secondary
organic aerosols and can worsen air quality (Laothaworn-
kitkul et al. 2009; Leung et al. 2011; Calfapietra et al.
2013; Cariñanos et al. 2017). Even healthy plants produce
bVOCs (Smith 1981), but exposure to drought, pol-
lutants, heat, and excessive sunlight, as well as physical
34 | WRI.ORG
injury or attacks by pests, may all induce the release of
additional bVOCs (Laothawornkitkul et al. 2009; Cal-
fapietra et al. 2013). Increases in global temperature may
increase bVOC emissions further (Laothawornkitkul et
al. 2009; Wang et al. 2014).
Allergenic pollen can undermine health (Smith 1981;
Beckett et al. 1998; Cariñanos and Casares-Porcel 2011;
Säumel et al. 2016; Eisenman et al. 2019; Hewitt et al.
2020). Climate change and air pollution have led to
the increased production of pollen in some tree species
(Cariñanos and Casares-Porcel 2011). Allergies due to
pollen can decrease the quality of life of urban dwell-
ers, and allergen exposure has been linked to ill health
conditions such as cardiovascular disease, pneumonia, and
asthma (Curtis et al. 2006).