TRANSFORMING SITKA SPRUCE PLANTATIONS
TO CONTINUOUS COVER FORESTRY
UCD AND TEAGASC FORESTRY RESEARCH
Forestry is entering a new phase where traditional management
objectives must be balanced with the need to enhance ecological
resilience. Predicted changes in our climate plus the very real threats
from introduced insect pests and diseases are making us look
again at the composition and structure of our forests. Currently
in Ireland and the UK we rely on a small number of productive
conifer and broadleaved species, and clear-felling remains the
dominant silvicultural system. Using a wider range of genotypes,
species and silvicultural systems will lead to a more diverse resource
and reduce environmental risks. However, the challenge for
foresters is to develop and adopt new approaches that increase
resilience while also sustaining timber production and other
Continuous Cover Forestry, commonly referred to simply as
CCF, is one of the most important options to emerge in recent
years. CCF is a f lexible and adaptable management system that
creates diverse and resilient forest stands. In this article, we
provide a brief overview and consider current research on CCF,
especially in production forests dominated by Sitka spruce.
DEFINITION OF CCF
Continuous cover forestry is defined as the use of silvicultural
systems where the forest canopy is maintained at one or more
levels without clear-felling. The guiding principles are to manage
and sustain the forest ecosystem, work within site limitations and
use natural processes wherever possible. Through single tree and
small group selection, production and regeneration take place
simultaneously. Gradually, as a result of successive interventions,
the stand develops an irregular structure (Figure 1).
There are several economic and environmental attributes of
CCF that may contribute to its wider adoption in Ireland and the
UK. In economic terms, CCF enables the forester to select individual
trees on the basis of their potential for high quality timber production.
Through good thinning practice, it is possible to control the
growth increment of high quality trees and schedule their harvest
across stand interventions to the point where they achieve their
target size or optimum value. Environmentally, several advantages
of CCF forests have been identified relative to even-aged systems,
that contribute to forest resilience: they can be more windfirm;
they maintain a more even carbon storage; they show lower soil
carbon losses during harvesting; there is a reduced risk of soil
fertility loss; they maintain higher humidity levels and they are
well-suited to both single and mixed species management. In
catchments and riparian zones, CCF can be used to regulate
water yield, stream temperatures and reduce the risk of siltation
or nitrate flushes that are often a concern downstream from clear-fell
sites. CCF is also desirable in multi-functional forests where
landscape, recreation and conservation values are important. In
effect, stands acquire old growth characteristics that enhance
TRANSFORMATION OF PLANTATIONS
In Ireland and the UK, most forest stands are plantations composed of
a single, or a very limited number of species. In order to increase
CONTINUOUS COVER FORESTRY (CCF)
Identifying potential crop trees and regular thinning are the keys to successful transformation of
plantations to resilient, continuous cover forests. By Edward Wilson, Ian Short, Áine Ní
Dhubháin and Paddy Purser.
Figure 1. Continuous cover forestry in a mature stand of Sitka spruce at Whinlatter Forest, Cumbria, England. The stand was planted in
1920 and is located at 313 m altitude; it has a Yield Class of 16. This stand has good stability, high quality trees across a range of
sizes and significant natural regeneration.
Photo: Gareth Browning/Forestry Commission 2018
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the area of woodland managed as CCF, it is necessary to identify
suitable even-aged stands that can be transformed from a uniform
to irregular structure. Transformation is achieved as a planned and
progressive series of stand interventions that emulate the successional
stages in natural woodland. The principles of CCF transformation are well
defined and the practice of CCF is now the standard approach to forest
management in many parts of Europe.
There are four well-defined stages, summarised as follows:
Stage 1: Preparation
Stands should be identified early in their development based on
their suitability for thinning and productivity. Poor quality trees
(inferior phenotypes) should be removed and better quality trees
(superior phenotypes) should be retained. Individual tree stability
can be increased through the thinning process (i.e., modifying
height: diameter ratios) and the selection process can then
continue with frequent light thinnings. Identifying good quality
stems, across a range of diameter classes, will by default lead to
greater irregularity in the canopy strata. This is best achieved
with “crown” thinning as opposed to “low” thinning, which is the
norm in most even-aged plantations. This approach allows the
forest manager to concentrate stand increment on high quality
stems (Figure 2).
Stage 2: Regeneration
As the stand matures and trees start to produce seed, consideration
can then include the regeneration process. Thinning should aim
to reduce basal area and enable appropriate levels of light to
reach the forest floor. Threshold basal areas for regeneration, and
separately for sustained growth, are now well recognised for
most of our productive species, and this information can act as a
guide for thinning prescriptions. Avoiding uniform removal of the
overstorey trees, and maintaining a degree of “clumping”, will
allow natural regeneration to become established in small
cohorts at irregular spacings throughout the stand.
Stage 3: Structural development
In this stage, tree selection focuses on removing high quality
(crop) trees at their desired target size, and maintaining good
quality smaller trees (i.e., future crop trees) from across a broad
range of diameter classes. Ideally, a maximum of 20% of basal area
and a volume no greater than the stand increment will be removed at
Stage 4: Structural maintenance
Finally, the stand will be transformed to an irregular structure
where the objective is to maintain a sustained yield of high value
trees while promoting the development of the understorey strata.
Trees should be marked for removal at their optimum economic
value, often called the target size, or if they are damaged and not
contributing to the development of the stand structure. Wind
damage, if it occurs, becomes part of the management system and
generally understorey trees are released to fill the gaps created
by blowdown of larger individuals.
CCF IN PRACTICE
In contrast with other areas in Europe, a relatively small area in
Ireland and Britain is currently managed on CCF principles. Research
by Lucie Vítková and colleagues, published in Irish Forestry (2013),
identified approximately 10,600 ha where CCF is being practiced
in Ireland. Experience with CCF is most advanced in broadleaved
woodlands and conifer species, such as Douglas fir, commonly
found on sheltered sites with favourable conditions for natural
regeneration (and where browsing animals are controlled).
Nonetheless, a survey of forestry professionals in Ireland found
there was significant interest in the wider use of CCF but that a
lack of experience, training and reliable measures of stand performance
were barriers to further progress. To promote greater understanding
of CCF, the Irregular Silviculture Network (ISN) has established
long-term research stands that provide information on stand
development, individual tree performance, costs and revenues,
and wider ecosystem services. Currently there are 6 sites in
Ireland, each enumerated on a 5-year cycle.
A critical element of CCF is the selection and marking of trees
to remove and retain at each stand intervention. A detailed under–
standing of tree development and quality criteria is required.
Linked to this is a need for regular, simple inventories of stand
structure. Information about diameter distribution, stem quality
classes, growth increment and basal area is necessary to prepare
tree marking prescriptions and monitor overall stand performance.
CONTINUOUS COVER FORESTRY (CCF)
Photo: Phil Morgan 2017
Figure 2. Sitka spruce in transformation to CCF at Bryn Arau Duon Forest, Wales. This large upland forest is at an altitude of 400m. The stand in
this image was established in 1962 and has a Yield Class of 16. It has been thinned 3 times on a 4 year cycle. The focus at each intervention is to
concentrate the increment on high quality trees. Natural regeneration is beginning to be established in many stands across the forest. Also note
that there is a well-developed infrastructure that supports access to the stands.
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CCF AND SITKA SPRUCE
Sitka spruce is the mainstay of the forest products sectors in
Ireland. In total, 335,000 hectares of Sitka spruce have been
planted. The vast majority of this resource is managed in
plantations less than 40 years in age. Transformation of even a
small proportion of Sitka spruce plantations to CCF would significantly
increase overall forest structural diversity and resilience.
Currently, there is limited guidance available for transformation
of Sitka spruce stands to CCF in Ireland. The closest large-scale
examples in upland locations are in Wales, such as Clocaenog
Forest, which is managed by Natural Resources Wales. Here a
variety of selection and shelterwood systems is being trialled on
sites at relatively high elevations. In the private sector, forestry
consultants Phil Morgan and Huw Denman have been working on
techniques in several woodlands (Figure 2).
Some of the considerations for transformation in Sitka spruce
plantations are now becoming more apparent. An early start to the
transformation process is especially important on upland sites and
where soils are relatively shallow to promote stand stability. Maintaining
good drainage across the site is also important to avoid an adverse rise
in the water-table. Extra care may be necessary in opening the canopy
after thinning operations; too much side light will stimulate epicormic
shoot development that can reduce sawlog quality.
CURRENT RESEARCH: THE TRANSSFOR PROJECT
There is much still to learn about CCF in upland Sitka spruce
plantations in Ireland. The COFORD-funded Low Impact
Silvicultural Systems project (LISS), led by University College
Dublin and completed in 2014, began the process of filling
knowledge gaps. As part of this study, a long-term thinning
experiment was established in two Sitka spruce stands on
contrasting soil and site types in Ireland (Figure 3). The first site
is located in Co Laois on a mineral gley soil, at an elevation of
300 m, in a forest managed by Coillte. The second site is in Co
Wicklow on a brown earth soil at a slightly lower elevation, in
privately-owned woodland. The experiment was designed with
three thinning treatments: low, crown and graduated density
thinning. The first two thinning interventions took place in 2011
and 2014. Initial work on this study was led by Dr Lucie Vítková
as part of her PhD project.
In 2017, funding was secured by Teagasc for a Walsh Fellowship
(2017-2021) that enables the study to continue on through the
third thinning operation, and to generate the first research papers
and operational guidance. This project is titled “Transformation of
Sitka spruce stands to continuous cover forestry”, more simply,
the TranSSFor project. The research is being supervised by Drs
Ian Short (Teagasc) and Áine Ní Dhubháin (UCD Forestry).
Thinning operations are scheduled for the first half of 2018 and
2019. Data will be collected on stand productivity, structural
development, tree stem quality and environmental conditions
within each treatment unit.
In addition to research on tree and stand development, it is
anticipated there will be professional development and knowledge
sharing events for foresters, contractors and woodland owners.
Site visits and training workshops on tree marking for CCF are
already being planned.
Ted Wilson, Walsh Fellow in Silviculture, Teagasc and UCD.
• Sanchez, C. 2017, Pro Silva silviculture: guidelines on continuous
cover forestry/close to nature forestry management practices.
Circular 2718. Forêt Wallonne, Namur, Belgium. 64 pp.
• Susse, R., et al. 2011. Management of irregular forests: developing
the full potential of the forest. Association Futaie Irrégulière,
Besançon, France. 144 pp.
• Teagasc, TranSSFor project:
• Pro Silva Ireland (PSI): www.prosilvaireland.wordpress.com
• Continuous Cover Forestry Group (CCFG): www.ccfg.org.uk
This article was written by Edward Wilson (Teagasc Forestry Development
Department and UCD Forestry), Ian Short (Teagasc Forestry Development
Department, Ashtown, Dublin 15), Áine Ní Dhubháin (UCD Forestry, University
College Dublin, Belfield, Dublin 4) and Paddy Purser (Purser Tarleton Russell Ltd,
Forest Sector Management, Consultancy & Research, Woodenbridge, Co. Wicklow).
Thanks to Phil Morgan (SelectFor Ltd) for helpful comments on a draft of this
CONTINUOUS COVER FORESTRY (CCF)
Figure 3. The TranSSFor thinning experiment, Co Laois. This study is comparing stand development under “low”, “crown” and
“graduated density” thinning regimes. The stand was established in 1992 and thinned in 2011 and 2014. The third thinning is
scheduled for spring 2018.
Photo: Anders Lundholm 2017
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