How will trees in Britain adapt to the changes in climate
expected over the next 100 years? Stephen Isaac explores the key
issues and looks at potential responses.
Our climate is changing. Everyone is aware of the threat to the
polar ice sheets, many people are aware of the threats to coral
reefs and Siberian permafrost, but how much is known about the
threats to biodiversity in the UK?
By 2040 the UK’s annual average temperature is expected to rise by
between 0.5°C and 1°C, depending on region and worldwide levels of
greenhouse gas emissions.
By 2100 the rise is expected to be between 1 and 5°C. The UK’s
climate has already warmed by 0.4°C and 0.9°C over the last century
and has experienced eight of the ten warmest years on record since
This year the UK Climate Impacts Programme (UKCIP) updated its
climate change scenarios from 2002 as part of UKCP09, in order to
predict the climate change effects from low, medium and high
greenhouse gas emission forecasts produced by the Intergovernmental
Panel on Climate Change.
The 2009 scenarios predict that all
areas of the UK will get warmer, and the warming will be greater in
summer than in winter.
There will be little change in the amount of precipitation (rain,
hail, snow) that falls annually, but it is likely that more of it
will fall in the winter, with drier summers, for much of the UK. Sea
levels are also expected to continue to rise, and will be greater in
the south of the UK than the north.
In 2009 UKCIP also published an updated report on recent climate
trends in the UK. The report shows that all of these climate changes
have occurred in the last two decades, albeit with regional
variation. It also shows that:
• severe windstorms have become more frequent in the UK in recent
• average annual and seasonal relative humidity has decreased in all
regions of the UK, except Northern Ireland, between 1961 and 2006,
by up to 5%.
• the annual number of days with air frost has reduced in all UK
regions between 1961 and 2006. There are now typically between 20
and 30 fewer days of air frost annually compared to the 1960s, with
the largest reductions in Northern England and Scotland.
The impact on woodlands
Woodlands and forests, like most flora and fauna, are
susceptible to climate variability, for example through increased
forest fire risk, pest and disease infestations and wind damage.
These changes are already happening in many parts of the world; most
people have seen the devastating effects of forest fire in countries
like Australia, the US and the drier countries of Europe.
We may now be seeing the first signs of them in Britain, with
recorded changes in earlier spring bud growth on British trees and
the increasing incidence of pest and disease attack on many tree
Many of the predicted impacts on woodland will be direct, others
will be indirect and as a consequence may be more difficult to
Direct impacts on trees
Increasing concentrations of atmospheric carbon dioxide (CO2) are
known to affect the productivity of flora through photosynthesis.
Controlled experiments on sessile, pedunculate and red oak saplings
have shown that tree size increases by 30-50% when the CO2
concentration is doubled.
Although evidence suggests that mature
trees are unlikely to respond as much in a forest environment, some
increase in productivity is likely and the forestry sector may
benefit from this. Large leaved species such as the non-native
sycamore may also out-compete native species that have a higher
biodiversity value. Other effects of increasing CO2 concentrations
are likely to include reduced leaf area water use, an increase in
leaf area, possible changes in timber quality and changes to the
nutritional quality of foliage for insect herbivores.
discussed previously, increased temperatures are likely to lead to
earlier bud growth in the year. This has already been shown to
affect some species. For example, oaks in Britain are now
experiencing budburst up to two weeks earlier than in the 1950s.
Sycamore, hawthorn and hornbeam are also budding earlier in the
Although the growing season is therefore expected to extend
for many species, this earlier budburst could also increase the risk
of frost damage early in the year. Warmer temperatures later in the
year may also cause delayed or incomplete winter hardening and
therefore increased susceptibility to frost damage. Additionally,
warmer winters may not fulfil the winter chilling requirements for
flower and seed germination, thereby affecting natural regeneration.
Long-lived woodland species may be less able to adapt to such
changing conditions as those with a shorter life cycle. Woodland
flora are also expected to respond to earlier springs, with
temperature sensitive species like garlic mustard and cow parsley
leafing earlier and dominating over snowdrops and bluebells. As a
result of higher peak temperatures the top-dying of Norway spruce is
likely to increase in England and eastern Scotland and Norway spruce
could cease to be a productive species over much of England.
Increasing heat and drought in the south and east are also likely to
increase tree loss, particularly among newly established trees and
mature trees in hedgerows and urban environments. Coniferous species
experience increased drought crack in central and southern England.
Changing rainfall patterns are likely to cause direct impacts on
In south-east England, reduced summer rainfall and an
increased evaporative demand are likely to lead to longer periods of
drought stress on trees. This is already known to affect beech trees
in south-east England. Conversely, increased winter rainfall could
increase the risk of water-logging, thereby reducing tree stability.
Water-logging could also cause fine root death and thus indirectly
affect the ability of trees to take up water, which in turn could
affect their ability to withstand summer droughts.
direct impact is likely to come from changing wind speeds. The
UKCP09 report on recent climate trends shows that storms have become
more intense in recent decades. If this trend continues woodland
will likely become more vulnerable to wind damage.
Indirect impacts on trees
Experts have predicted that the majority of insect pests that
currently affect UK forestry are likely to benefit from climate
change as a result of increased activity and reduced winter
mortality. Pests that are currently found in low or medium levels,
such as defoliating moths, the horse chestnut leaf miner and bark
beetles may become more prevalent. The range of certain pest species
may also expand, with a northward movement of species that currently
have a southern distribution and the probable appearance of new
species from continental Europe and beyond.
Outbreaks of bleeding
cankers, green spruce aphid and pine weevils are also likely to
increase as the climate warms. Fungal pathogens such as Phytophthora
ramorum, responsible for the condition known as ‘sudden oak death’
may also thrive. Up to early December 2006 a range of tree species
in Cornwall including beech, southern beech, horse chestnut, sweet
chestnut, sessile oak, Turkey oak and sycamore have all been found
with potentially lethal infections of this fungus. Red band needle
blight is another fungal disease that could become more widespread
in the UK.
It was first spotted in the UK in the 1950s but has
only become a serious problem recently. The disease, recognisable by
a distinctive red band around each needle, can kill trees, affect
their rate of growth and reduce the quality of their timber. It
thrives on Corsican pine, but has also attacked indigenous species
such as Scots pine.
Mammalian tree pests such as the grey
squirrel and various deer species may also be more likely to survive
the winter under warmer conditions, with deer additionally
benefiting from earlier spring growth of ground vegetation.
Forest fires pose a considerable threat to woodland worldwide.
Although the risk may be higher in warmer and drier countries, parts
of the UK are certainly vulnerable. Although the earlier growth of
ground vegetation could reduce the risk of forest fires in spring
and early summer, extended summer droughts, such as those of 1976
and 1995, result in a build up of dry, dead and highly flammable
Flooding associated with sea level rise or fluvial
inundation may also pose a significant threat to trees in coastal
areas or river floodplains, either directly or gradually and
indirectly through habitat change. In some cases wet woodland
species may benefit from increased fluvial inundation.
Mitigation and adaptation Woods and forests of the world have an
important role to play as carbon ‘sinks’, essentially absorbing
atmospheric carbon dioxide that would otherwise contribute to
Woodland is also important in that it provides
biomass, an alternative to burning fossil fuels and the use of short
rotation forestry can provide timber that often provides a practical
alternative to using fossil fuel intensive materials.
In order to
manage the response to climate change there are several ways in
which the forestry sector and landscape managers can adapt to the
changing circumstances. For example, climate change predictions
should be taken into account when choosing stock.
at the edge of their range or with poor capacity to reproduce may
need to be discounted.
The introduction of more southerly or
drought resistant species could help revitalise existing woodlands
and establish more robust new woodland.
Increasing the species
mix of the woodland can also enhance its adaptive capacity and help
to alleviate the threat from pests and diseases that target specific
Areas of land vulnerable to sea level rise or changed
hydrological conditions may also need to be discounted from new
On a landscape level, larger, better connected
woodland will enhance the ability of woodland and individual tree
species to adapt to a changing climate, for example through
migration within the woodland area. This may include managing,
protecting and connecting woodlands to increase resilience and
reduce the impacts from other external pressures that might have a
cumulative or synergistic impact with climate change.
The risk of
fire can also be reduced through management of fuel loads, managing
for appropriate age and species structures of woodlands, and the use
of fire breaks.
The forestry sector could potentially come into conflict with nature
conservation bodies over the choice of species and the way in which
woodlands are managed but the two sectors are not mutually
There is potential to increase the biodiversity value
and climate change resilience of commercial forestry and
non-commercial woodland by utilising the latest climate change
research and promoting effective co-ordination between planners and
the wider public.
Research into the effects of environmental and
climate change on forestry and how the forestry sector can adapt to
and mitigate climate change is now being actively progressed through
bodies such as Forest Research, an agency of the Forestry Commission
which has recently developed a "Centre for Climate Change."
The outputs from the UKCP09 climate change scenarios can be used as
a vital tool for predicting and responding to the likely impacts on
woodlands and trees in Britain. As well as providing useful data for
landscape managers they could also provide a vital baseline input to
policymakers and environmental professionals assessing climate
impacts on biodiversity using tools such as Environmental Impact
Assessment, Strategic Environmental Assessment and Habitats
By understanding the climate challenges
ahead we can better inform our responses to it and give
wildlife-rich British woodland a fighting chance of survival.
Article written by Stephen Isaac AIEMA
Halcrow Group Ltd