Woodland actions for biodiversity and their role in water ...
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Research Report
Woodland actions for biodiversity and their role in water managementMarch 2008
Research Report
Woodland actions for biodiversity and their role in water managementMarch 2008
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INTRODUCTION
The highly fragmented semi-natural habitats
of the UK are vulnerable to climate change.
There is a need to develop landscapes that
are resilient, i.e. able to absorb and respond
to changes, thereby sustaining biodiversity
and ecosystem goods and services.
Woodland actions for biodiversity have an
important role to play ecologically and may
have considerable potential to contribute
to economic and other benefits.
Increasing attention is being given to the
interactions between woodland and water,
as integrated land and water resource
management seeks to address a number of
water issues, including the threats posed by
climate change. While a wide range of
projects have researched or reviewed
particular aspects of water management on
which trees and woodland have an impact,
there is a need for an accessible
overarching synthesis. Technical terms are
asterisked where they appear for the first
time and a glossary is provided at the end
of the document.
AIMS
This report reviews national and international
peer-reviewed and ‘grey’ literature on the positive and
negative, direct and indirect, impacts of trees and
woodland in temperate systems on water resources in
relation to:
� Water quality: turbidity*/siltation* and riverbank stability;
eutrophication*; pesticides and other chemicals;
acidification; water colour/dissolved organic carbon*.
� Water quantity: streamflow*; groundwater recharge*; soil
infiltration* and run-off pathways; base or low flows*; peak
flows*; flood frequency, intensity and risk.
Projected impacts of climate change are taken into
account.
Cover photographs:Top- WTPL/David Rodway. Lower- WTPL/Pete Holmes
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The report considers the implications for water
resources at a catchment scale, regionally and nationally in
the UK, of:
� Maintaining the existing area of native* and ancient
woodland*.
� Restoring non-native conifer plantations on ancient
woodland sites to native woodland.
� Converting other non-native conifer plantations to native
woodland.
� Planting/regenerating native woodland on arable land,
improved pasture and within urban areas, including
riparian*, wet and floodplain woodland.
� Restoring semi-natural open-ground habitats from
conifer plantations.
Relevant ongoing research is identified, as are
knowledge gaps and research priorities yet to be
addressed.
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LITERATURE REVIEW
Water quality
Forestry can benefit and threaten water quality 2,11,29,93,86,110.
Unmanaged or well-managed woodland is generally beneficial
for protecting water quality.Worldwide, there is growing
designation of native forests as ‘Protection Forests’, often
aimed at stabilisation of steep slopes and reducing avalanche
damage, as well as preserving the quality of drinking-water
supplies. Benefits of forests for water quality are at the
forefront of payment schemes being developed for
ecosystem services in Europe and elsewhere113. Most
instances of woodland causing problems for water quality
arise from poor management or inappropriate design of
non-native plantations and exploitation of timber.
Native woodland aids water quality, as it protects soils from
disturbance due to its management having a generally low
impact. Benefits are greatest where compared to more
intensive land uses, such as agriculture and urban
development. However, native woodland can pose
potential threats linked to interactions between the canopy
and the atmosphere.
Turbidity/siltation and riverbank stability
Woodland can reduce soil erosion and sediment entering
streams by: improving soil structure and stability; increasing
soil infiltration rates; reducing rapid surface run-off; and
providing shelter from wind53,83.The semi-permanent canopy
and benign management, typical of native woodland, minimise
soil exposure and disturbance. One study found less than
1 per cent of observed bare, eroding ground within the
erosion-prone catchment of Bassenthwaite Lake in north
England was associated with native woodland83.
Overseas studies demonstrate the effectiveness of woodland
in reducing soil erosion and maintaining high water
clarity29,22,110.The main risks of woodland increasing turbidity
and siltation follow large-scale windthrow or harvesting,
track construction and cultivation for new planting, which
are not generally associated with native woodland and can be
minimised by best practice.
Many overseas studies demonstrate sediment in water,
draining from adjacent land, is prevented from entering
streams by woodland riparian buffers. For example, one study
found a 30m-wide buffer effectively removed around 80 per
cent of solids suspended in run-off into a Pennsylvanian
stream70, while another reported a 20m-wide buffer
significantly reduced sedimentation following a major storm
event in Newfoundland24. An open woodland canopy,
providing sufficient light to maintain a vigorous understorey
and ground cover, is often regarded as the most effective
land use for retaining sediment7. Large woody debris-dams
within streams have a beneficial effect, trapping and delaying
downstream movement of sediment67.
Native riparian woodland is widely acknowledged as
beneficial for protecting riverbanks and improving
hydromorphological* conditions106.Tree roots bind and
stabilise stream banks, reducing erosion and siltation20. More
stable banks help to maintain deeper channels that favour
fish. Alder is particularly effective at bank protection, as it is
deep-rooting25,93. It is often planted for this purpose but its
dense shading, potential contribution to acidification and
susceptibility to Phytophthora disease118 limit large-scale
planting in many locations.Trampling of riverbanks by farm
animals accessing drinking water is a major cause of bank
erosion, turbidity and siltation. Exclusion of livestock from
native riparian woodland enhances its protective function.
Eutrophication
Water draining from native woodland has a lower nutrient
content than that draining from more intensive land uses29.
This reflects nutrient inputs from the atmosphere usually
matching demand by native woodland, lack of fertiliser
applications and low levels of soil disturbance. Few native
woodland streams are monitored in the UK but studies
elsewhere have found they contain low concentrations of
nitrogen (nitrate and ammonium), phosphate and potassium.
One study found streams draining broadleaved woodland in
the UK were characterised by low nitrate concentrations
with the exception of those draining large areas of alder
woodland (covering 50-80 per cent of the catchment),
although nitrate concentrations were still much lower than
in streams draining intensive farmland41.
Another study looked at the effect of oak woodland on the
quality of groundwater recharge at Clipstone Forest in the
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English Midlands and found concentrations of nitrate,
phosphate and potassium were 13 per cent, 5 per cent and
58 per cent respectively of those in water draining a grass
ley11. Different research found nitrate concentrations were
very variable in groundwater recharge beneath beech and
ash woodland in southern England but averaged less than
half those draining fertilised grassland49. Concentrations
were lowest under ash and highest within woodland
clearings where the beech had been felled or wind blown.
Surprisingly, concentrations were also low close to the
woodland edge, which was thought to be due to the strong
uptake by edge trees.
Tree canopies capture atmospheric pollutants, which can
promote high levels of nitrate in surface and groundwaters in
highly polluted areas.This can be a localised problem where
conifer forests downwind of intensive livestock units
‘scavenge’ ammonia, especially at the woodland edge109, but
this is less of an issue for native broadleaved woodland due
to its weaker scavenging ability49,11.
Broadleaved woodland can provide an effective nutrient
buffer for water draining adjacent land, especially in riparian
zones74,53,93,22. It is effective at removing nitrate in drainage
water, particularly when flow is through the upper soil. For
example, one study found a 30m riparian woodland buffer
removed nitrate to less than detection levels in shallow
groundwater by the River Garonne in France90. Another
study similarly demonstrated that 99 per cent of nitrate
draining from arable fields in southern England during winter
was retained within the first 5m of a buffer planted with
poplar51. Nutrient uptake is strongest during younger stages
of growth and declines rapidly with age. Riparian woodland
buffers are also effective at intercepting phosphate in
drainage waters, especially that carried by sediment.
Catastrophic windthrow or clearfelling of woodland over a
large proportion of a catchment could contribute to
eutrophication, as felling studies have shown sudden
cessation of nutrient uptake can lead to significant increases
in nitrate concentrations in water draining from woodland77.
Pesticides and other chemicals
Native woodland streams are generally free of pesticides and
other chemical pollutants due to absence of inputs.
Herbicides may be used to establish trees in new native
woodland or restocking.This usually entails a single annual
application for two or three years to reduce weed
competition, especially on former agricultural sites.
Herbicides are also used to control rhododendron, laurel,
Himalayan balsam and Japanese knotweed. Best-practice
guidelines place emphasis on selecting chemicals with least
risk of off-site impacts (often glyphosate) and, increasingly,
non-chemical methods of control38. Applications of herbicide
in forestry commonly involves spot treatments by hand and
Bassenthwaite Lake.
Frontal cloud
Wind direction
Emission of pollutantsfrom industry, carsand agriculture
Lowland
Pollutant depositionin rain and snow similarfor forests and other
types of vegetation
Dry depostion of somegaseous pollutants
(HC1, HNO3 and NH3)
N uptake in forest growth
Cap cloud
Rain passes through capcloud and carries S and Npollution to the ground
Pollutants in cloudand mist captured more
easily by forests (becomingimportant above c. 300m)
In many upland areas, base-poor(acidic) soils and impermeable bedrockresults in acidity being quickly passed
to streams in run-off water
Upland
S and N pollutants lifted in aerosol
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Literature review
wide buffer zones*, which also minimise risk of water
contamination. No monitoring assessments of herbicides
have been undertaken in the UK and the few overseas
studies have looked at different chemicals.
Riparian woodland provides an effective buffer for protecting
streams and groundwaters from pesticide applications on
adjacent land. It is particularly efficient at intercepting aerial
drift of pesticides and trapping pesticides bound to sediment
in run-off69. Pesticide residues can be removed from drainage
water by a number of natural processes within woodland
soils, as well as by tree uptake22.
Acidification
Forestry has been implicated in acidification of stream
waters in the UK since acid rain became an issue in the late
1970s85. The primary cause of acidification is the emission of
sulphur and nitrogen pollutants from the combustion of fossil
fuels. However, the quantity deposited, and impact on soil
and water, is strongly influenced by the nature of vegetation
cover, as well as climate and the underlying geology2,81.
Tree canopies ‘scavenge’ pollutants from the atmosphere,
which can contribute to increased acidification within areas
with base-poor soils and geology. Broadleaved woodland
captures less sulphur and nitrogen deposition than conifers
and the impact on surface water acidification is smaller75.
One study assessed the effect of broadleaved woodland on
surface water acidity at a catchment scale and found a link
between the proportion of broadleaved woodland and the
degree of stream acidification of ten acid-sensitive
catchments across the UK41. Broadleaved woodland at less
than 30 per cent cover had no significant effects on
streamwater chemistry.The risk of acidification due to
enhanced pollutant capture will be similar for native
pinewoods and conifer plantations.
Water colour/dissolved organic carbon
Forests can increase water colour in streams draining peaty
soils due to cultivation, drainage and tree growth enhancing
mineralisation of organic matter. Greater colouration can
affect drinking water treatment and represents a loss of soil
carbon38. UK studies have focused on assessing the impact of
upland conifer forests on water colour.These have shown
levels are only marginally higher than in nearby moorland
streams81,77. Broadleaved woodland is unlikely to have a
significant impact on water colour due to absence of drainage
treatments and lower water use compared to conifers.
Water quantity
Plants use water by two processes: transpiration*, whereby
water is taken up from the soil by roots and evaporated
through pores in the leaves; and interception*, involving
direct evaporation from the surfaces of leaves during
rainfall and, in the case of trees and shrubs, from branches
and trunks82.
Figure 1: Acid deposition and forest cover38.
© Crown copyright. Reproduced with permission of the Forestry Commission from Forests & Water Guidelines (2003).
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Trees and woodland can use more water than shorter
vegetation95,114,82,15.Trees have deeper roots and keep
transpiring longer during dry periods, while interception by
woodland can be more than 10 times greater, as trees are
tall, increasing aerodynamic turbulence and thereby
evaporation15.
In the UK, the principal reason for woodland using more
water than shorter vegetation is interception. Evergreen
species maintain high interception rates all year round,
particularly when conditions are wettest and windiest46,18,1,82.
Deciduous trees are typically only in full leaf between June
and September. For around six months of the year, water loss
by deciduous woodland is limited to: interception from
trunks and branches; transpiration from ground vegetation;
and evaporation from soil.
Broadleaved woodland in the UK typically intercepts
10-25 per cent of annual rainfall, compared to 25-45 per cent
for conifer stands46,12,82. Interception is greatest when trees
are in leaf, averaging 40 per cent or more101, in contrast with
3-12 per cent when trees are leafless49,101.Trees with lighter
canopies, such as ash, only intercept 10-15 per cent of annual
rainfall compared to 15-25 per cent by oak and beech49,101.
The deeper rooting of trees can sustain potentially higher
transpiration rates than shorter vegetation in drier parts of
the UK.This effect can be partly limited by the pores on the
needles and leaves of some tree species being very
responsive to dry atmospheric conditions and able to
control water losses99,19. Annual losses from transpiration
occur primarily during the growing season and are generally
in the region of 300-350mm irrespective of woodland type
or species98. Higher values of 390-410mm have been derived
for native broadleaved woodland in southern England, slightly
greater than those from more recent studies (360-390mm)101.
Other factors influencing use of water are woodland
structure and age. Structural diversity in broadleaved
woodland increases aerodynamic roughness and thus
evaporation. However, the impact of this effect is
generally limited to within 20m of the woodland perimeter76.
Variation in tree height, tree density and canopy gaps may
exert an influence but studies of partial conifer thinning
suggest small openings may be unimportant. In Wales, even
the removal of one in three rows of trees only led to a
minor reduction in interception loss from 38 per cent to
36 per cent, perhaps due to the increased canopy ventilation
being compensated by the reduction in leaf area13.
Species diversity affects woodland leaf area, although the
impact on interception is likely to be limited.The presence of
an understorey can make a marked contribution to the
amount of water intercepted but the potential increase in
leaf area tends to be offset by a more open tree canopy
necessary for its development31.
Tree age influences leaf area and efficiency of water use,
which decline with old age51,115,100,104. In theory, this could be a
significant factor for native woodland, especially ancient
woodland, which can have more, older trees. In south-east
Australia, transpiration rates in eucalyptus forest are clearly
related to forest age61, with 230-year-old trees transpiring
less than half that of 50-year-old ones, mainly due to less
conducting sapwood.This greatly outweighs increases in
interception (19 per cent versus 23 per cent) associated with
greater height and structural diversity30. In the USA,
increased water use by young stands regenerated after
timber harvesting has also been recorded5.
Arable crops tend to have the lowest water use due to the
relatively short crop cycle, with significant periods of low
evaporation associated with young growth, ripening and
fallow phases. Annual water use is 370-430mm48, overlapping
with the low end of the range for native broadleaved
woodland of 400mm, assuming 1,000mm annual rainfall81.
However, the difference between woodland and arable crops
Older trees use less water.
Phot
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is even smaller and possibly reversed where arable crops
receive irrigation, which increases water use by as much as
100mm per year.
Annual evaporation from short grass crops well supplied
with water is the basis of the Penman potential evaporation
rate, widely recognised as a hydrological benchmark88.Water
use by grass predominantly results from transpiration. Annual
values (400-600mm) overlap with those of native woodland.
Factors limiting water use by grass include a colder climate,
drought and heavy grazing.
Other shorter vegetation with distinctive water use includes
bracken and heather. Bracken has a high annual interception
loss (20 per cent), exceeding that of some native broadleaved
trees119, and its water use (600-800mm for 1000mm annual
rainfall) tends to exceed that of broadleaved woodland.
Heather also has a significant interception loss (16-19 per
cent) due to its fine branch structure, although this is offset
by regulation of its transpiration rate leading to overall water
use of 360-610mm.
Streamflow
Some authors claim native woodland is able to maintain and
possibly increase water yield* compared to short
vegetation22,86. However, evidence indicates the opposite in
most cases54,32,29,33,43,108. A review of historic catchment studies
on forest hydrology, which have been conducted
predominantly in the USA, found that woodland removal was
almost always associated with increased water yield5.
A 10 per cent reduction in woodland cover increased
average annual water yield by 25mm for broadleaved trees
compared to 40mm for conifers but water yield usually
reduced rapidly as the woodland regenerated.These findings
have been confirmed by more recent catchment studies in
Europe but changes are sometimes small and difficult to
separate from climatic variations44,40. Other studies have
found replacement of broadleaved woodland with conifers
produces a highly variable and unpredictable response at a
catchment level. For example, one study concluded that it
was impossible to detect a significant difference in water
yield between maule native forest and plantations of
Monterey pine in central Chile89.
Although no relevant catchment studies have been
undertaken in the UK, modelling of Loch Katrine, a major
water-supply catchment in mid Scotland, predicted
regeneration of native birch-oak woodland over 40 per cent
of the area would only reduce average annual water yield by
2 per cent (and over 70 per cent of the area by 3 per cent)92.
When considering the impact of native woodland on water
yield, absolute changes decline in line with rainfall and run-off
but percentage changes increase. Consequently, absolute
differences are more relevant to wetter climates when
dealing with water supply or hydroelectric requirements,
while percentage reduction is more important in dry regions
with regard to minimum environmental flows.
Groundwater recharge
There have been several major studies in the UK of the
impact of native woodland on groundwater. Assessments of
beech-ash woodland on chalk and ash woodland on clay in
southern England indicated annual recharge beneath
70-year-old beech and 50-year-old ash on chalk, England’s
primary aquifer, exceeded nearby grassland by 17 per cent
and 25 per cent respectively (although not using
contemporary data), while recharge below 70-year-old ash
on clay exceeded grassland by 14 per cent49. A repeat study
of beech on chalk compared with grass (using concurrent
data) found only 13 per cent difference in recharge over an
18-month period101. However, the researchers recently
concluded there may be ‘little overall difference between
broadleaved woodland and grass, either in soil-water
abstraction or in evaporation’ where they overlie chalk, as
it maintains sufficient upward water movement to sustain
high transpiration rates by grass during summer months100.
Other factors involved are the longer growing season of
grass, especially in early spring, and the low interception
loss of ash woodland. Modelling work, however, indicates
that recharge from beneath native woodland could be lower
than grassland in wetter climates because of higher
interception losses101.
Another study compared groundwater recharge beneath
grass, heather, pedunculate oak and Corsican pine overlying
Triassic sandstone in the English Midlands, the UK’s second
most important aquifer11. Estimates based on field and
modelling assessments indicated that, unlike on chalk and
clay, recharge on sand was 16-48 per cent greater under
grass than 60-year-old oak woodland.This may be due mainly
to oak sustaining higher transpiration rates on the drought-
prone sandy soils12,45, as it roots more deeply than grass.
Applied to the likely impact on groundwater resources of
creating a new Community Forest in Nottinghamshire,
increasing woodland cover from 9-27 per cent, recharge was
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predicted to reduce by 3-6 per cent for oak, compared to
10-14 per cent for Corsican pine.
In the wetter parts of the UK, high interception losses
predominate over possible differences in transpiration,
leading to more water use and less groundwater recharge
under native woodland than grassland.
Soil infiltration and run-off pathways
It has been noted that soils can affect water use and yield by
influencing transpiration rates. Soil type and condition are
also important for determining pathways and consequent
timing of water draining from soils into streams and rivers.
Shallow, poorly draining soils have superficial pathways and a
fast catchment response to rainfall, resulting in high peak
flows. Deep, freely draining soils promote deeper water
pathways and a delayed, attenuated response.
Woodland protects soils and often improves their condition,
as compared to more intensive land uses35. Native woodland
rarely involves the formation of continuous cultivation
channels or drainage treatments to aid establishment. Lack of
disturbance helps to increase soil organic matter and
improves soil structure, resulting in an increased ability to
receive and store water, commonly referred to as a ‘sponge
effect’*84. Recent studies at Pont Bren in Wales found
soil-infiltration rates were up to 60 times higher under
young native woodland than heavily grazed pasture4.
Soil-infiltration rates appeared to improve by 90 per cent
within two years of stock removal and woodland planting.
Base or low flows
Woodland has long been associated with augmenting base
flows in rivers due to the ‘sponge effect’122,64,94. However,
others believe greater water use by woodland is likely to
outweigh higher infiltration rates and delayed release of
water from woodland soils97. Many studies have found felling
of native woodland increases catchment water yield and base
flows. For example, one study found removal of natural
woodland in Taiwan increased low flows by 91 per cent57,
while another demonstrated natural regeneration of
broadleaved woodland over the last 50 years has led to
long-term reduction in dry-season flows in the Dragonja
catchment, Slovenia44.
The most detailed study of the impact of woodland on flows
analysed 28 catchments across Europe102. The authors
concluded that while there are specific local situations where
woodland management (drainage and clearfelling) can
increase, or growth of conifers can decrease, low flows by
10-20 per cent, the effect is relatively small at the European
or regional scale.The partial felling of central European
mixed native broadleaved and Mediterranean native open
forests had no detectable effect on base flows.This is
supported by another study that found low flows in upland
catchments were largely determined by local geology and
soils, not land use85.
Peak flows
The woodland ‘sponge effect’ is commonly associated with a
reduction in peak flows73,53. Many felling experiments have
demonstrated an increase in peak flows for several years
after woodland clearance, until new trees become
established101,105. Some have argued this is due to soil
compaction and ground damage from timber extraction but
the effect has been replicated where best practice has been
Phot
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Intensively managed soils can have lower infiltration rates.
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employed.These short-term studies limit assessment to
annual or lesser peak flows.The impact of woodland on
more extreme events has not been evaluated.
Research has only considered upland conifer afforestation
and clearfelling, which was reviewed in 2003 as part of a
wider EU study102.This found that growth of conifer forests
could reduce annual peak flows by 10-20 per cent in
completely afforested headwater catchments*, while forest
drainage and felling had the opposite effect. As with base
flows, partial clearance of native mixed broadleaved or
Mediterranean open forests had no detectable effect on
peak flows.
Flood frequency, intensity and risk
Most UK studies have focused on upland conifer forests but
found no significant effect at the headwater or large
catchment scale103. A review of regional flood studies in
Britain determined woodland area was not significant in
flood prediction, although percentage woodland cover was
generally small79. Other reviews found woodland reduces
small ‘muddy’ floods at hillslope or headwater catchment
scales but little evidence of significant impact on extreme
events or flood risk at regional or large catchment
scales71,102,87. A number of worldwide assessments have also
failed to discover evidence of any forest type preventing
large-scale major floods, even in predominantly forested
catchments, and considered the impact of forests on
floods to be limited to catchments less than 100 km2 in
area33,15. A recent global study6 , analysing data from satellite
observations at the country scale, has claimed flood
frequency and duration decline with increasing native
woodland cover but the opposite for non-native plantation
forest. However, this study excluded extreme flood events
and the robustness of the approach has been questioned by
other workers.
Floodplain woodland can have a mitigating effect on large
flood events, absorbing and delaying release of flood
flows53,92,22,97,55. Mathematical modelling found greater
hydraulic roughness created by a 2.2km reach of floodplain
woodland on the River Cary in south-west England increased
flood storage by 71 per cent and delayed the flood peak
progressing downstream by 140 minutes for a one-in-a-
hundred-year flood event111.This was considered significant in
potentially protecting downstream sites from inundation.
Field testing is required. Studies of riparian woodland have
shown large woody debris-dams in headwater streams can
retard generation of flood flows67. However, there are
concerns that wash-out of woody debris could block bridges
and other structures, increasing risk of downstream flooding.
Floodplain woodland can reduce and delay flood flows.
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KEY CAVEATS TO THE LITERATURE REVIEW AND ITS APPLICATION TO THE UK
Native woodland and forestry in the UK
A distinction is made between conifer forest and broadleaved
woodland in the UK, and between UK forestry and forest
management elsewhere in the temperate zone. Most conifer
forests in the UK have been planted in the last 90 years as
monocultures for commercial timber production.
Concentrated in the uplands, the wettest parts of the UK,
sites for conifer planting have been cultivated and, crucially,
drained to ensure establishment. Silviculture can involve
frequent thinning on more stable sites, and often clearfelling.
These intensive forestry operations have the potential to
have a high impact, although this is increasingly controlled by
improved forest design and best management practices. By
contrast broadleaved woodland in the UK has generally been
around longer and is predominantly lowland. Broadleaved
woodland is rarely drained and silvicultural systems are
generally low impact. Elsewhere in the world, temperate
forestry is focused on a long-established forest resource and,
unlike in the UK, is more often based on selective harvesting
and regeneration rather than afforestation and clearfell. It is
important to understand this distinction in drawing
conclusions from studies across temperate zones and
applying their findings to the UK.
Quantification
The impact of native woodland on water resources is
affected by many factors and there has been limited
quantification in the UK. Overseas research needs interpreting
with caution due to differences in climatic conditions, soil
types and woodland species (e.g. many studies consider
mixed conifer and broadleaved stands).The majority of
historic catchment studies on forest hydrology have been
conducted in the USA. No studies have measured the effect
on streamflow and water yield of changing land use from
short vegetation to mature broadleaf woodland due to the
timescale involved. Research has focused on the impact of
woodland clearance on catchment water yield. Impacts of
woodland clearance cannot simply be used to infer changing
water yield for new woodland as it matures.This is because
of: the lack of a suitable control (comparison only possible
with clearfelled site); the partial nature of felling treatments,
in many instances; and the potential for harvesting to
complicate results, due to levels of disturbance.
Scale
Empirical evidence of the impacts on water resources of
trees and woodland comes from studies of individual trees,
stands and localised research. Extrapolating findings is
complicated by a range of variables as scale increases.
Within individual sites, diversity of woodland structure and
tree species tend to increase with area, as does the
proportion of open habitats, roads and tracks. Moving to a
larger catchment scale or beyond, land use patterns and
management practices become ever more complex, as do
variations in topography, geology, soils, rainfall, snowmelt
and run-off pathways80,84, woodland type, age and growth
rates116,59.This limits understanding of interactions between
land use and land-management practices on water at the
river-basin scale.
As the percentage of woodland cover declines, its signature
is diluted by other land uses. Similarly, it may be difficult to
identify impacts on water against natural background
variation when less than 20 per cent of a catchment is
subject to woodland creation or removal24. Smaller-scale
woodland creation may nevertheless have a discernible
impact on both flood flows and water quality at a local level,
if it is appropriately targeted (e.g. within riparian zones)83.
This is particularly relevant, as the UK has less than 12 per
cent woodland cover.
Seasonality
Seasonality is a significant factor affecting water use and its
impact on water resources.Transpiration loss from grass
starts earlier in the year than broadleaved woodland;
however, drought can lead to early ageing of grass and
shorten the growing season12.This can influence seasonal
flows, although timing depends on the amount of water
stored in underlying soils and rock and the lag in drainage
waters reaching rivers.Phot
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12
POLICY CONTEXT
Impacts of climate change on woodland and water
Climate change projections for the UK are generally for
warmer, wetter winters and hotter drier summers with
more prolonged droughts, particularly in the south, and
evaporation totals increasing in all regions82,17,61.The potential
impacts on water resources and flood control are significant
and are expected to become more challenging in the future91.
Water supplies are already severely stretched in some parts
of the UK, particularly the south, while at the same time
growing pressure for housing development on floodplains
and rising property values are increasing the flood hazard27.
Impacts on water quality could also be significant with less
water to dilute pollutants. Rising temperatures will increase
thermal stress for freshwater life.
Implications for woodland and water in the UK
Projected changes in climate are expected generally to lead
to increased evaporation and reduced water yields from both
woodland and non-woodland areas.Warmer temperatures
increase potential evaporation rates and lengthen the
growing season and higher atmospheric carbon-dioxide
levels could enlarge total leaf area82. Ultimately, if potential
evaporation rates far exceed rainfall inputs, water use by
all vegetation-types will converge16. Nevertheless in
water-scarce areas, for the foreseeable future, large-scale
woodland creation may need to be restricted50 and species
with a high water demand possibly avoided.
Most climate change scenarios lead to projected increases in
flood magnitude, although its significance and timing is less
certain17.The presence of woodland could serve as a buffer
against excessive run-off and reduce annual peak flows
(e.g. in the West Weald,West Sussex53) but this may only be
significant in relatively small catchments where woodland is
predominant29.Targeted floodplain woodland creation could
alleviate flood risk55,84.
The main evidence of an effect of climate change on water
quality is the widespread rising trend in dissolved organic
carbon concentrations in many upland streams across the
UK and Europe.Thought to result from increased
mineralisation of soil organic matter due to a warming
climate38, dissolved organic carbon concentrations, and
associated water colour, tend to be marginally greater in
conifer forest streams but are unlikely to be affected by
broadleaved woodland. Climate change may also affect water
quality indirectly by stimulating land use changes that bring
diffuse pollution to new areas56.This might be reduced by
targeting woodland creation to source areas, pollutant
pathways* and as a buffer to riparian zones. Planting
riparian woodland would have the additional advantage of
providing shade to help moderate the impact of rising
water temperatures on sensitive freshwater life, especially
salmonid fish.
Climate change is likely to have a significant bearing on future
planting and management of native woodland.The Climate
Change Programme and Energy Reviews that are underway
may help to clarify woodland’s role in mitigation and
adaptation. There is significant scope for harvesting of more
biomass from native broadleaved woodland.
EU Water Framework Directive
Woodland could play an important role in helping meet
objectives of the EU Water Framework Directive, including:
to protect and improve the status of Europe’s rivers, lakes,
groundwaters, estuaries and coastal waters; to promote the
sustainable use of water resources; and to help reduce effects
of flooding and drought.
Tewkesbury 2007: major flooding put the spotlight on water policy.
13
A key target of the EU Water Framework Directive is for
rivers to achieve good ecological and chemical status by 2015.
This is a major challenge, with 93 per cent of rivers in
England and Wales at risk of failing to achieve it (Figure 2).
The two most important threats are diffuse pollution and
physical changes, often associated with agriculture and urban
development.The main sources of diffuse pollution responsible
for at-risk water bodies are: sediment delivery (21 per cent),
pesticides (21 per cent), phosphorus (47 per cent) and
nitrate (38 per cent). Some 48 per cent of rivers are also at
risk from physical degradation of river channels and banks.
New native woodland creation has potential to reduce
pressures by protecting soils and riverbanks, reducing rapid
surface run-off and intercepting pollutants before they reach
watercourses. Although this is scale-dependent, benefits
could be maximised by targeting high risk soils and careful
placement of woodland to intercept pollutant pathways.This
Figure 2: Percentage of water bodies in England and Wales at risk of notachieving EU Water Framework Directive objectives31a.
would require better integration of farming and woodland
within catchments. River-basin management planning is a key
mechanism within the EU Water Framework Directive for
delivering improvements to the water environment.
If due care is taken, native woodland has significant potential
to sustain water quality and alleviate flooding through its
effects on run-off pathways and flood flows, without posing
additional problems for water resources and droughts.The
Environment Agency in England and Wales has developed
Catchment Abstraction Management Strategies and Catchment
Flood Management Plans for catchments identified at risk
from low flows and flooding, and for groundwater bodies at
risk of not meeting good quantitative status.
UK Biodiversity Action Plan
The UK Biodiversity Action Plan sets targets for priority
species and habitats to guide conservation action. Climate
change is now recognised as a significant factor that was not
taken into account when the original UK targets were set.
The targets were reviewed in 2005-6 and are designed to
improve the long-term viability of habitats and species
populations.
The following targets have been set that relate to woodland:
� Maintain the extent of native woodland in the UK (no net
loss of one million hectares).
� Maintain the current extent and distribution of ancient
semi-natural woodland, which qualifies as native woodland
in the UK (no change in the existing area of 403,000ha).
� Restore 50,300ha of non-native plantations on ancient
woodland sites to native woodland in the UK by 2015.
� Expand the current native woodland resource in the UK
by 134,500ha by 2015 through a combination of
converting (restocking) existing plantations not on ancient
woodland sites and creating native woodland on former
agricultural land.
� Expand semi-natural open-ground habitats (which will
include restoration where planted with non-native
conifers), e.g. lowland heathland by 7,600ha by 2015.
Pressures Rivers Lakes Estuaries Coastal Waters Groundwater
Point discharges 23.1 20.1 48.5 18.2 3.9
Diffuse pollution 82.4 53 25 24.2 75.3
Abstraction 10.7 2.1 14 Not applicable 26.1
Physical changes 48.2 59.3 89.7 77.8 Not applicable
Alien species 21.1 9.3 98.5 45.5 Not applicable
Overall % ofwater bodies at risk 92.7 84 98.5 84.5 75.3
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LIKELY IMPLICATIONS FOR WATER RESOURCES OF WOODLAND ACTIONS FOR BIODIVERSITY
Maintaining the existing area of native and ancient woodland
The main impacts on water resources can be
summarised as:
� Preservation of high quality drainage waters with low
nutrient, pesticide and sediment concentrations due to
lack of soil disturbance.
� Maintenance of good or high ecological status of water
bodies draining catchments dominated by native
woodland, except possibly for Scots pine within acid
sensitive areas.
� Reduction in water use and increased water yield as
younger woodland matures.
� Maintenance of water yield, and probably base flows,
across large parts of central and southern England
overlying chalk or clay soils (likely to be within plus or
minus 10 per cent of that from grassland).
� Reduction in water yield, and probably base flows, in dry
parts of England (less than 750mm annual rainfall)
overlying sandy soils (likely to be 20-50 per cent less than
from grassland).
� Reduction in water yield in wet parts of the UK (greater
than 1,500mm annual rainfall) (up to 10 per cent less than
grassland).
� Reduction in small (less than one in every five years)
floods (10-20 per cent less than grassland).
Effects on water yield may be reduced with increasing size
of woodland, as structural and species diversity grows and
edge effects have proportionally less significance. It is
unlikely that small differences in water use expected
between different native broadleaved tree species could
ever justify species management to limit impacts on water
quantity. Conversion of native pinewoods to birch could
yield significantly more water but within its native range
there is no demand.
Based on the review of impacts of trees
and forests on water resources, likely
implications for water resources of
woodland actions for biodiversity in the
UK are outlined below.
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Restoring non-native conifer plantations on ancient woodland sites to native woodland
This would benefit both water quantity and quality, except
where restoration is to native conifer woodland. Main
impacts might be:
� Increase in water yield, and probably base flows (by
20–50 per cent in dry regions and up to 10 per cent in
wet parts of UK), unlikely to be noticed at the level of a
large surface or groundwater body but could be locally
significant, although slow to develop if restoration is
gradual.
� Increase in small (less than one in every 5 years) floods
(less than 10 per cent) due to lower water use and greater
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run-off from broadleaved woodland, especially during
winter and early spring when floods are most frequent.
� No effect on extreme floods.
� Reduction in threats to water quality following
restoration, although these would have been greatly
constrained by the Forests & Water Guidelines38.
� Local improvements in water quality, greatest in areas
subject to continued acidification, where the weaker
scavenging effect of native broadleaved woodland will
reduce the impact of acid deposition.
� Sizeable reduction (up to 90 per cent) in nitrate
concentrations in very dry regions (less than 600mm
annual rainfall); elsewhere, little change.
16
Likely implications for water resources of woodland actions for biodiversity
Converting other non-native coniferplantations to native woodland
The effects of converting plantations more generally are
expected to be similar to those outlined immediately above.
Once again, changes to both water quantity and quality are
unlikely to be significant where conifer plantations are
converted to native pinewoods. An anticipated additional
impact is an increase in flood retention where conifer
plantations on floodplains are converted to native woodland,
as hydraulic roughness would be increased by development
of a shrub layer, ground cover and increasing levels of
deadwood. Ancient woodland on ground liable to flooding is
rare, as it was historically the most valuable land as
meadow95, but conifer plantations on sites without a long
history of woodland cover occur on floodplains
Throughout the UK, consideration is being given to continuous
cover forestry as a means of managing large areas of conifer
plantations.This silviculture creates a certain degree of shrub
cover and age diversity in the crop but evidence would suggest
limited benefits for water yield.
Restoring semi-natural open-ground habitats from conifer plantations
Restoration can be expected to benefit water quantity, and
to a lesser extent water quality, depending on how these
areas are subsequently managed. Adverse effects could result
from large-scale felling to waste, chemical and cultivation
treatments to control conifer regeneration, burning or use
of livestock for vegetation management, although good
practice will minimise these risks. In general, impacts are
expected to be:
� Local improvements in water quality greatest in areas
subject to continued acidification, where removal of the
scavenging effect will reduce the impact of acid
deposition.
� Substantial reduction (up to 90 per cent) in nitrate
concentrations in dry regions (less than 600-650mm
annual rainfall), as conifers have a marked evaporation-
concentration effect.
� Reduction in speed of surface run-off from blocking forest
drains and cultivation channels on peatland but countered
by increased volume of run-off and possible reduction in
available soil water storage.
Heathland restoration from conifer plantation. Phot
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Planting/regenerating native woodland on arable land, improvedpasture and urban areas
Available evidence suggests native woodland creation on
more intensively managed land will benefit water quality
status but may pose issues for water resources. Impacts are
expected to be:
� Improvement in water quality, as it removes the need for
regular soil disturbance/cultivation and fertiliser and
pesticide treatments, especially where it replaces
potentially damaging land uses on sensitive soils (e.g.
improved grassland and arable overlying soils prone to
erosion and nutrient loss).
� Reduction in sediment, nitrate, phosphate and pesticide
concentrations (by as much as 90 per cent possible) by
aiding soil infiltration, retaining suspended particles,
binding nutrients and pesticides and promoting nutrient
removal via tree uptake.
� Reduction in nitrate concentration linked to scale of
planting and significant decrease in sediment, phosphate
and pesticide levels achieved by small-scale targeted
planting of source areas and pollutant pathways, where
surface run-off emerges or seepage occurs close to the
surface (e.g. downslope boundaries of steep fields, springs,
purpose-built infiltration basins/swales, riparian zones and
associated wetlands).
� Retention of chemical pollutants on brownfield sites,
through use of trees in sustainable urban drainage
systems* to aid soil infiltration, increase soil organic
matter levels, reduce drainage volumes and protect soil
from disturbance (taking care to avoid mobilising some
pollutants through soil acidification).
� Increase in water yield and probably base flows
(by 50-100 per cent in dry regions and up to 20 per cent
in wet areas).
� Increase in small (less than one in every five years) flood
events (less than 20 per cent).
� Effects of small-scale restoration (less than 20 per cent of
the area of the surface/groundwater body or catchment)
unlikely to be detected at the level of a main surface or
groundwater body.
� No effect on extreme flood flows.
WT
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Likely implications for water resources of woodland actions for biodiversity
� Shade provided by new riparian woodland moderating the
impact of rising water temperatures on sensitive
freshwater life, especially salmonid fish.
� Reduction in surface run-off and groundwater recharge
where woodland created on arable and brownfield land, as
woodland generally uses more water, unless the arable
relies on irrigation. Lighter-foliaged trees, such as ash,
would limit the difference, although reduction would still
be expected on drought-prone soils and in wetter areas.
Planting on grassland would also reduce surface run-off
and groundwater recharge, except on chalk or clay soils in
areas of intermediate rainfall where they could be
marginally increased.
� Little change to water yield, and probably base flows,
where woodland created on grassland across large parts
of central and southern England overlying chalk or clay
soils (likely to be within plus or minus 10 per cent).
� Reduction in water yield, and probably base flows, in dry
parts of England (less than 750mm annual rainfall).
overlying sandy soils (likely to be 20-50 per cent less than
grassland).
� Reduction in water yield where native pinewoods or yew
woodland created, regardless of existing land cover.
� No significant effect on water yield, base or peak flows
where scale of planting is less than 20 per cent of surface
or groundwater body.
� Reduction in small (less than one in every five years)
floods (10-20 per cent), as a result of improved soil
infiltration reducing local peak flows (where water
resources are not in short supply, selection of species
with higher water use, such as poplar and willow, could
further aid attenuation of summer floods)
� Possible reduction in extreme floods downstream by new
riparian or floodplain woodland, although local upstream
properties would be at risk from the backwater effect and
potential for large woody debris release to block critical
structures downstream (e.g. bridges and culverts).
� Improvements to riverbank and in-stream physical habitat
through bank protection, woody debris-dam formation,
leaf fall and shading.
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KNOWLEDGE GAPS AND RESEARCH NEEDS
A combination of targeted field and modelling studies are
required to:
� Quantify the impact of upland native woodland on water
quantity and water quality at the catchment scale, as most
hydrological studies have focused on conifer plantations.
� Field test models and further quantify the impact that
new native floodplain woodland can have on mitigating
large flood events.
� Further quantify effects of targeted planting of native
woodland on diffuse pollution within agricultural
catchments, specifically in relation to infiltration
basins/swales, riparian buffers, source areas and pollutant
pathways.
� Develop best practice on managing floodplain woodland
in terms of benefits and potential threats (e.g. from the
release of large woody debris) to flood defence.
� Quantify the water use of a wider range of native
woodland species and the effect of woodland design and
structure on woodland evaporation120.
� Quantify the effects on flood flows and diffuse pollution
control of using woodland within sustainable urban
drainage systems.
� Quantify the economic costs and benefits of native
woodland impacts on water and evaluate the case for
payments for water services in the UK.
� Develop an improved climate change water use impacts
model that can take account of species differences under
present and projected climate scenarios to support
operational decision-making.
� Monitor the long-term effects of native woodland on the
freshwater environment.
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CONCLUSION
The key interactions between woodland and water
management are summarised below:
� Broadleaved woodland can substantially improve
water quality, as it removes the need for regular soil
disturbance/cultivation and fertiliser and pesticide
treatments, especially where it replaces potentially
damaging land uses on sensitive soils (e.g. improved
grassland and arable overlying soils prone to erosion
and nutrient loss)
Delivery of targets in the UK Biodiversity
Action Plan for maintenance, restoration
and expansion of native woodland, as well
as restoration of semi-natural
open-ground habitats from conifer
plantations, could make important
contributions to meeting some of the
objectives of the EU Water Framework
Directive and provide some opportunities
to help meet the EU Floods Directive.
21
ACKNOWLEDGEMENTS
This work was funded by the Woodland Trust and the
Forestry Commission.We are grateful for comments
received on drafts of the report from: Steve Gregory, Helen
McKay, Derek Nelson and Sallie Bailey (Forestry
Commission); Mark Diamond (Environment Agency); Nick
Collinson, Fran Hitchinson and Mike Townsend (Woodland
Trust). The document is a review and is not intended to
reflect the policy positions of any of the organisations
involved.
� Annual water yield from broadleaved woodland is
expected to be greater than from conifer plantations
but potentially less than from grassland or arable. Risk
of woodland creation reducing water yield depends
on climate, geology and woodland design and can be
managed by selecting species that use less water
� Woodland has the potential to reduce low flows. Risk
is greatest for conifers on deeper lowland aquifers
and lowest for broadleaved woodland on shallower
upland aquifers
� Broadleaved woodland can reduce small ‘muddy’
floods at a local scale and on floodplains can mitigate
large flood events, absorbing and delaying release of
flood flows. Models suggest the impact of floodplain
woodland could be significant but field testing is
required.
The impacts of woodland on water quantity tend to be
related to the extent of woodland cover within a
catchment. Effects are very difficult to detect when
woodland creation or removal involves less than 20 per
cent of the area. Scale tends to be less important for
water quality due to the localised nature of many
pollution sources and the success of targeted measures.
There is an urgent need for further research to quantify
the relative impact of native woodland on water
quantity and quality, as compared to other land cover. It
is vital that the effect of continuing native woodland
expansion on the status of water bodies and
groundwater supplies is assessed to build on current
limited evidence.
An opportunity is presented at Loch Katrine, one of the
most important public-water-supply reservoirs in
Scotland, where Forestry Commission Scotland is
initially planning to increase native woodland cover to
2,000ha (18 per cent cover) with significant scope for
more. Planting of demonstration floodplain woodland
across the UK is also required to facilitate research and
aid communication.
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22
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BIBLIOGRAPHYGLOSSARY OF KEYTECHNICAL TERMS
Ancient woodland Believed to have been continuously wooded since atleast 1600 AD. Before this planting was uncommon, soit was likely to have developed naturally.
Base or low flows Extreme minimal flow of water in a water body.
Buffer zone Area of natural or specifically selected vegetationbetween a water body and adjacent land use whosepurpose is to protect water quality from potentialthreats.
Dissolved organic Compounds found in water derived from organiccarbon materials (e.g. decomposed plant matter)
Eutrophication Pollution caused by excessive plant nutrients (primarilyphosphorus, nitrogen and carbon). Growth of algaepromoted by these nutrients potentially changes waterquality leading to oxygen depletion and fish deaths.
Groundwater recharge Process by which groundwater is replenished by watersoaking into the ground
Headwater catchment The area drained by small streams in the upperreaches of land that feeds into a water body such as astream, river or lake.
Hydromorphology Physical characteristics of a water body, includingbanks and bed.
Interception Direct evaporation of rainfall from surfaces of leaves,branches and trunks.
Native woodland Predominantly composed of native species; covers aspectrum from pure broadleaved through to nativepinewoods, which include an element of broadleaves.
Peak flows Extreme maximum flow of water in a water body.Magnitude of peak flow events has traditionally beencharacterised in terms of the ‘return period’: thelonger the return period, the larger the peak event.
Pollutant pathways Known or identified avenues for the transport ofpollutants.
Riparian zone Land adjacent to a water body.
Siltation Sediment input to a water body.
Soil infiltration Process by which water on the surface enters the soil,governed by two forces: gravity and capillary action.Smaller pores offer greater resistance to gravity butvery small pores pull water through capillary action.
Sponge effect Lack of disturbance helps to increase soil organicmatter and improves soil structure in woodland,resulting in an increased ability to receive and storewater.
Streamflow Total discharge of water from a catchment in streamsand rivers.
Sustainable urban Aim to mimic as closely as possible the natural drainage systems drainage of a site to minimise the impact of urban
development on the flooding and pollution ofwaterways.
Transpiration Process whereby water is taken up from the soil bytree roots and evaporated through pores in the leaves.
Turbidity A measure of light intercepted by water due topresence of suspended and dissolved matter andmicro-organisms. Increasing turbidity decreases lightpenetrating the water column. High levels of turbidityharm aquatic life.
Water yield Quantity of water as an output from a catchment,including both groundwater recharge and surfacewater components.
23
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