Studies of the impact of forests on peak flows andbaseflows: a European perspective
M. Robinsona,*, A.-L. Cognard-Plancqb, C. Cosandeyc, J. Davidd, P. Durande,H.-W. Fuhrerf, R. Halla, M.O. Hendriquesd, V. Marcb, R. McCarthyg, M. McDonnellh,
C. Martini, T. Nisbetj, P. O’Deag, M. Rodgersh, A. Zollnerk
aCentre for Ecology and Hydrology, Wallingford, Oxon OX10 8BB, UKbLaboratoire d’Hydrogeologie, Universite d’Avignon et des Pays de Vaucluse, U.F.R. des Sciences Exactes et Naturelles,
33, rue Louis Pasteur, 84000 Avignon, FrancecLaboratoire de Geographie Physique, URA 141 du CNRS, 1, Place A. Briand, 92190 Meudon Cedex, France
dDepartmento de Engenharia Florestal, Universidade Tecnica de Lisboa, Tapada da Ajuda, P-1349 Lisbon Codex, PortugaleINRA, Unite Sol et Agronomie de Rennes-Quimper, 65 route de St Brieuc, F-35042 Rennes Cedex, France
fHessen-Forst, Prof. Olkers Str. 6, 34346 Hann. Munden, GermanygCoillte, Dublin Road, Newtownmountkennedy, Co Wicklow, Ireland
hSchool of Engineering, National University of Ireland, Galway, IrelandiUMR 6012 ‘‘ESPACE’’ (CNRS et Universite de Nice), Departement de Geographie, 98 Boulevard Edouard Herriot,
BP 3209, F-06204 Nice Cedex 03, FrancejForest Research, Alice Holt Lodge, Wrecclesham, Farnham, Surrey GU10 4LH, UK
kBayer. Landesanstalt WF, Am Hochanger 11, 85354 Freising, Germany
Received 9 August 2002; received in revised form 12 March 2003; accepted 22 April 2003
Abstract
Most of the scientific studies of forest impacts on stream flows have been conducted in North America. Many were primarily
concerned with felling effects. These have generally found forests to be associated with reducing both peak and low flows. Their
results, however, may not necessarily be directly applicable to European forests due to differences in tree species, forest
management, catchment physiography and climate. Forests are a major land cover in Europe, and there are plans to promote and
further expand the area of European forests. The recent droughts and floods in different parts of Europe have heightened interest
in the role of forests on river flow regimes, particularly flood peak and dry weather baseflows. This paper presents the
hydrological results from 28 basins across Europe sampling a wide range of forest types, climate conditions and ground
conditions. The aim was to determine if forestry can have significant impacts on stream flows and to identify particularly critical
situations. The findings highlighted coniferous plantations on poorly drained soils in NW Europe and eucalyptus in Southern
Europe as the situations where the most marked changes to flows are likely to occur. In contrast, other forest types, and changes
in forest cover at a regional scale will be likely to have a relatively small effect on peak and low flows.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Forests; Land use; Flooding; Low flows; Peak flows
Forest Ecology and Management 186 (2003) 85–97
* Corresponding author. Tel.: þ44-1491-38800; fax: þ44-1491-692424.
E-mail address: [email protected] (M. Robinson).
0378-1127/$ – see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0378-1127(03)00238-X
1. Introduction
Forests are a major land use in the European Union,
accounting for approximately 37% of the total land
area (FAO, 2001). Over the last 50 years the forest area
has expanded for timber production, environmental
protection, recreation and amenity. Further forest
expansion is likely as a part of Common Agricultural
Policy reforms to reduce agricultural overproduction.
Continued technological developments in agriculture
could enable adequate volumes of food production
from under half of the current farmed area in the EU
(Bouma et al., 1998); potentially an extra 15–20 Mha
could be afforested, increasing the present forest area
by a third. At a series of Ministerial Conferences on
the Protection of Forests in Europe (MCPFE), the
Forest Ministers of European governments set guide-
lines for the sustainable management of Europe’s
forests and agreed to promote forest conservation,
replanting and further afforestation (Mayer, 2000).
The benefits of planting forests on former agricultural
land include binding carbon, improved ecology and
wildlife, and improved water quality (due to less use of
fertilisers and pesticides than on farmland).
Previous concerns about the hydrological impacts
of a large increase in forest cover centred upon the
enhanced total annual evaporation loss, and its nega-
tive impacts upon water resources. It is now widely
accepted that forests, particularly coniferous, can
increase total water use relative to shorter vegetation
due to their greater aerodynamic roughness enhancing
interception losses. A number of predictive models are
available to account for this (e.g. Calder, 1990). In
addition, it is known that certain species such as
eucalyptus can have a very high water use. Concerns
relating to water resources have now moved to try to
reduce the uncertainty about the impacts of forests
upon extreme flows, both peak flows and baseflows
(EEA, 2001). It has often been claimed that forestry
moderates flood flows and there are strong rival claims
that it can deplete or enhance low flows in drought
periods (McCulloch and Robinson, 1993).
Changes to extreme flows may have considerable
economic and social impacts. Over the 5 years
(1995–1999) the annual cost of flood damage in
Europe averaged 2000 M Euro (Swiss Re, various
dates). Insurance industry surveys show that although
flood damages vary widely from year to year, they are
steadily increasing over time, even allowing for infla-
tion. This rise is due to higher property values, and
a greater concentration of property and buildings in
flood-prone areas (Munich Re, 2000).
It is often claimed in the literature that forests can
reduce flooding downstream and so are a ‘natural’
solution to flood problems. Serious flooding of the
Rhine and Meuse in the early 1990s highlighted
the possible effect of land use change upon river
flow regimes (e.g. Waterloopkundig Laboratorium,
1994; Mendel, 1996). In February 1995, the Environ-
ment Ministers of France, Germany, Luxembourg
and the Netherlands adopted the ‘‘Declaration of
Arles’’ to take measures to reduce future flood risks,
which include land management and forestry (WMO,
1995).
Although less dramatic than floods, reduced
recharge of groundwater reserves and lowered river
baseflows can also have serious economic conse-
quences. Following recent droughts in parts of Europe
(Vogt and Somma, 2000) there is increased concern
that the generally higher rates of evaporation from
forests compared to grass or crops may exacerbate the
depletion of dry weather flows of streams. The costs of
providing alternative sources of water in drought
prone areas are less easy to quantify than flood
damages, but indicative estimates can be made. For
example, at a regional scale the total development
costs of a single new water supply borehole and the
associated water treatment and distribution network
could be up to 5 M Euro (J. Sanders, United Utilities
plc, pers. comm.).
The earliest scientific study in Europe of the
impacts of forestry on river flows was undertaken in
Switzerland (Engler, 1919). Flows from two mountain
basins, one forested and the other under grass, were
compared. The forested basin had lower peak flows,
but higher baseflows, suggesting that forests were
beneficial both for reducing flood flows as well as
sustaining baseflows. However, the forested basin
had much deeper soils, and so it was unclear to what
extent the differences in flows were due to soils or to
vegetation effects.
There have subsequently been a large number of
basin studies concerned with forest impacts, most
notably in the USA. Hornbeck et al. (1993) reviewed
11 long-term studies of forest clearing in the North-
eastern USA. They found that this resulted in an
86 M. Robinson et al. / Forest Ecology and Management 186 (2003) 85–97
increase in annual water yields for up to 10 years
before regrowth increased evaporative losses to pre-
vious levels. The extra flow occurred largely as aug-
mented baseflows in the summer months. Increases in
summer low flows after forest harvest have also been
noted in other parts of the US (e.g. Rothacher, 1970;
Troendle, 1983). Various studies have found large
increases in peak flows after felling, and attributed
this not just to the absence of the trees but also to the
ground disturbance and soil compaction associated
with the different methods of felling (e.g. Cheng et al.,
1975; Harr et al., 1979).
Studies of forested catchments in Europe include
Plynlimon in Britain (Kirby et al., 1991), Lange
Bramke in Germany (Herrmann et al., 1987), Real
Collobrier and Mont Lozere in France (Cosandey et al.,
2002). These basin studies have generally been con-
cerned more with the impact of forestry on total water
yield and/or water quality, than with flow regimes and
extremes of flow.
2. Experimental design and objectives
To tackle specifically the concerns about forest
impacts on peak and low flows in a European context,
the FOREX project (Forestry and Extreme Flows)
collected and analysed data at 28 small basins across
Europe. The objectives of the work were:
� To study the impact of forestry on both peak flows
and baseflows.
� To study a representative range of European con-
ditions (climate, location, soils, tree species, etc.).
� To recognise that most European forests are man-
aged, and to look at different silvicultural activities
(planting, growth, felling).
Forestry across Europe is quite diverse, reflecting
both the ranges of natural conditions (climate, geology
and soils), as well as national differences (history,
land tenure and legal systems, planning regulations,
population density and economic development).
Since not every combination of forest type, forest
activity and soil/geology/climate could be investi-
gated within the resources of this project, the study
design sampled a range of representative conditions.
This used the EU Biogeographical zones classifica-
tion (EEA, 1995) as the broad framework for dividing
the present distribution of forestry across Europe into
three groups:
� ‘Atlantic’ North West Europe, where conifers are
most important,
� ‘Continental’ central Europe, where there are
important mixed hardwood forests,
� ‘Mediterranean’ southern Europe, with open forests
and eucalyptus plantations.
The study basins were located on two transects
along climatic gradients (Fig. 1): (a) Oceanic to Con-
tinental, and (b) Mediterranean to Cool Temperate.
These encompass the major agricultural regions of
Western Europe (Kostrowicki, 1991) and sample a
range of climates and soils. The lengths of flow record
varied from 5 to 30 years and in total, nearly 400
station-years of flow records were analysed.
The data collected at the research basins cover the
main forest types and included the key silvicultural
and forest changes likely to impact upon extreme
flows (Table 1). One aspect of forestry, which has
often not been sufficiently emphasised in the scientific
literature, is that most forests are commercially man-
aged. In Europe, approximately 80% of the forested
Fig. 1. Location of the two transects of research sites across
Western Europe. (A) From Oceanic to Continental climate (G:
Glenturk, Ireland; W: Woodburn, N. Ireland; C: Coalburn and
Kershope, N. England; P: Plynlimon, Wales; K: Krofdorf, central
Germany). (B) From Mediterranean to Cool Temperate (S: St.
Antonio, Portugal; L: Mt. Lozere; H: Chiemsee, S. Germany).
M. Robinson et al. / Forest Ecology and Management 186 (2003) 85–97 87
Table 1
Summary of the study catchments, showing the forest treatment studieda
Site Soils and vegetation Precipitation
(mm per year)
Type of study and forest treatment during
the period of hydrological measurements
1 2 3 4
North West ‘Atlantic’ climate
(mainly conifers)
Glenturk, S. Ireland Deep Blanket peat, Picea
sitchensis, Pinus contorta
1500 1970–1980 tree establishment, 1980–1985
tree growth; 1996 growth and clear felling.
Two forested basins with drains at
15 and 75 cm depth
– Y Y Y
Woodburn, N. Ireland Brown earths and surface
water gleys, Picea abies
1200 1959–1973; 4 basins 3 forest growth,
1 grass control
– Y Y –
Coalburn, N. England Blanket peat and peaty gleys,
Picea sitchensis
1350 1966–1972 grasses, 1972 deep
(90 cm) drainage, 1973–2000 forest growth
Y Y Y –
Kershope, N. England Peaty gley, Picea sitchensis 1440 1981–1985; 4 forest basins, 3 clear-felled,
1 control basin
– – – Y
Plynlimon, Wales Peaty gley soils, Picea
sitchensis, with Picea abies
and Pinus contorta
2400 One forest basin and one grass basin
control (both with 3 sub-basins).
Commercial felling of 40þ years trees
commenced in 1983 and is continuing
– – Y Y
Chiemsee, S. Germany Basin peat, Picea abies 1400 Four basins, including two grass controls.
Young trees studied up to their felling.
No forest drainage
– Y Y Y
Central Europe ‘Continental’ climate
(mainly mixed hardwood)
Krofdorf, Central Germany Deep brown earth, over tight
palaeozoic bedrock Fagus
sylvatica with Quercus petraea
670 Four forest basins with old growth stands
(�120 years). In two basins the trees were
clear felled and natural regeneration
occurred. Soil compaction was minimised
by cutting when ground was dry
– – Y Y
Southern Europe ‘Mediterranean’ climate
(open forests and eucalyptus)
Mt. Lozere, S. France Rankers and brown soils, over
granite bedrock Sapine, Fagus
silvatica, Latte*, Picea excelsa
Cloutasses, grass control
1950 1980–2000; Three basins: 1 grass control
and 2 under forest. *The spruce forest
suffered from bark beetle attack and was
felled 4 years later, and is now mostly a
genista moor
– – Y Y
St. Antonio, S. Portugal Humic cambisols, E. globulus 750 1982–2000; 2 basins (13–15 ha).
Two cycles of commercial clear felling
(coppicing) on each basin
– Y Y Y
a The forests are worked using standard commercial methods and techniques that are representative of their area, except for the felling of non-commercially mature trees at Mt.
Lozere (latte basin) due to severe insect damage. (1) Pre-forest; (2) forest establishment; (3) forest growth; (4) forest felling.
88
M.
Ro
bin
son
eta
l./Fo
restE
colo
gy
an
dM
an
ag
emen
t1
86
(20
03
)8
5–
97
land is exploitable or managed forest. A major objec-
tive of this study was to include, as far as possible,
the different stages of a plantation forest cycle: site
preparation before planting, young forests, mature
forests, forest felling and post-felling conditions.
3. Results and discussion
The principal method of analysis was ‘paired catch-
ment’ approach, with the flow changes of the forest
basins over time being compared with those of bench-
mark or control basins to remove the influence of
climatic variability. This enabled the magnitude and
duration of changes to be identified.
In this study the terms ‘peak’ or ‘flood’ flows are
taken to be high flows generated from the landscape,
and are typically the five highest flows in a year. These
may not necessarily lead to over-bank flooding since
the carrying capacity of the channels downstream may
depend upon chance local obstructions and blockages
due to debris or to constrictions due to inadequate size
of bridges or culverts. The catchments studied were all
‘upland’ basins in the sense that they generate runoff
to river channels. It was outside the scope of this study
to consider, for example, the practicability or other-
wise of planting forests on floodplains to ‘hold back’
floodwaters, as has sometimes been advocated. The
term ‘low’ flows generally refers to dry weather sum-
mer flows, rather than frozen conditions, since this is
the most important time of stress for water supply.
The principal low flow measure used was the flow
exceeded for a certain percent of the time (generally
95% of the year although in some cases where flow
ceased for a long time in the summer a higher threshold
was adopted). A secondary measure, the Baseflow
Index (Gustard et al., 1993) which is the proportion
of the total annual flow occurring as baseflow was also
used.
3.1. ‘Atlantic’ North West European conifers
Conifers are widely grown on a commercial basis in
northwestern areas of Europe, often on peaty soils of
low permeability. The management cycle of the tree
crop may be as short as 30 years but is more commonly
about 40–60 years. Cultivation and land drainage are
often an integral part of site preparation for new
planting, and can have a very important hydrological
impact upon both peak flows and low flows. The
basins studied in this category are summarised in
Table 1. The Coalburn catchment is one of the longest
continuously running research sites in Europe, and its
30-year record includes the change from agriculture
(upland pasture) through planting to full canopy cover
of a conifer plantation (Robinson, 1998; Robinson
et al., 1998). The Glenturk study encompasses a
similar period of time in three separate periods of
5–10 years duration (McDonnell, 1999). There are no
data prior to planting, but due to the fast forest growth
there are records of the forest felling. Other major
basin studies include the effect of forest growth
and partial felling at Plynlimon in mid-Wales. The
Chiemsee catchments, in the German PreAlps, have
many similarities in terms of tree species, peaty soils
and high precipitation, and although not geographi-
cally within the NWAtlantic zone, have been included
in this category.
3.1.1. Forest establishment and growth
3.1.1.1. Peak flows. Drainage of peaty soils by open
ditches or furrows can significantly increase peak
flows and shorten the rise time of flood hydrographs.
The enhanced flood risk can last for 20 years in slow-
growing coniferous tree crops. The economic con-
sequences can be assessed locally, to determine if
floods exceed the design capacity of channels and
culverts downstream. After drainage and afforesta-
tion at Coalburn, England, peak flows immediately
downstream were increased by about 15% over the
short to medium time scale (Fig. 2).
Once a closed forest canopy has become estab-
lished, continued tree growth and associated drain
in-fill leads to a decrease in peak flows over time.
Reductions of 10 or 20% are typical. Due to the great
natural variability in the weather-dependent occurrence
and magnitude of storm events, this gradual reduction
in peak flows is not necessarily easily evident from an
examination of time series of peak flows alone. It is
essential to compare flows against a nearby ‘control’
catchment. At Chiemsee, S. Germany, where conifer
forests were planted on pipe-drained farmland a non-
linear (logarithmic) model best-fitted the decline
in flow peaks over time (Fig. 3). The forested areas
initially had a higher peak runoff than the unplanted
M. Robinson et al. / Forest Ecology and Management 186 (2003) 85–97 89
moorland (ratio of forest/moor peak flow >1) due to
factors including extensive open water surfaces in the
moor. The reduction was most rapid in the early years,
and then progressively slowed. The computed ‘half
life’ in this case was approximately 12 years. As the
land had already been drained there was no forestry
drainage and this decline was predominantly due to the
effect of the trees. In contrast, at Coalburn where there
was extensive forestry drainage, a remedial drainage
experiment involving the recutting of a couple of
0
3
6
9
12
15
18
1967-71 1973-77 1979-81 1981-83 1984-88 Post1990
Rel
ativ
e in
crea
se (
%)
Fig. 2. Increase in calculated design flood peaks 1973–1999 (relative to pre-forestry baseline period 1967–1971) following the drainage and
conifer planting of 90% of the Coalburn catchment in 1972.
Ratio = -1.148Ln(Age) + 4.49R2 = 0.83
Ratio = -0.748Ln(Age) + 3.00R2 = 0.90
0
1
2
3
0 5 10 15 20 25 30 35
Forest Age (years)
Rat
io: F
ores
t Qpk
/ M
oor
Qpk
Fig. 3. Ratio of the average peak flow (Qpk) of two replicated forested catchments at Chiemsee (& and ^) relative to a moorland control
catchment. The initial rapid decline is due to forest growth as there was no forest drainage.
90 M. Robinson et al. / Forest Ecology and Management 186 (2003) 85–97
infilled drainage ditches in the 25-year-old forest
increased their peak outflow rates, thus indicating that
drain infill had played a key role in the observed
reduction in peak flows at the catchment scale (Nisbet,
unpublished data).
3.1.1.2. Low flows. Forestry drainage channels or fur-
rows can augment baseflows by providing a deeper
outlet for soil profile gravity drainage. This increase
may amount to a redistribution of as much as 10–15%
of the annual flow, and in small upland catchments
with very small baseflows this can result in as much as a
doubling of the previous ‘natural’ baseflows. Although
the absolute volume of the increase may be quite
small this is a large relative increase, and it can be
very important for headwater streams.
As the tree crop grows, baseflow levels decline over
the years in a non-linear manner as forest interception
losses and transpiration increase and the ditches
become progressively blocked (initially by weed
growth and then after canopy closure by the annual
accumulation of forest leaf litter). The overall impact
on baseflows will thus depend upon the balance
between these effects and the original drainage
enhancement.
Soil water measurements generally show drier
soil conditions under established forest than nearby
grass (Hudson, 1988; Robinson and Cosandey, 2002).
This reduces the soil moisture reserves to sustain
baseflows in dry weather periods and delays autumn
rewetting. However, soil properties may be altered by
forest rooting, leading to an increase in the effective
thickness of the upper, more permeable horizons. This
process may help to sustain baseflows in the drainage
system, so slowing the rate of decline with forest
growth. The drain deepening experiment at Coalburn
and the comparison of the different drain depths at
Glenturk both indicated that once a mature forest had
become established differences in drain depth had
little effect on low flows (Nisbet, unpublished data;
McDonnell, 1999). At both sites the forest had low-
ered the soil water table to below typical drain depths.
The drying out of the soil over time results primarily
from the ‘biological’ drainage (forest interception
losses and transpiration) rather than by ‘technical’
drainage by the artificial drains. The pattern of base-
flows from afforested sites will depend upon the
balance between the opposing influences of the drains
(increasing low flows when trees are still small) and
the growing forest (reducing soil moisture and low
flows).
3.1.2. Forest cutting
The studies have shown that, although forest felling
is visually very dramatic, its impact on extreme flows
is relatively small and difficult to detect.
0
0.05
0.1
0.15
0 5 10 15 20 25 30Hours
Pro
port
ion
of fl
ow p
er h
our
8
15
0
Fig. 4. Average storm unit hydrographs for drained conifer forest at Glenturk, comprising 8-year-old forest in 1972 (long dash line), 15-year-old
forest in 1980 (solid line) and after forest felling in 1999 (short dashes).
M. Robinson et al. / Forest Ecology and Management 186 (2003) 85–97 91
3.1.2.1. Peak flows. Studies at Glenturk indicate that
complete clear felling can increase moderate peak
flows immediately downstream (Fig. 4). Partial
felling at Plynlimon produced only a weak tendency
for higher peaks to increase, while the smaller peaks
were actually reduced. The lack of response may be
due to the limited change in the interception capacity
between the standing forest and that of felled areas
covered by large amounts of tree brash. The presence
of debris dams in streams in the clearfelled areas
may also have acted to attenuate surface runoff and
hydrograph peaks.
3.1.2.2. Low flows. The impact of forest felling on low
flows can be relatively small and may be difficult to
detect as very important role of local factors such as
geology and soils, can mask the effect of differences in
vegetation cover between basins. Furthermore, even
where a forest reduces soil moisture recharge and the
absolute levels of low flows, the shape of the streamflow
recessions may not be affected. This is because the rate
of release of water from storage is controlled by
subsurface hydraulic properties. Weather variations
may also obscure the detection of a felling effect.
The clearfelling of a mature forest crop at Kershope
increased the baseflows of the three experimental
catchments (Fig. 5). This increase appeared to be
short-lived, but unfortunately definite conclusions
cannot be made as there was only a limited period
of observations after the felling. The partial felling at
the Plynlimon catchment study increased low flows
for about 5–10 years (Robinson and Dupeyrat, 2003).
3.2. Central European broadleaf forests
Old growth mixed hardwood forests are extensive
in many parts of central Europe. They provide an
important supply of high quality timber and many
other benefits. The life cycles of these commercially
managed forests are much longer than those of the
fast-growing conifer plantations described above, and
the trees may be 100 years or more when they are
felled. They tend to form a continuous forest cover,
often comprising two species with the faster growing
crop being thinned out and replanted first, letting
the sub-canopy crop become dominant until felling.
Generally, only small areas within an individual basin
are thinned or felled each year. It is unusual for these
0.6
1.0
1.4
1.8
1981 1982 1983 1984 1985
Year
Rat
io: F
ores
t Low
flow
/ C
ontr
ol b
asin
CUT
0.6
Fig. 5. The proportion of the annual streamflow occurring as baseflow at three adjacent forested catchments (&, ~, ^) at Kershope. Values
are shown relative to the baseflow at a control catchment. Baseflow increased after felling in 1983 and remained higher for about 3 years.
92 M. Robinson et al. / Forest Ecology and Management 186 (2003) 85–97
forests to have significant artificial drainage. Due to
the very long growth period to maturity there are no
long-term studies of establishment and growth of these
forests on their water balance and patterns of flows. At
the Krofdorf study in central Germany, flows have
been measured from four beech covered basins and
the effect of partial felling upon water yield has been
studied (Brechtel and Fuhrer, 1991; Fuhrer, 2002).
3.2.1. Forest cutting
3.2.1.1. Peak flows. The felling at Krofdorf caused a
significant increase in the annual water yield, but the
impact upon peak flows was limited. Medium and
large daily stream flows increased by about 10%, but
the very highest instantaneous peak flows did not
increase significantly after felling. The main effect
was a tendency to somewhat greater flood volumes
after the felling, with a lengthening of the duration of
high flows (Fig. 6).
3.2.1.2. Low flows. Results for Krofdorf indicate a
complex situation for low flows. The felling enabled
summer soil moisture reserves to be replenished
quicker and an earlier return to field capacity than
for the standing forest, but there was little evidence of
a change in baseflows.
3.3. ‘Mediterranean’ Southern European
woodlands
In this region it is necessary to distinguish between
the intensively managed eucalyptus plantations and
the lightly managed open mixed forest, including
spruce and mountain pine.
3.3.1. Eucalyptus plantations
The planting of eucalyptus plantations has been
promoted in many parts of southern Europe as a means
to develop areas where agricultural productivity is low
and there is high rural unemployment. The wood is
used mainly for pulpwood production. One of the most
common species is Eucalyptus globulus, which can
grow extremely rapidly and is usually coppiced with a
10–12-year rotation period. Data from St Antonio in
southern Portugal provide information covering two
After:A1 = 1.62 B1 + 0.845
R2 = 0.83
Before:A1 = 1.44 B1 - 0.3888
R2 = 0.97
0
10
20
30
40
50
Control Basin B1 (mm)
Exp
erim
enta
l Bas
in A
1 (m
m)
0 10 20 30 40
Fig. 6. Flood volumes (mm) at Krofdorf catchments A1 (felled) and B1 (control basin). Comparison of periods before felling (1972–1981; ~,
solid line), and after completion of felling (1986–1992; ^, dashed line).
M. Robinson et al. / Forest Ecology and Management 186 (2003) 85–97 93
cycles of coppicing on a pair of adjacent catchments
(David et al., 1994).
3.3.1.1. Peak flows. The four clearcuttings in the St.
Antonio basins all showed the same effect—unless
the weather was very dry there was an increase of
about 50% in peak flows after the cutting, and this
effect lasted for 1–2 years (Fig. 7). Because regrowth
is so quick, it is difficult to separate the hydrological
effects of the cutting (ground disturbance, brash
residues) from those of the rapid new growth. The
increase in peak flows was due to larger stormflow
volumes, rather than to an increase in the peakiness
of the flood hydrographs, and this may be related to
the hydrophobic properties of eucalyptus (Ferreira
et al., 2000), with the cut leaves left on the ground
(they were not burnt) creating both a physical and
a chemical barrier to storm runoff infiltration. The
short period of increased peak flows means that
the impact of cutting could be controlled by good
forest management practices, for example, through
phased cutting of sub-areas within an individual
basin.
3.3.1.2. Low flows. Changes in low flows could not
be determined with confidence due to the absence of
sustained baseflow at the study sites. Streamflow only
occurred for about a third of the time in each year.
Forest cutting produced a small increase in low flows
in one of the basins, but little change in the other. No
surface basin feature could be identified to account for
this difference, suggesting that the variable response
was most likely due to differences in subsurface
hydrogeology. Overall, the evidence indicated that
forest cutting may have resulted in only a limited
increase in baseflows. This would accord with an
increase in the hydrophobicity of the soils and the
published world literature of the high rates of water
use by eucalyptus (Calder et al., 1997).
3.3.2. Mediterranean open forest
The Mediterranean open forests are not intensively
managed for commercial purposes but they are widely
perceived as having a high conservation value and
being beneficial for tourism. These forests are subject
to frequent disturbance due to recurrent forest fires.
At Mont Lozere, the open spruce forest in the Latte
Before cutE = 0.517 C + 0.137
R2 = 0.72
After cut E = 0.972 C + 0.200
R2 = 0.81
0
1
2
3
4
5
0 1 2 3 4 5 6 7 8
Control Basin, C (mm h-1)
Exp
erim
enta
l Bas
in, E
(m
m h
-1)
Fig. 7. Comparison of control basin and experimental basin peak flows (mm h�1) at St. Antonio Portugal before (~, solid line), and after
(^, dashed line) cutting the eucalyptus.
94 M. Robinson et al. / Forest Ecology and Management 186 (2003) 85–97
catchment was severely attacked by bark beetle (Den-
drochtonus micans (Kug)). This damage necessitated
the partial clear cutting of the forest which modified
the hydrochemical and hydrological behaviour of the
basin. Cosandey (1993) showed that the felling
increased the annual discharge by 150 mm (10%).
3.3.2.1. Peak flows. The cutting of the spruce forest
led to an apparent increase in peak instantaneous
flows when compared to a control basin (Fig. 8).
However, the difference between the pre-felling and
felling periods was relatively small, given the wide
dispersion of the points and uncertainty in the
regression lines. In addition, the post-felling period
was much wetter than the calibration period, with some
exceptionally large daily rainfall, so it is not possible
to attribute the increase in peak flows solely to the
forest cutting. There was no change to the highest
daily flows after cutting, but the small to medium daily
flows were increased by about 10%. New vegetation
growth resulted in a rapid restoration (<4 years) of the
pre-felling hydrological behaviour.
3.3.2.2. Low flows. A comparison of the flow duration
curves showed no apparent change in low flows after
felling of the spruce forest, when compared with a
control catchment. This was confirmed by a rainfall-
runoff model, fitted to data before and after felling,
which showed no change in the shape of the recession
curves (Cosandey and Robinson, 2000). The flow
recessions are controlled by the catchment physio-
graphy and geology rather than by surface factors.
The use of a flow mixing model (with parameters
estimated or adjusted to fit 18O isotope concentra-
tions) indicated that low flows were fed from a deep
reservoir where the residence time of the water is about
12 months (Marc et al., 2001).
4. Conclusions
Whilst the effects of forests and forest manage-
ment on the extreme flows of rivers may be thought
to be uniquely site-specific, this study has found a
relative consistency of results between regions and
Before:Latte = 0.6 Cl - 0.0553
R2 = 0.81
After:Latte = 0.97 Cl - 0.2997
R2 = 0.94
0
2
4
6
8
10
0 2 4 6 8 10
Cloutasses control basin (mm h-1)
Latte
exp
erim
enta
l bas
in (
mm
h-1
)
Fig. 8. Peak flows (mm h�1) in the forested Latte basin before (~, solid line), and after (^, dashed line) the felling. Peaks increased relative
to the grass Cloutasses control basin, but the scatter of points was very large.
M. Robinson et al. / Forest Ecology and Management 186 (2003) 85–97 95
sites which gives confidence in the generality of the
findings.
At the local scale there are specific situations where
forest impacts are potentially significant on peal flows
and low flows. However, at the broad European or
regional scale, forestry generally has a relatively small
impact on extreme flows. The main findings for the
three broad forest types investigated are:
(a) Commercial conifer plantations on peaty soils in
NW Europe:
� Pre-planting forest drainage increases peak
flows in the early stages of the forest cycle,
and the effects may last for 10 years or longer,
� Forest drainage enhances baseflows from a
young forest; the duration of this increase
depends upon the balance between the oppos-
ing effects of the drains and the growing forest,
� Peak flows from a mature forest cover, may be
little different from unforested land,
� Forest cutting leads to short-term increases in
both peak flows and baseflows at the local
scale, although this may not be detectable at
the larger catchment scales.
(b) The effects of partial harvesting of central
European mixed broadleaved and Mediterranean
open forests were relatively small, with little
difference detectable in either peak flows or
baseflows after felling.
(c) The coppicing of Eucalyptus plantations led to
an immediate increase in peak flows and some
evidence of short-term enhancement of baseflows.
Flood flows appeared to be increased for only 1 or
2 years following coppicing as a consequence of
the very rapid regrowth of this crop and short-
lived changes to the soil structure, either due to
soil compaction during the tree cutting and/or to
the release of natural water repellent chemicals
from the eucalyptus residues.
For all the forest types studied the changes to
extreme flows will be diluted at the larger basin scale,
where forest management is phased across a catch-
ment, or only a part of the basin is forested. Overall,
the results from these studies conducted under realistic
forest management procedures have shown that the
potential for forests to reduce peak and low flows is
much less than has often been widely claimed. Con-
sequently, other than at a local scale, for the particular
cases of managed plantations on poorly drained soils
in NW Europe and Eucalyptus in Southern Europe,
forestry appears to probably have a relatively small
role to play in managing regional or large-scale flood
risk or influencing drought flows across Europe.
Acknowledgements
The European Commission financially supported
this work under project FAIR-0235, The Impacts of
Forestry and Silvicultural Practices upon the Extreme
Flows of Rivers. The authors are also indebted to
colleagues in the various organisations who were
involved in the data collection.
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