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: mr@ceh.ac.uk (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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