many countries, it is widely believed that forests encourage water
to infiltrate into the soil, increasing the recharging of groundwater
and reducing surface runoff and erosion. Thus, forests are often
claimed to enhance dry weather baseflows and subdue peak runoff.
But there is contradictory evidence that, in dry weather periods,
forest evaporation may dry out the soil and reduce baseflow, whilst
in wet weather, forests may have little impact on flood flows.
important consideration is that many forests and woodlands are
managed by people for timber production. Local silvicultural practices
may disturb the soil and alter water flow pathways, with variable
effects on extreme flows.
the European Union, the land area under forest cover is expanding:
as an alternative to agriculture, to assist reform of the Common
Agricultural Policy, for environmental protection and enhancement,
and to reduce the impact of climate change through carbon fixation.
objective is to quantify the effects of forestry (broadleaf and
conifer) on extreme flows - both dry weather flows and peak flood
flows. This covers a range of forest conditions and includes:
stages of forest growth (planting, growth, harvesting);
forest types (conifers, broadleaves, eucalyptus);
silvicultural practices (drainage, harvesting);
different climate regions in Europe (oceanic, continental, Mediterranean).
partnership structure includes universities, governmental research
organisations and national forest bodies responsible for commercial
forest management and timber production. The project team includes
scientific experts with extensive appropriate academic experience,
as well as participants who work closely with forest managers,
so that the results and experimental studies are relevant to real
are nine field sites across Europe, covering a range of forest
types, climates and soils. Tree species studied include Sitka
spruce, Lodgepole pine, Eucalyptus, Maritime pine, spruce, beech
and some oak.
programme of work includes:
analysis of existing long-term catchment records;
detection of effects on streamflow of felling, coppicing or planting;
measurement of plot parameters such as transpiration, interception
losses, soil water, soil moisture, meteorological variables;
production of a general descriptive model.
results of forestry are complex, which accounts for the differences
in opinions and observations. Nevertheless some general observations
can be made.
Pre-planting drainage is widespread on peat ground in northern
European countries to aid tree establishment. Forest drainage
can increase peak flows, with effects varying temporally and spatially,
depending on the scale of operation. There is the potential for
increased erosion from the open drains, which may block watercourses
downstream and further aggravate flooding. However, deep drains
may augment baseflows during the early growth of trees by providing
an outlet for a deeper thickness of soil than the former natural
The generally deeper rooting of trees may potentially cause greater
depletion of soil water in rainless periods, and lead to a reduction
in stream baseflow
Intercepted rainwater on the forest canopy evaporates at a faster
rate than from short vegetation, and will also reduce soil water
Due to the higher evaporation from trees, forest growth can reduce
low flows and peak flow rates, especially for evergreen tree species.
Changes over time are most rapid when the trees are still small.
Once canopy closure has been reached, further changes in extreme
flows are more difficult to detect.
Forest cutting (harvesting) can increase stream baseflow and peak
flow - or can have no clear effect. Care must be taken to identify
and distinguish the effect of associated work, such as soil compaction
by forest machinery.
project brochure (FOREX Impacts of forestry on extreme flows)
has been produced; copies are available from the EU, the coordinator,
or from any of the project participants.
addition, approximately 10 scientific reports have been published
or are going to press.
+44 1491 69 23 22
+44 1491 69 24 24
Université d'Avignon et des pays de Vaucluse
Rue Louis Pasteur 33
Tel.: +33 4 90 14 44 00
Fax: +33 4 90 14 44 89
Universidade Tecnica de Lisboa
Tapada da Ajuda
Tel.: +351 21 350 20 93
Fax: +351 21 364 50 00
Forestry Commission - Forestry Authority
Alice Holt Research Station
UK-GU10 4LH Farnham
Tel.: +44 1420 22255
Fax: +44 1420 23653
The Irish Forestry Board - Coillte Teoranta
Sidmonton Place 1-3
IRL-Bray, Co. Wicklow
Tel.: +353 12 86 77 51
Fax: +353 12 86 81 26
Hessische Landesanstalt fuer Forsteinrichtung
Prof. Olkers Straße 6
D-34346 Hann Muenden
Tel.: +49 5541 70 04 24
Fax: +49 5541 70 04 73
Bavarian State Institute for Forestry
Am Hochanger 11
Tel.: +49 8161 71 48 81
Fax: +49 8161 71 49 71
National University of Ireland
Tel.: +353 91 52 44 11
Fax: +353 91 52 57 00