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database constructed for the characterisation exercise lists in Category 1a four rivers
and eight lochs where forestry is the primary pressure. Overall, forestry is considered
to be a risk factor in 110 rivers and 28 lochs, equivalent to 16% of the total number
of river and 40% of loch water bodies at risk from diffuse pollution in Scotland.
Table 1:
Characterisation of rivers and lochs in Scotland assessed at risk of
diffuse pollution due to forestry
Category 1a
Category 1b
Number Length (km) Number Length (km)
Primary River 4 63.497 18 181.026
Loch 8 – 4 –
Secondary River 24 189.054 64 622.409
Loch 8 – 8 –
MINIMISING THE RISK FROM FORESTRY
How does the Forestry Commission prevent these potential risks of diffuse pollution
becoming a reality? The Forests and Water Guidelines, which were first published
in 1988 (Forestry Commission, 1988), form the key supporting document for the
UK Forestry Standard and include the equivalent of Best Practice Guidance. These
went through a substantial revision with the 4th edition, which was published in 2003
(Forestry Commission, 2003). The revision was done in collaboration with both SEPA
and EA.
The Guidelines deal with the following aspects of diffuse pollution: acidification;
siltation; nutrient enrichment; high colour, iron and manganese concentrations;
pesticides; and fuel oils. All must be borne in mind by forest practitioners but
some must also be tackled earlier at the planning stage and addressed at the
catchment scale. These include: acidification, associated with both new planting and
restocking; nutrient enrichment; and as a pollution control measure, the design and
management of riparian buffer areas. The most important issues are acidification,
nutrient enrichment and siltation.
Acidification
Acidification of fresh waters occurs where the inputs of sulphur and nitrogen
pollutants exceed the buffering (neutralising) capacity of the soils and the underlying
rocks through which water passes before entering streams, rivers and standing
water. The buffering capacity of these receiving waters is also an important factor.
The most acidified areas in the UK are in the uplands where catchments with basepoor,
slow weathering soils and rocks coincide with high pollutant inputs in the form
of large volumes of moderately acidic rainfall. Although most pollutant inputs are now
declining due to emission control, surface water acidification remains a particular
problem in parts of central and south-west Scotland.
The quantity of sulphur and nitrogen pollutants deposited at a given site is strongly
influenced by the nature of the vegetation layer. Forest canopies can significantly
increase the capture of some of these pollutants in the atmosphere. This increased
capture, often termed scavenging, is a function of the stand structure which creates
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