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actual susceptibility of individual waters to a forest scavenging effect within both
exceeded and adjacent critical load squares.
Planting
For new planting, the Forestry Commission, taking advice as necessary from the
appropriate water regulatory authority, will determine the need for the more detailed
assessment depending on the size of the planting scheme, species mix, altitude, the
proportion of forestry already in the catchment, soils, geology, and the sensitivity of
local water uses. Broadleaved woodland poses less of an acidification threat due
to the smaller scavenging effect, but the impact of larger planting schemes merits
consideration.
In some cases, assessment will be possible on the basis of existing information.
Where sufficient information is not already available, assessment is likely to involve the
collection of one or more water samples at high flow (preferably in January, February,
or March, when conditions tend to be most acidic) from the main watercourse
receiving drainage from the area proposed for new woodland. The results from the
chemical analyses will be used to calculate the receiving water’s critical load. This
will then be compared with the 1995–97 total pollutant deposition of S and N for the
appropriate critical load grid square. Where the deposition exceeds the critical
load, approval of a planting grant is unlikely until there are further reductions in
pollutant emissions.
Restocking
Harvesting temporarily eliminates pollutant scavenging until restocked crops approach
canopy closure at around 15 years of age. By this time, agreed emission reductions
are predicted to protect many catchments from the risk of further acidification. The
combination of these factors means that in the future forest replanting will be less
likely to contribute to the exceedance of freshwater critical loads compared to the
first rotation. Nevertheless there are some circumstances where restocking plans
require greater scrutiny, particularly higher altitude stands (> 300 m) at which level
the scavenging of pollutant cloud deposition is known to increase markedly. Where
conifer forest occupies a large proportion of the catchment above this altitude,
consideration should be given to selective deforestation, subject to a detailed
catchment-based assessment as part of an Environmental Impact Assessment
under environmental impact regulations applying in the different countries. Also in the
case of catchments designated as cSACs in critical loads exceedance and adjacent
squares, a detailed catchment-based assessment is required for forest replanting
under the Habitats Directive regardless of altitude.
The short-term release of nitrate that can follow the large-scale harvesting of some
forest sites may pose an additional acidification threat within acid-sensitive areas and
there may be a need to carry out a site impact assessment depending on catchment
size, the timing of felling operations, species mix, local soils and geology, and the
presence of fish. Sites found to be at risk will require the size of felling area to
be reduced and the adoption of management practices designed to minimise
nitrate losses.
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