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turbulent air mixing. The effect therefore becomes more important as trees grow and
the height of the stand increases. The enhanced capture of mist, which can contain
large concentrations of sulphur and nitrogen, is greatest at high altitude because of
the increased duration of cloud cover and high wind speeds.
Forest growth is usually limited by nitrogen availability, and drainage water from
forests and woodlands generally has very low nitrate concentrations. Consequently
nitrogen deposition would not normally be expected to pass through undisturbed
forest ecosystems and result in acidification of water. However, nitrate leakage
from older forest stands has been identified in areas of high nitrogen deposition,
e.g. in south Scotland. There is concern that forest soils are becoming increasingly
saturated with nitrogen and that this could lead to a marked rise in nitrate losses and
thus acidification.
Significant nitrate leakage is also known to result from harvesting operations. This
is due to increased rates of mineralisation and nitrification in the soil in the absence
of uptake by the trees. This pulse may last for 2–5 years, depending upon the rate
of re-vegetation. While an increase in nitrate concentrations in soil and stream
waters poses only a very small risk of exceeding environmental quality standards, of
more concern is the short-term increase in hydrogen ion concentration which may
contribute to acidification and increase aluminium solubility.
The increased capture of acidic pollutants by forests could delay the recovery of
acidified waters or even lead to further acidification in the most sensitive areas.
Large-scale conifer afforestation represents the greatest threat while the replanting
of existing forests can also be a cause for concern. In order to protect the freshwater
environment, the Guidelines require the Forestry Commission to take the scavenging
effect into account when considering new planting or restocking plans. Both the
Forestry Commission and applicants must identify which areas are most at risk.
An indication of a site’s sensitivity is obtained by the maximum pollutant load that a
given ecosystem can tolerate without suffering adverse change – the critical load. For
fresh waters, critical loads can be calculated which, provided they are not exceeded,
should ensure the maintenance of water chemistry suitable for the protection of
populations of fish and other freshwater biota. Defra have calculated critical loads
for fresh waters for 10-km 2 grid square samples incorporating the role of N as well
as S. Having compared these with total pollutant inputs of S and N, maps have
been derived that indicate where critical loads for total acidity for fresh waters are
exceeded.
The Forestry Commission has built in three additional safety margins to the Guidelines.
We have used the deposition data from 1995–97 (ECRC, 2001) even though pollutant
depositions have reduced significantly since then and are expected to continue to do
so up to 2010 and beyond. The impact of new planting and restocking plans must be
considered not only in exceedance squares but also in all adjacent squares. Lastly in
view of the requirement to protect cSACs), site-specific data, if available, are used to
refine the assessments of acidification risks for designated river catchments.
The indicative nature of the 10-km scale critical load exceedance map means that a
more detailed catchment-based assessment might be required for determining the
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