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Although nitrate leaching can be reduced by whole-tree harvesting, a much larger,
longer-term threat of soil and water acidification is presented by the greater removal
of base cations in harvested produce. Consequently, whole-tree harvesting is not
recommended in critical load exceedance or adjacent squares, except on steep
sites where whole trees have to be extracted to roadside.
Biological recovery of streams showing chemical improvement in response to emission
reductions may benefit from the opening-out of stream sides. Where practicable,
these areas should be targeted for earlier clearance of stands casting heavy shade
and cleared zones linked to aid upstream migration of fish and invertebrates.
Nutrient enrichment
There is concern that the nutrient, and hence the ecological, status of fresh waters,
particularly standing waters, may be significantly changed following the aerial
application of phosphate fertilisers in their catchments. Fertilisers may be accidentally
sprayed or blown into watercourses, or may be transported indirectly via subsequent
leaching or run-off. Waters vary in their sensitivity to nutrient enrichment from forestry,
with nutrient-poor (oligotrophic) waters most at risk of nutrient pollution.
Aerial phosphate fertiliser must be carefully planned to ensure that phosphate losses
from consecutive applications in a given catchment do not exceed environmental
quality standards in receiving lakes or reservoirs. Applications exceeding a total
area of 50 ha in any 3-year period may pose a problem; the effect will depend on the
soil properties, timing of application, size of the catchment and the characteristics of
the water body. Early consultation with the water regulatory authority will establish
whether a more detailed site assessment is required. Assessments need to include
phosphate releases from large-scale felling operations, which can be significant on
some soil types.
Fertiliser usage is declining in forestry in line with the reduction in new planting and
a shift to more fertile soils. A move to more hand applications and better targeting
of aerial treatments through the use of granulated fertiliser and improved helicopter
guidance systems, are also reducing the risk of nutrient pollution.
The generally higher nutrient status of lowland soils means that lowland woodlands
rarely require fertiliser applications. Thus woodland planting on ex-agricultural land
may help to reduce leaching losses and tackle nutrient pollution within sensitive
areas such as Nitrate Vulnerable Zones (NVZs). Opportunities exist for maximising
this benefit through the targeted planting of riparian and downslope buffer areas.
Forest type is an important factor, with conifer forests less able to reduce nutrient
concentrations in drainage waters. This is because of their ability to capture nitrogen
pollutants from the atmosphere and concentrate nitrate levels in groundwater,
particularly in areas of low rainfall. High nitrogen inputs can result where forests are
downwind of local pollutant sources, such as intensive pig- and poultry-rearing units.
This is likely to be an increasingly important consideration with the expansion of
NVZs. The main areas at risk are those receiving low rainfall where the concentrating
effect of evaporation is disproportionately large. Consideration should be given to
avoiding large-scale conifer planting within NVZs receiving < 650 mm annual
rainfall.
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