Responses of forest ecosystems to nitrogen deposition in the United Kingdom

Most of UK's woodland exceeds the critical nitrogen (N) loads. We reviewed current evidence from forest monitoring and experimental research on the measured impacts of N deposition on UK's forests. Forest monitoring (>20 years) suggests a significant decline in nitrate deposition in some lowland areas and in both ammonium and nitrate deposition in upland forests but no change in areas with high N deposition and near N pollution sources. Some ground flora response to decline or enrichment of N deposition has been identified, but forest age, canopy management, previous land use and edge effect need to be considered. Soil surveys suggest that UK's forest soils have accumulated N under conifers which may be released during forestry operations and pose a risk to water's quality and emissions of greenhouse gases. Tree species and soil type play a vital role in the response of the forest to N inputs. Forest edge effect introduces both high spatial variability in N inputs and forest ecosystem responses to N deposition. Strong relationships between tree N uptake, litterfall, growth and insect damage have been related to N deposition. The generation of N through insect infestations of the forest canopy has been quantified and canopy N transformations have been demonstrated through stable isotopes studies. Dry and wet N deposition is found to be a significant predisposition factor for Acute Oak Decline. Clear thresholds of N deposition for ectomycorrhizal community's changes have been determined. Nitrogen input needs to be included in predictive modeling of biodiversity and tree health in forest ecosystems.