Estimating the effect of abiotic factors on modifying the sensitivity of vegetation to nitrogen deposition: an application of endorsement theory

Abstract: Many natural and semi-natural vegetation communities are sensitive to eutrophication; most eutrophication is caused by human activities. Critical loads have been developed in Europe to provide an effects-based approach to pollutant abatement including nitrogen deposition. Critical loads to protect ecosystems from eutrophication from excess nitrogen are only specified for very broad habitat types (eg "dry heaths") and as a range of values (eg between 10 and 20 kg N ha-1 yr-1). There may be considerable variation in vegetation communities within a broad habitat and there is a requirement (eg. from conservation agencies etc.) for more precise critical loads for more clearly specified receptors (habitats, vegetation communities). This paper demonstrates the use of Endorsement Theory to rationalise incomplete, qualitative and conflicting information on abiotic parameters (eg, climate, management) that may influence the vegetation response to nitrogen deposition and hence the critical loads. The results are shown for 22 heaths described in the National Vegetation Classification, identifying whether the critical load should be nearer the upper or lower limit of the published ranges. The policy implications for critical loads exceedance for European Dry Heaths are also discussed.