『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
(e.g., ‘dry heaths’) and as a range of values (e.g., between 10
and 20 kg N ha-1 year-1). There may be considerable
variation in vegetation communities within a broad habitat and
there is a requirement (e.g., 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 (e.g., climate,
management) that may influence the vegetation response to nitrogen
deposition and hence, the critical loads. The endorsement theory
approach is tested for 22 heathland community types in the UK
(as described by the National Vegetation Classification of Rodwell,
Cambridge University Press, vol. 3, 1991) to determine if the
critical load for a community should be nearer the upper or lower
limit of the published ranges. The results give a ‘confident’
endorsement for one heath community and a ‘likely’ endorsement
for a further 13 communities for a critical load at the lower
limit. The endorsements suggest that the critical load for most
heaths should be at the lower end of the range meaning that current
estimates of the exceeded area calculated using the mid-point
of the range is an underestimate.
Keywords: Endorsement theory; Critical loads; Nitrogen; Eutrophication;
Abiotic factors; national vegetation classification』
1. Introduction
2. Methods
2.1. Compiling the abiotic data for the heathland communities
2.2. Nitrogen critical loads and modifying factors
2.3. Compiling the evidence for setting critical loads for heathland
vegetation communities
2.3.1. Toxic effects
2.3.2. Productivity and competition
Climatic conditions
Nutrient conditions
Base status
2.3.3. Removal of excess nitrogen
Leaching
Denitrification
Management
2.4. Endorsing a decision for critical loads
2.5. Developing an alternative weighting scheme
3. Discussion
3.1. Comparison with Ashmore and Hicks
3.2. Comparison with modelling studies
3.3. Implications for policy of adopting an endorsement approach
4. Conclusions
Acknowledgements
References