Ecological ranges for the pH and NO3 of syntaxa: a new basis for the estimation of critical loads for acid and nitrogen deposition

Question: Can the abiotic ranges of syntaxonomic units (associations) in terms of pH and nitrate concentration be estimated and then in principle be used to estimate critical loads for acid and nitrogen deposition? Location: Europe. Methods: Using splines, abiotic ranges of syntaxonomic units were estimated on the basis of measured soil pH and nitrate concentration and vegetation relevés. Owing to limited calibration data, this yielded responses for only a few syntaxa. In a second attempt, we used a 160 000-relevé training set; the syntaxon of each relevé was known but not their soil pH values and soil nitrate concentrations, so for each relevé, the soil pH and nitrate concentration were estimated by inference from species composition. We again estimated abiotic ranges using the spline method, including the 5th and 95th percentiles as a proxy for the ecological range. Results: The second (indirect) method yielded ranges for soil pH and for nitrate concentration for many of the associations. The means and percentiles were corrected for regression to the mean. Conclusions: It is not yet possible to directly estimate ranges for syntaxa for pH and nitrate on a large scale from the data available. However, indirectly estimated soil pH and nitrate concentrations are sufficiently available to derive ranges for many associations. The lower (5th) percentile of the indirectly estimated pH ranges for associations may be used as the starting point for the estimation of critical loads for acid deposition. The 95th percentile should still be regarded as a rather uncertain estimate of the maximum nitrate concentration