Bimodal spectra of nutrient indicators reveal abrupt eutrophication of pine forests

Five hundred and eight phytosociological relevés from pine forests on sand, calcareous gravel and rock in NE and S Germany were analysed with respect to the frequency of Ellenberg indicator values of vascular plants for nutrients (N). Principal component analysis revealed that after the average nitrogen value (mN), the distribution shape and modality are the second most important sources of variation in the N-spectra of relevés. Of the five spectral types defined by combinations of mN and modality, the unimodal low nutrient type (66.5%) prevailed, followed by bimodal distributions with many indicators for low and high N-supply, with few in the intermediate classes 4 and 5 (27.4%), whereas spectra with a single mode at high (3.9%) or intermediate (2.2%) N- values were rare. Two explanations for the frequent coexistence of vascular plant indicators of N-deficiency with those indicating eutrophication are discussed: (a) Bimodality may be a consequence of the low capacity of pine forests to sequestre the excess input of anthropogenic nitrogen from the atmosphere, and/or (b) the natural dynamics of humus accumulation and mineralization following disturbance. To avoid misinterpretation of mN, inspection of modality of the N-spectra should be standard practice when analysing pine forest or other long-lived vegetation with low N-sequestration. Predominance of high N- over low N-indicators in relevés may be interpreted as a signal of advanced anthropogenic eutrophication, N-saturation and increased risk of N-leaching to groundwater. Bimodal spectra with prevailing deficiency indicators, on the other hand, may be either due to short-term N-release or indicate the beginning of eutrophication.