ON THE STATISTICAL-ANALYSIS OF VEGETATION CHANGE - A WETLAND AFFECTEDBY WATER EXTRACTION AND SOIL ACIDIFICATION

A case study is presented on the statistical analysis and interpretati on of vegetation change in a wetland subjected to water extraction and acidification, without precise information on the environmental chang es. The vegetation is a Junco-Molinion grassland and the changes in ve getation are evaluated on the basis of releves in 1977 and 1988 of 20 plots in a small nature reserve on moist oligotrophic, Pleistocene san ds in the Netherlands. The changes are attributed to water extraction (since 1972) and soil acidification and the effect of the environmenta l changes on the vegetation is inferred from data on water depth and a cidity collected in 1988. Many species typical of wetlands decreased i n abundance, including rare species such as Pamassia palustris, Selinu m carvifolia and Ophioglossum vulgatum. Some species increased, notabl y Anthoxanthum odoratum, Holcus lanatus and Plantago lanceolata. A sig nificant decrease was found in the mean Ellenberg indicator values for moisture and acidity. The mean indicator value for nutrients did not change significantly. Multivariate analysis of the species data by Red undancy Analysis demonstrated the overall significance of the change i n species composition between 1977 and 1988 (P < 0.01, Monte Carlo per mutation). The spatial and temporal variation in the species data was displayed in ordination diagrams and interpreted in terms of water dep th and pH. A simple model is developed to infer the change in water de pth and pH from the releve data and recent data on water depth and pH. Because the correlation between water depth and pH made a joint estim ation of the changes useless, the change in pH was estimated for a ser ies of likely changes in water depth. For the most likely change in wa ter depth, significant acidification was inferred from the change in v egetation. The model is more generally applicable as a constrained cal ibration method.