On the statistical analysis of vegetation change: a wetland affected by water extraction and soil acidification

A case study is presented on the statistical analysis and interpretation of vegetation change without precise information on environmental change. The changes in a vegetation of a Junco-Molinion grassland are evaluated on the basis of relevés of 1977 and 1988 (20 plots) from a small nature reserve on moist oligotrophic, Pleistocene sands in The Netherlands. The changes are attributed to water withdrawal (since 1972) and soil acidification. Their effect on the vegetation is inferred from data on water depth and acidity collected in 1988. Many species typical of moist oligo- or mesotrophic and neutral haylands and several rare species such as Parnassia palustris, Selinum carvifolia and Ophioglossum vulgare decreased in abundance. A few species increased, especially Anthoxantum odoratum, Holcus lanatus and Plantago lanceolatum. A significant decrease was found in the mean Ellenberg indicator value for moisture and for acidity. The mean indicator value for nutrients did not change significantly. Multivariate analysis of the species data by redundancy analysis demonstrated the overall significance of the change in species composition between 1977 and 1988 (P<0.01, Monte Carlo permutation). The spatial and temporal variation in the species data was displayed in ordination diagrams and interpreted in terms of water depth and pH. A simple model is developed to infer the change in water depth and pH from the relevé data and recent data on water depth and pH. Because the correlation between water depth and pH made joint estimation of the changes useless, the change in pH was estimated for a series of likely changes in water depth. For the most likely change in water depth, significant acidification was inferred from the change in vegetation.