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Surveys of ground layer vegetation are a reliable tool for monitoring spatial and temporal variations of the forest environment. In 1991, 47 vegetation plots first sampled in 1972 in a forest of the Lorraine Plain, North‐East France, were resampled. This beech–oak forest had both acidic and calcareous soils depending on the thickness of alluvial deposits above the calcareous bedrock. The mean number of species per sample increased from 32 to 39 during this period and the frequency of nitrophilic species increased throughout the forest. On acidic soils, acidophilic species increased in frequency, while on calcareous soils there were no systematic changes. Multivariate statistical analysis clearly distinguished two gradients in this set of vegetation samples: acidity and nitrogen availability of soils. Along the first gradient, a shift of the plots towards a more acidic state occurred during the past 19 years on acidic soils …