Ecological behaviour of herbaceous forest species along a pH gradient:: a comparison between oceanic and semicontinental regions in northern France

Aim On the basis of 2402 phytoecological releves, with complete species lists, and real pH measurements resulting from chemical analyses of the top layer of forest soils, this paper compares quantitatively the ecological response of 46 herbaceous forest species along a pH gradient in two regions of northern France. Location The two regions investigated are oceanic north-western France (NW) and semicontinental north-eastern France (NE). Methods For each of the 46 species with more than 50 occurrences in both NW and NE regions, an ecological response curve was computed with simple logistic regression models, and two synthetic numerical values were derived: ecological optimum (OPT) and ecological amplitude (AMP). A comparison of the ecological behaviour of species present in both regions was performed in terms of shift in optimum and/or amplitude. Results All 46 species did reveal a reaction to pH. Our main observation was the relative stability of the behaviour of most species with reference to pH conditions, which is consistent with results from some previous studies. Slight differences regarding the two synthetic parameters (OPT and AMP) were however observed between the NW and NE regions, probably due to a form of competitive release in neutral environments in the NW. Main conclusions Among the main possibilities that could explain a geographical shift in the ecological behaviour of herbaceous species, competitive effects are, we suggest, the most plausible explanation. In the light of former studies, it seems that further analyses of geographical shifts in the ecological behaviour of forest plant species over large areas are needed, such as for example over the European continent.