Niche overlap and species assemblage dynamics in an ageing pasture gradient in north-western France

The first axis of the CA arranged samples according to the pasture ageing gradient and separated “pioneer” (CA1−) from “old pasture” (CA1+) species assemblages. The second axis segregated two different assemblages (CA2− and CA2+) that were consistently represented along the ageing gradient and was assumed to represent intra-plot assemblage heterogeneity. Niche overlap according to soil organic C, C:N ratio and root biomass was higher than expected by chance (EBC) in most assemblages, and was higher when calculated for the whole regional species pool than for local assemblages. Morphological dispersion was random or lower than expected by chance for the regional species pool and both CA1− and CA1+, and higher than expected by chance for both CA2− and CA2+. These results indicate that: (1) habitat and dispersal constraints act as filters by allowing only those species with similar prerequisite traits into assemblages; (2) inter-specific competition limit composition in a further step by calling for a minimal level of overdispersion in morphological traits.