Quantification of the intra-plot correlation in plant abundance data: A possible test of the neutral theory

A method for summarising the degree of spatial covariance (intra-plot correlation) among different plant species using plant abundance data is presented. The method will provide an alternative test of the hypothesis of neutrality in uniform environments, and we apply it in this study to data derived from two different grasslands. There is a significant positive intra-plot correlation in both grasslands that may be due to either sampling effects or deterministic ecological effects. For the calcareous grassland, we also applied the method to a subset of plant species that are expected to be less influenced by sampling effects, and they also has a significant positive intra-plot correlation, which suggests that sampling effects are playing a minor role in the calcareous grassland as an explanation for the spatial aggregated of species. Overall, the results suggest that the deterministic ecological processes that do not contradict the neutral theory are more important than any possible deterministic ecological processes that contradict the neutral theory. Consequently, the hypothesis of neutrality could neither be rejected nor corroborated in the investigated grasslands. We suggest that the spatial aggregation in the sampled calcareous grassland is mainly caused by small local differences in abiotic factors or in the timing of gaps for recolonisation. This in turn may lead to different local niches (ecological adaptation or phenology of establishment from seeds) and possibly local exclusion of plant species by interspecific competition.