Regional effects on competition/productivity relationship: a set of field experiments in two distant regions

We studied the effect of productivity on competition intensity and the relationship between competition intensity and community species richness, using a removal experiment with the perennial plant Solidago virgaurea. The experiment was conducted in 16 different communities from two geographically distant areas (western Estonia and northern Norway). The results were compared with the results of previous experiments with Anthoxanthum odoratum from the same areas. Removal of neighbors had a positive effect on the biomass of both Solidago and Anthoxanthum, and this response was stronger in communities with higher productivity. Thus, the corrected index of relative competition intensity, CRCI, increased with increasing community productivity. Species richness was negatively correlated with CRCI in Estonia but not in Norway and not in the case of the pooled material. The results suggest that competitive exclusion operates at least in these communities which species pool is large. Our results indicate that the relationship between competition intensity and productivity is non-linear. In our data, competition prevails in communities where living plant biomass exceeds 200 g m2, whereas in less productive communities