Local plant species richness increases with regional habitat commonness across a gradient of forest productivity

We tested the prediction that forest habitat types with relatively high productivity are not only relatively low in species richness but are also regionally uncommon. This relationship was supported by an analysis of data from 146 forest communities in southern Ontario, Canada. Potential forest habitat productivity was determined based on a classification scheme developed for the Canadian Land Inventory (CLI) project. Vascular plant species richness approximated a unimodal distribution across forest productivity classes with the lowest mean species richness recorded for the two most productive classes. The contemporary regional commonness of forest habitat productivity classes were also displayed as a unimodal frequency distribution. Hence, mean species richness per CLI class was positively correlated with the regional area of land encompassing each of these productivity classes and this relationship was increasingly significant at increasingly larger spatial scales of regional CLI class land areas. These results are consistent with the species pool hypothesis, which postulates that species richness is relatively low in highly productive habitats because such habitats have been relatively uncommon in both space and time and hence, have had relatively little historical opportunity for the origination of adapted species.