Species richness in deciduous forests: Effects of species pools and environmental variables

The study was conducted in deciduous forests of two Swedish regions, Öland and Uppland. It had two objectives: to (1) test the species pool hypothesis by examining if differences in small-scale species richness are related to differences in large-scale species richness and the size of the regional species pool, and (2) to examine the relationship between species richness and productivity and its scale-dependence. The first data set comprised 36 sites of moderate to high productivity. In each site, we recorded the presence of vascular plant species in nested plots ranging from 0.001 to 1000 m2 and measured several environmental variables. Soil pH and Ellenberg site indicator scores for nitrogen were used as estimators of productivity. The second data set included 24 transects (each with 20 1-m2 plots) on Öland in sites with low to high productivity. Species number, soil pH and relative light intensity were determined in each plot. The forest sites on Öland were more species-rich than the Uppland sites on all spatial scales, although environmental conditions were similar. Small-scale and large-scale species richness were positively correlated. The results present evidence in favour of the species pool hypothesis. In the nested-plots data set, species number was negatively correlated with pH and nitrogen indicator scores, whereas a unimodal relationship between species number and pH was found for the transect data set. These results, as well as previously published data, support the hump-shaped relationship between species richness and productivity in Swedish deciduous forests. Two explanations for the higher species richness of the sites with moderate productivity are given: first, these sites have a higher environmental heterogeneity and second, they have a larger ‘habitat-specific’ species pool. Nomenclature: Karlsson (1997).