In this study we analysed the relationships between the species composition/richness of moist forests and environmental variables (light conditions, soil pH, S-value, base saturation, C/N-ratio, lime content, soil moisture, thickness of A- and G _r-horizons) on the basis of 38 soil sample plots (northern Germany, Schleswig-Holstein) and a principal component analysis. In addition, we tried to explain the species richness of moist forests in the context of the species pool hypothesis by comparing the local species richness with the regional species pool. Taking moist forests as an example our results confirm the findings of other studies that the species richness of basiphilous forest communities is due to the great number of basiphilous species in the Central European forest flora. In deciduous forests in northern Germany in general the species richness increases with increasing base supply. However, particularly in moist forests the species composition/richness is affected mainly by the soil moisture, as this site parameter is responsible for a certain site heterogeneity and niche availability, and therefore often masks the influence of other site factors. The lime content in the subsoil and the base supply are of subordinate importance for the species composition and are little (negatively) or not correlated with the species richness, respectively. Light conditions and the C/N-ratio have almost no influence on the species richness. The results present evidence in favour of the species pool hypothesis.