Pattern of local plant species richness along a gradient of landscape topographical heterogeneity: result of spatial mass effect or environmental shift?

Several processes are hypothesised to mediate the relationship between local (microsite) plant species richness and the topographical heterogeneity of the surrounding landscape. In a topographically heterogeneous landscape with various habitats occurring close to each other, local species richness may be enriched by species from surrounding habitats due to the spatial mass effect (sink‐source dynamics). In contrast, increased habitat fragmentation due to spatial heterogeneity may have a negative effect on local species richness. The spatial mass effect is thought to be more pronounced in communities with a higher ratio of generalists, as generalists are more likely to establish viable populations in sink habitats. To reveal the pattern of local species richness along a gradient of landscape topographical heterogeneity at middle altitudes of the Bohemian Massif, we used 2551 forest vegetation plots stored in the Czech National Phytosociological Database. We developed an analytical approach relating the pattern of local species richness of vegetation types to the gradient of landscape topographical heterogeneity. An increase or decrease in species richness with increasing landscape heterogeneity was related to changes in the generalist/specialist ratio, and also to changes in soil reaction and productivity estimated through Ellenberg indicator values. Local species richness along a gradient of increasing landscape heterogeneity increased in nutrient‐poor vegetation and decreased in nutrient‐rich vegetation. Nutrient‐poor vegetation types, such as thermophilous and acidophilous oak forests, also had a high proportion of habitat generalists, supporting the hypothesis that increased richness in heterogeneous landscapes may result from the spatial mass effect. However, the same pattern may be explained by a shift in environmental conditions along the landscape heterogeneity gradient, such as increasing productivity of nutrient‐rich vegetation types or increasing soil reaction of most vegetation types in more heterogeneous landscapes. We discuss available evidence and conclude that these two explanations need not be mutually exclusive.