Taxonomic, functional and phylogenetic diversity peaks do not coincide along a compositional gradient in forest-grassland mosaics

Ecosystems with forest and grassland patches as alternative stable states usually contain various closed, semi-open and open habitats, which may be aligned along a vegetation cover gradient. Taxonomic diversity usually peaks near the middle of the gradient, but our knowledge on functional and phylogenetic diversity trends along gradients is more limited. We investigated the eight main habitats of Hungarian forest-grassland mosaics, representing various vegetation cover values, and compared their species composition as well as their taxonomic, functional and phylogenetic diversity. We found a compositional gradient ranging from large forest patches through smaller-sized forest patches and edges to closed and open grasslands. Species richness peaked at the middle of the gradient (at edges). Shannon diversity was high near the middle and at the open end of the gradient. Functional diversity was high throughout woody habitats (in forests and at edges) and was significantly lower in grasslands. When considering all species, phylogenetic diversity tended to peak at north-facing forest edges. When excluding non-angiosperms, this peak disappeared. The high taxonomic diversity at the middle of the gradient is in line with the edge-effect theory. Our results suggest that community assembly in grasslands may be dominated by environmental filtering, while competition may be decisive in woody habitats. The low phylogenetic diversity of grassland habitats can be explained by their young evolutionary age compared to forests. Synthesis. Functional and phylogenetic diversity do not necessarily coincide with taxonomic diversity along vegetation cover gradients. In ecosystems where forest and grassland patches represent alternative stable states, the trends of taxonomic diversity may be similar to those revealed here, but functional diversity patterns may be more system-specific for some traits. Trends in phylogenetic diversity may vary according to the evolutionary age of the habitats.