Topographically controlled soil moisture drives plant diversity patterns within grasslands

Grasslands are recognized as biodiversity hotspots in Europe. However, protection and management of these habitats are currently constrained by a limited understanding of what determines local grassland plant diversity patterns. Here, we combined vegetation records (8,639 inventory plots) from 258 semi-natural grasslands with fine-resolution topographic data based on light detection and ranging technology to investigate the importance of topography-particularly topographically controlled soil moisture-for local and regional grassland plant diversity patterns across a 43,000 km(2) lowland region (Denmark). Specifically, we examined the relationships between five vegetation measures representing species composition and richness as well as functional composition (Ellenberg indicator values) and four functional topographic factors representing topographic wetness, potential solar radiation, heat balance and wind exposure. Topography emerged as an important determinant of diversity patterns in both wet and dry grasslands throughout the study region, with topographic wetness being the strongest correlate of the main local (within-site) and regional (among-sites) gradients in species composition and species' average preferences for soil moisture. Accordingly, topography plays an important role in shaping grassland plant diversity patterns both locally and regionally throughout this lowland European region, with this role mainly driven by topographically controlled soil moisture. These findings suggest hydrology to be important to consider in the planning and management of European grasslands.