Topographically determined water availability shapes functional patterns of plant communities within and across habitat types

Plant-environment relationships can be assessed through functional traits, but we have little understanding of how they vary on larger scales due to limited sampling. Using a fine-grained digital elevation model and vegetation survey data from a national monitoring program, we now have the chance to investigate the importance of topographically determined water availability in shaping the functional structure of vegetation of different habitat types across Denmark. Plant community responses to hydrology were detected through community-weighted Ellenberg F values and six community-weighted functional traits. We used mixed-effect models to account for the variability related to unknown site-specific factors such as management regime and regional species pool. Additionally, we evaluated whether we can trust a remote-sensing-based topographically determined water availability index (TWI) that calculates how water accumulates on the surface of the landscape to represent actual hydrology. Remote-sensing-based topographically determined water availability represented actual local water availability as indicated by a positive correlation with community-weighted Ellenberg F values (P < 0.001), showing that this is an effective method of measuring water availability at large scale. The strength and direction of vegetation-TWI relationships differed between habitat types. Functional responses were also habitat dependent and to a certain degree explained by non-considered site-specific factors which presumably include historical land use and current management. This study contributes to the understanding of plant-water relationships which is highly relevant, as the hydrological regime might change rapidly in the near future with potential prevalence of extremes in the hydrological environment.