Topographically controlled soil moisture is the primary driver of local vegetation patterns across a lowland region

Topography is recognized as an important factor in controlling plant distribution and diversity patterns, but its scale dependence and the underlying mechanisms by which it operates are not well understood. Here, we used novel high-resolution (2-m scale) topographic data from more than 30500 vegetation plots to assess the importance of topography for local plant diversity and distribution patterns across Denmark, a 43000 km(2) lowland region. The vegetation data came from 901 nature conservation sites (mean size = 0.16 km(2)) distributed throughout Denmark, each having an average of 34 plots (five-meter radius) per site. We employed a variety of statistical measures and techniques to investigate scale dependence and mechanistic drivers operating within the study region. Ordinary Least Squares (OLS) multiple regression modeling scaled at different spatial resolutions (2x2, 10x10, 50x50, 100x100 and 250x250 m) was used to identify the horizontal resolution yielding the strongest vegetation-topography relationships. Using data scaled at this resolution, we quantified local (within-site) and regional (among sites) relationships between elevation, mechanistic topographic factors (slope, heat index, potential solar radiation, wind exposure, wetness index) and 10 vegetation measures representing species composition, richness and functional composition (average plant preferences along key environmental niche axes). We also investigated how overall site-level environmental characteristics affect the strength of these local relationships. Topography exerted the strongest effects at the 10x10 m horizontal resolution scale. Elevation exerted the strongest influence on vegetation, followed by slope and wetness. Topography generally affected all vegetation measures and exhibited the strongest local relationships with the main species-compositional gradient, the main functional gradient and the plant's average soil moisture preference. The strength of these relationships was strongly influenced by habitat and site-level average moisture conditions, with the strongest relationships found in wet habitats. Our findings show that fine-scale topography can strongly influence local vegetation patterns across a wide range of habitat types even in low-relief lowland regions. Notably, topography exhibited a consistently strong relationship with the main local floristic and functional compositional gradients. While a plurality of underlying mechanisms may contribute to the relationship between topography and vegetation patterns, topographically controlled soil moisture exerts primary control on the relationship.