DRAINAGE BASIN MORPHOLOGY AS A FOUNDATION FOR PLANT COMMUNITY ORGANIZATION

The distribution and composition of plant communities are in large part determined by moisture-nutrient gradients. Topographic structure of drainage basins plays a large part in determining the moisture-nutrient gradient. Landscapes are made up of valleys and ridges, with hillslopes occupying areas in between them. Since hydrological flow paths are directed according to slope gradient, processes which determine slope morphology and arrangement are influential in plant community organization and dynamics. Wetness values, which encapsulate flow routing and water storage, are a function of soil transmissivity, contributing area (hillslope length is often used in its place) and slope gradient. Nutrient transport and storage along hillslopes in large part follow wetness values. Plant species have different tolerances to wetness and nutrients, which are reflected in plant tolerance curves.

Despite the dynamic nature of landscape morphology, attempts have been made in the literature to put forth robust topographic scaling relations which define landscape structure in certain situations. In this paper, connections between landscape structure and associated plant community organization are examined. We explore how hillslope length and steepness, both of which control species distribution and abundance via soil moisture (along with substrate), are arranged in basins. Hillslope length decreases with increasing drainage density, such that landscapes with greater dissection will have lower moisture values at the slope base (all else being equal). Following from this, it becomes critical to ask if there is a relation between drainage density and slope gradient. In fact, both positive and negative relations have been put forth in the literature for landscapes of different overall relief. Based on these relations, we construct speculative moisture gradient curves (based on wetness indices) to determine the proportion of the basin over which particular species exist. We then compare these proposed moisture gradient curves with those calculated using digital elevation models (DEMs) for a variety of landscapes and regional settings to assess how drainage basin structure affects patterns of wetness and vegetation.