The relationship of plant distribution to geomorphic processes

Background/Question/Methods Traditionally, ecologists have sought to describe plant distribution by gradient analysis, for example, describing abundance of a particular species along a moisture gradient from dry to wet. Tolerance curves obtained from direct and indirect analysis are generally symmetrical and unimodal; however, some studies have found skewed, and bimodal distributions as well. One element common to all gradient analyses has been that the structure of the terrain as it relates to soil moisture has not been considered in any real way. We use a terrain evolution model to create terrains acted upon by different geomorphic processes such as soil creep, threshold landslides, overland flow, saturation-excess flow and erosion thresholds, and use a topographic index to show how soil moisture depends upon the processes at work on a terrain. Hypothetical Gaussian plant tolerance curves are then distributed and analyzed one at a time across each terrain, and direct gradient analysis performed for randomly selected transects along a path of steepest descent towards the channel network. The transects are then averaged for each terrain, and a tolerance curve is returned. 
Results/Conclusions

Results show that the distribution of soil moisture is related to geomorphic processes through the patterns of slope and contributing area sculpted by those processes at work on the terrain. The same plant sampled from different terrains often produces only a portion of the original Gaussian plant tolerance curve, leading to the skewed distributions found in the literature. We thus show that traditional methods of gradient analysis are inadequate as they fail to recognize that different terrains have different distributions of soil moisture.