Mechanisms of Competition for Nutrients and for Light

Regarding the mechanism of competition for nutrients in soils among terrestrial plants, Tilman's theory relies on the resource-reduction assumption, that is, the species that can reduce the average concentration of nutrient in soil solution to the lowest level R* and persist at this level will be competitively superior for that nutrient. But this theory does not take into account the diffusion of nutrients in soils, which limitation in dry soils often limits uptake and create nutrient-depletion zones around roots. Using a spatially explicit, raster-based model that simulates the diffusion and uptake of nitrate in soil, Craine, Fargione, and Sugita showed in 2005 that plants outcompete other plants by preempting the supply, and not by reducing the average concentration. Differences among plants in the reduction of soil moisture, which primarily affects nutrient diffusion, can cause R* to predict competitive success incorrectly.

Size asymmetry in plant light acquisition complicates predictions of competitive outcomes in light-limited communities. Therefore, it can be argued that, much like interference competition, competition for light is more asymmetric than competition for nutrients or water, and thus is more likely to result in local exclusion. In contrast to the resource-reduction model of symmetric competition for a single soil resource, models of plant competition assuming such asymmetry predict that outcomes of asymmetric competition for light will sometimes depend on the timing of establishment and the consequent hierarchy among species in canopy position.

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