Plant size is generally seen as a key trait influencing competitive ability, due to the nature of size-asymmetric competition. The degree to which larger individuals gain a competitive advantage over smaller individuals (size-asymmetric competition) can drive changes in community assembly through the competitive exclusion of small and slow growing individuals. However, the factors which govern the degree of size-asymmetric competition within a community are fairly poorly explored and can be roughly summarized as competition for light is size-asymmetric, while competition for soil resources is size-symmetric. This binary view of size-asymmetric competition, however, has been challenged. There are predictions that under certain nutrient distributions, where larger individuals could pre-empt nutrients, or if there is nutrient sharing between individuals through microbial interactions, competition for soil resources could alter the degree of size-asymmetric competition. Here, using experimental mesocosms, we test these predictions by examining how soil fertility, nutrient distribution (i.e. high- and low-fertility patches), and the initial suppression of microbial communities alters the degree of size-asymmetric competition within a three-species community. We manipulated both initial plant size and soil resources to determine how the competitive response of individuals change as a function of their initial size under the various soil treatments.
Results/Conclusions
The intensity of competition changed in response to the soil treatments; however, the degree to which the competition was size-asymmetric depended on which species in the community was examined. Overall, soil fertility and nutrient distributions significantly altered the degree of size-asymmetric competition within two species, while the initial microbial suppression had no effect on any species. Our findings do not support the predictions of how soil resources could alter the degree of size-asymmetric competition. We found no effects of microbial communities and nutrient distributions where larger individuals could pre-empt nutrients (i.e. high-fertility patch) either had no effect or decreased size-asymmetric competition. On the other hand, nutrient distributions where a low-fertility patch was present led to a significant increase in size-asymmetric competition. This suggests a need for a revision of the theory behind the size-asymmetry of belowground competition. Based on our data, we propose competition for soil resources can be size-asymmetric when nutrients are heterogeneously distributed, but due to larger plants constraining smaller neighbor root growth to low-fertility patches and not due to their pre-emption of high-fertility patches. These results provide some first steps to understanding the size-asymmetry of belowground competition, which aids our understanding of plant interactions and community assembly.