Evidence for size asymmetry of belowground competition

Theory and previous experimental results suggest that competition for light is size asymmetric, while competition for soil resources is generally size symmetric. However, root competition may be size asymmetric if resources are patchily distributed and larger plants are more likely to reach resource-rich patches. I tested for size symmetry of belowground and total competition in three herbaceous species growing in unhomogenized field soil. In both types of competition, larger individual Bromus inermis neighbors (those germinating two weeks before the target) had greater per-gram effects on Bromus targets than smaller individual neighbors (those germinating two weeks after the target), indicating size asymmetric competition. Size asymmetry was not detected for Bromus neighbors with Hieracium caespitosum and Achillea millefolium targets. The role of resource heterogeneity was not tested directly, but results from this experiment suggest that patchy resources may favor larger plants. Most importantly, this study demonstrates that belowground competition can be size asymmetric.