Larger Triticum aestivum plants do not preempt nutrient-rich patches in a glasshouse experiment

Plant competition belowground generally appears to be size-symmetric, i.e. larger plants only obtain a share of belowground resources proportional to their size, and therefore do not suppress smaller individuals. The experimental evidence for size-symmetric belowground competition comes primarily from experiments with homogenous soil conditions. It has been hypothesized that the presence of high nutrient patches that can be pre-empted by larger plants can make competition belowground size-asymmetric. We tested this hypothesis by growing Triticum aestivum individuals singly and in pairs in containers with aboveground dividers so that competition occurred only belowground. Plants grew in either a homogenous soil mixture, or in the same mixture with a band of enriched soil between them. Initial size differences were generated by a seven day difference in sowing date. There was no evidence of size-asymmetric competition with or without soil heterogeneity. Large plants did not have a disproportionate effect on smaller plants, nor did they perform disproportionately better when paired with a small neighbor. Our results suggest that in heterogeneous soil conditions, roots of larger plants that reach nutrient patches first are not able to prevent roots of smaller plants that arrive later from obtaining resources from the patch.