Can root competition be size asymmetric?
There is much evidence that plant competition below ground is generally size symmetric, i.e. that competing plants share contested resources in proportion to their sizes. Several researchers have hypothesized that a patchy distribution of soil nutrients could result in size-asymmetric root competition, in which larger plants obtain an "over proportional" share of the contested resources by reaching and pre-empting nutrient rich patches before the roots of smaller individuals arrive. We tested this hypothesis in a greenhouse experiment with Triticum aestivum (wheat) individuals of different initial sizes grown alone or with below-ground competition from one neighbor. Plants were grown in tall, narrow containers under three different soil treatments: (1) a nitrogen-poor sand mixed field soil, (2) nitrogen fertilizer added uniformly within the upper 50 cm of the container, and (3) the same amount of fertilizer added in a layer 20-30 cm below the surface. We measured total leaf length of individuals non-destructively throughout the course of the experiment, as well as final aboveground biomass at harvest. We also measured root depth and root frequency over time in a subset of containers that were transparent.
Root competition was very strong: competing plants were close to half the size of non-competing plants. Root competition was partially size-asymmetric. Per-unit-size effects of neighbors larger than a target were greater than those of neighbors smaller than the target. Size variation increased over time for competing pairs of different initial sizes, while variation decreased for noncompeting pairs. A high nutrient patch did not increase the degree of size asymmetry. On the contrary, the high nutrient patch reduced competition and the degree of size asymmetry temporarily. Size asymmetry in poor, deep soils may result from directionality in resource interception as roots compete for limiting nutrients by growing deeper. In conclusion, partially size-asymmetric root competition was observed in pairs of plants growing in a relatively deep, nutrient-poor soil. Spatial heterogeneity of resources did not increase the degree of size asymmetry. Root competition can be partially size asymmetric, but not to the same degree as competition for light.