Symmetry of below-ground competition between Kochia scoparia individuals

Competition among individual plants is usually asymmetric, i.e. larger plants are able to obtain a disproportionate share of the resources (for their relative size) and suppress the growth of smaller individuals. There is evidence that the asymmetry of competition is primarily due to shading, but there is very little information about the symmetry of competition below ground. We grew Kochia scoparia individuals singly and in pairs in containers for 54 d with dividers above ground so that competition could occur only below ground. Initial size differences were generated by a 10-d difference in sowing date. There was no evidence that larger individuals had a disproportionate effect on smaller individuals; the effect of a small neighbor on the growth rate of a plant was similar for large and small plants, as was the effect of a large neighbor. The results demonstrate that competition for resources below ground can be symmetric. When competition is symmetric, it will not exacerbate initial size differences.