Competition for patchy soil resources reduces community evenness.

The presence of small-scale patches of soil resources has been predicted to increase competition, because multiple species will proliferate roots in the same small area, and therefore decrease plant diversity. I tested whether such patches reduced species evenness in a community of four old-field species, both with and without interspecific interactions. In species mixtures, patches reduced evenness, while in "communities" constructed via combined monocultures, in which species did not compete, patches increased evenness. Therefore, the reduction in evenness in response to patches was due to changes in competition. Community-level changes may be attributable to plant foraging traits-in species with low foraging precision, competition reduced abundance much more in patchy soils than in even soils, while in species with high root foraging precision, the effect of competition was similar in patchy and even soils.