Resource use patterns predict long-term outcomes of plant competition for nutrients and light

An 11-year competition experiment among combinations of six prairie perennial plant species showed that resource competition theory generally predicted the long-term outcome of competition. We grew each species in replicated monocultures to determine its requirements for soil nitrate (R*) and light (I*). In six pairwise combinations, the species with the lower R* and I* excluded its competitor, as predicted by theory. In the remaining two pairwise combinations, one species had a lowerR*, and the second had a lower I*; these species pairs coexisted, although it is unclear whether * I resource competition alone was responsible for their coexistence. Smaller differences in R* or I* between competing species led to slower rates of competitive exclusion, and the influence of differences on the rate of competitive exclusion was more pronounced on low-nitrogen soils, while the influence I* of differences was more pronounced on high-nitrogen ( low-light) soils. These results were not explained by differences in initial species abundances or neutrality. However, only a few of our paired species coexisted under our experimentally imposed conditions ( homogeneous soils, high seeding densities, minimal disturbance, regular water, and low herbivory levels), suggesting that other coexistence mechanisms help generate the diversity observed in natural communities.