Abstract
It remains unclear how competitive exclusion is avoided between two ecologically, economically, and culturally important codominant grass species in the tallgrass prairie of the Great Plains, Andropogon gerardii and Sorghastrum nutans. These functionally similar C4 grasses appear to coexist despite considerable niche overlap, and asymmetric competition and drought tolerance in favor of A. gerardii. According to the stress gradient hypothesis, it may be that the sum of interactions between these species, which is typically negative (competitive) due to similar resource requirements, shifts to positive (facilitative) as abiotic stress increases. For instance, if the canopy cover of the stronger competitor reduces losses of subcanopy humidity or shallow soil moisture, recruitment of S. nutans tillers may be extended further into the drought event than would occur in the absence of A. gerardii. As later months of the growing season are drier on average where these species are codominant, such a mechanism may enable S. nutans to recover from early season asymmetric competition and stabilize their codominance. We tested this hypothesis in a greenhouse experiment in which we manipulated community composition and water availability in the latter half of the growing season. We found no evidence that a shift from a negative to positive interaction occurs, with each species performing similarly in mixed communities and monocultures. The similarities of the two species in their functional traits and responses to water limitations may limit such a shift in interaction net effects and suggests that other mechanisms are determining coexistence of these co-occuring C4 grasses.
A Shift From Competition To Facilitation With Abiotic Stress is Limited For Two Codominant Grass Species
