In mesic regions of the Great Plains, two grass species, Andropogon gerardii and Sorghastrum nutans, account for the majority of plant individuals and determine ecosystem structure and function. Despite functional similarities, fluctuating population sizes, and shared resources, seldom is either species locally extirpated. This co-existence is thought to be mediated by differential timing of tiller emergence of each species. Under current climate conditions, A. gerardii tiller emergence completes during a relatively brief window early in the growing season. In contrast, although the majority of S. nutans tillers also emerge early in the growing season, tiller emergence can occur throughout the growing season. A. gerardii’s up-front tiller investment may provide an early-season advantage, but only in years when water availability is sufficient to sustain growth. Delayed tiller emergence might give S. nutans the advantage with delayed water availability. Thus, with climate changes, such as increased variability in precipitation regimes, the ability of these species to coexist may be altered. To examine processes contributing to mutual persistence of these ecologically important species and whether these are modified by climate change, we assessed population dynamics of these species in a 9-yr experiment that heated and increased the timing between rainfall events without changing total rainfall amount. Late- and early-season tiller densities were measured and intra- and interannual density dynamics were analyzed in plots receiving ambient and altered growing season precipitation.