Local and regional differences in abundance of co-dominant grasses in the shortgrass steppe: a modeling analysis of potential causes

The shortgrass steppe is co-dominated by two C-4 perennial grasses, Bouteloua gracilis and Buchloe dactyloides. At our site and throughout eastern Colorado Bouteloua gracilis has higher cover and biomass than Buchloe dactyloides. We hypothesized that the interaction between seedling recruitment differences and disturbance regime and tolerance to drought conditions were the most likely causes of the observed differences in relative abundances. We used a simulation model to investigate the relative importance of the three factors. We studied plant biomass of these two species in 18 simulated treatments that resulted from a factorial combination of seedling recruitment differences, disturbance regime, and tolerance to drought conditions. Analysis of the simulation outputs with ANOVA indicated that biomass of each species responded positively to increases in recruitment probability. Increasing disturbance frequency favored Buchloe dactyloides over Bouteloua gracills, whereas the susceptibility Buchloe dactyloides to drought favored Bouteloua gracilis. The results indicated that differences in drought tolerance and seedling recruitment probabilities along with their interactions with disturbance regimes exert a major control on the biomass of the species. In the absence of or with intermediate disturbance, a higher recruitment probability and greater tolerance to drought of Bouteloua gracilis than Buchloe dactyloides yielded patterns of relative biomass similar to the patterns observed in the shortgrass steppe.