Climatic variation and simulated patterns in seedling establishment of two dominant grasses at a semi-arid-arid grassland ecotone

The objective of this study was to predict the effects of climatic variation at multiple temporal frequencies on seedling establishment by two congeneric C-4 perennial grasses (Bouteloua gracilis and B. eriopoda) at the ecotone between shortgrass steppe grassland and Chihuahuan desert grassland in central New Mexico, USA. The approach was to use a daily time-step simulation model to determine the occurrence of a recruitment event in each year based upon the amount and timing of soil water required for establishment. Historical weather data were used to predict effects of seasonal and interdecadal variation in climate on establishment. A sensitivity analysis was used to predict effects of directional climate change on establishment. Bouteloua gracilis had a broad pattern of simulated establishment from May through September that included periods with high year-to-year variation in precipitation. B. eriopoda establishment events occurred primarily in July when precipitation amounts were most reliable. Climatic conditions from 1949 through 1968 were more favorable for B. eriopoda establishment compared to the cooler, wetter conditions from 1969 through 1988 that favored B. gracilis. Establishment of B. eriopoda was lowest in El Nino years whereas B. gracilis establishment was highest in La Nina years. Establishment of B. gracilis was most sensitive to temperature when precipitation was higher than current amounts. The greatest response to temperature by B. eriopoda for all precipitation amounts occurred at cooler temperatures than found currently. These results indicate that climatic variation at multiple frequencies has differential effects on seedling establishment for these two perennial grasses, and may account at least in part for patterns in dominance at this biome transition zone.