Submitted to: Journal of Vegetation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/1998
Publication Date: N/A
Interpretive Summary: The geographic distribution of plant species is often related to patterns in climate and may be determined by climatic effects on the recruitment, growth, or mortality of plants. In our study, we evaluated if geographic distributions of two perennial grass species could be explained by patterns in recruitment as affected by climate. We used a simulation model of soil water dynamics to determine the number of years that temperature and soil water conditions were sufficient for the germination and establishment of seedlings for 16 sites along a climatic gradient from northern Colorado to southern New Mexico. We found that establishment of blue grama decreased from north to south, and that establishment increased from north to south for black grama. These patterns are similar to differences in the geographic distributions of these species and indicate that recruitment is an important process affecting species dominance. We also predict that the distributions of each species will shift northward under conditions of global climate change. Our results are important to the management of these ecosystems since blue and black grama respond very differently to grazing by cattle. It is important to understand the controls on their dominance as well as to predict how their distributions of dominance may change under a change in climate.
Technical Abstract: Our objective was to evaluate if establishment patterns predicted from a soil water model could explain the dominance patterns of two perennial, C4 grasses along a climatic gradient from northern Colorado to southern New Mexico, U.S.A. under current climate. We also predicted changes in establishment patterns under climate change scenarios. Bouteloua gracilis dominates the shortgrass steppe from northeastern Colorado to southeastern New Mexico. Bouteloua eriopoda dominates desert grasslands in central and southern New Mexico. Simulated establishment rates for each species were evaluated at 16 sites along the gradient using a daily time step, multi- layer soil water model (SOILWAT) to determine the percentage of years that temperature and soil water criteria for germination and establishment were met. Simulated establishment rates decreased from north to south for B. gracilis, but increased from north to south for B. eriopoda, comparable to observed dominance patterns. The 95% confidence interval around the point at which simulated establishment rates were equal for the two species was near the location of the shortgrass steppe-desert grassland ecotone where both species are abundant. The intersection in establishment rates for the two species was predicted to move farther north when climate was scaled using three Global Circulation Models (GCMs), indicating a possible northward expansion of B. eriopoda. Our results suggest that recruitment by seed is an important process in determining, at least in part, the geographic distribution of these two species. Changes in climate that affect establishment constraints would result in shifts of species dominance that may or may not be accompanied by changes in species composition.