Submitted to: Journal of Vegetation Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2000
Publication Date: 8/1/2000
Citation: PETERS, D.C. CLIMATIC VARIATION AND SIMULATED PATTERNS IN SEEDLING ESTABLISHMENT OF TWO DOMINANT GRASSES AT A SEMIARID-ARID GRASSLAND ECOTONE. JOURNAL OF VEGETATION SCIENCE. 2000. V. 11(4). P. 493-504. Interpretive Summary: The recruitment of new plants is an important process in determining patterns in species dominance on rangeland, especially for areas where two different plant community types can exist. These areas are called ecotones, and a better understanding is needed of the processes that determine dominance by one species over another. We analyzed the effect of fvariation in climate through time on recruitment of two perennial grasses that dominate different rangeland types: blue grama dominates shortgrass steppe rangelands and black grama dominates desert rangelands. We used a simulation model of soil water dynamics to determine the effects of seasonal and 20-year cycles of climate variation on recruitment. We also analyzed the importance of directional changes in climate on recruitment by these two species. We found that blue grama recruitment occurred mainly in September and October when year-to-year variation in precipitation was high. By contrast, black grama recruitment occurred in July when precipitation amounts are most reliable. Climatic conditions from 1949 to 1968 were found to favor black grama, whereas conditions from 1969 to 1988 favored blue grama. A long-term increase in precipitation was predicted to shift the boundary between shortgrass steppe and desert grasslands to the north of its current boundary in southern Colorado. Our results indicate that differences in recruitment patterns between these two species through time may account for observed patterns in species dominance and boundaries between rangeland types.
Technical Abstract: Seedling establishment by two congeneric C4 perennial grasses (Bouteloua gracilis and B. eriopoda) that dominate semiarid and arid North American grasslands was simulated by modeling multiple temporal frequencies of climate at their ecotonal boundary. The occurrence of a recruitment event in each year was determined by comparing simulated soil water content through time with amount and timing of soil water required for establishment based on data in the literature for each species. The two Bouteloua species had different regeneration strategies. B. gracilis had a broad pattern of establishment that occurred from June through October with a peak in September and October when year-to-year variation in precipitation was high. B. eriopoda had a narrow distribution of establishment events that peaked in the middle of summer (July) when precipitation amounts were most reliable. Interdecadal variation in climate had species-specific effects on establishment. Climatic condition 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 negatively affected by El Nino events, whereas B. gracilis establishment was positively affected by conditions associated with El Nino events. A long-term directional increase in annual precipitation resulted in a small shift in the geographic location of the ecotonal boundary based upon the location of similar establishment probabilities for the two species. These results indicate that temporal partitioning of soil water required by seedlings may account for species coexistence at an ecotonal boundary.