Competition amongst perennial bunchgrasses in a semi-arid grassland

Interplant competition structures many plant communities and determines the response of these communities to environmental change. However, the importance of competition in arid and semi-arid systems is debated. To address this controversy, we investigated the importance of competition in shaping vegetation patterns on north- and south-facing slopes in a semi-arid grassland at Wallula Gap Biological Station, WA. Using overdispersion as an indication of competition, we characterized the spatial patterns of large bunchgrass (Pseudoroegneria spicata, Festuca idahoensis, and several species of Poa) and rabbitbrush species (Ericameria nauseosa and Chrysothamnus viscidiflorus). We hypothesized that competition between bunchgrass and rabbitbrush species limits their establishment and growth. Additionally, to investigate competition as a potential cause of the higher abundance yet smaller size of bunchgrass plants on north-facing slopes relative to south-facing slopes, we had two competing hypotheses: (1) Greater perennial plant density on the north-facing slope causes greater intensity of competition on this slope. (2) More difficult seedling establishment on the south-facing slope causes greater competition at the time of seedling establishment on this slope. Surprisingly, our findings showed overdispersion to be rare and clumping to be common, regardless of slope aspect. Therefore, we concluded that competition does not determine spatial patterns of bunchgrass and rabbitbrush species in this semi-arid grassland. The clumped distribution we observed could result from processes including clumped plant die-off events and locally variable seedling recruitment caused either by an uneven distribution of seeds in the landscape or by environmental heterogeneity. In comparing the north- and south-facing slopes, we observed greater clumping of bunchgrass and rabbitbrush species on the hotter and drier south-facing slope. This suggests that ongoing climate warming may lead to an even greater degree of clumping on the north-facing slope, as the north-facing slope warms and becomes more like the south-facing slope.