COS 64-4 - Comparative responses of native and invasive perennial bunchgrasses to fire, grazing, and their interaction

Abstract: Crested wheatgrass is a C3 perennial bunchgrass that was seeded extensively across the western USA for livestock forage. It competes strongly with native species, often reducing native species diversity in areas where it invades. It also has the potential to reduce grassland productivity over the long-term because it has lower root biomass than native species, reducing the amount of nitrogen and carbon in grassland soils over time. We assessed the potential to alter competition between crested wheatgrass and native grasses by prescribed burning, burning followed by a single grazing event, and repeated grazing after burning in a replicated field study. We measured productivity, forage quality, soil moisture, soil microbial, and plant photosynthetic responses in each treatment and compared plant-level response variables across species. Grazing and fire together reduced crested wheatgrass biomass. There was slightly lower native bunchgrass biomass in burned than mowed treatments but the difference was not substantial; there were no differences across grazing treatments. Fire enhanced photosynthesis for both species, but that effect lasted longer for the native bunchgrass. Grazing had no effect on photosynthesis in either species. This suggests that despite a slight fire-related reduction in native bunchgrass biomass, a potential longer-term positive impact of fire on native bunchgrass productivity was not realized in the current study. This is likely because of the short duration of the growing season following burning and overriding drought effects on the productivity of both species. The implications are that fire and grazing could enhance native bunchgrass competition with crested wheatgrass in years with sufficient moisture for continued growth or when fire is conducted sufficiently early to permit a longer period of regrowth before senescence.