Plant community responses to historical wildfire in a shrubland–grassland ecotone reveal hybrid disturbance response

Abstract

Most ecotones include structural and taxonomic elements from both adjacent communities, but it remains unclear how these elements function and interact within ecotones. We investigated long-term plant community responses to wildfire in a 7000-km2 ecotone between mixed-grass prairie and sagebrush steppe ecosystems, which have dramatically different historical fire regimes. We asked whether plant community responses to wildfire in the ecotone were more similar to mixed-grass prairie, sagebrush steppe, or a hybrid of these two. We sampled plant community composition at 70 pairs of transects located inside and outside of wildfires that burned from 1937 to 2012. We determined whether (1) wildfires predicted plant community composition, (2) plant community response to fire varied based on abiotic factors, and (3) effects of wildfire varied based on time since fire. Plant community responses to wildfire did not vary substantially across the study region, despite continuous plant community variation in response to abiotic factors. Overstory responses were characteristic of sagebrush steppe. Burned transects had <10% as much big sagebrush cover as unburned transects, and cover did not increase with time since fire. In contrast, understory plant community responses to fire were similar to mixed-grass prairie. Burned sites had high forb cover in the short term and perennial grass cover in the long term. Within an ecotone, different components of the plant community can maintain functional fidelity to their home ecosystems, despite being spatially juxtaposed. The idea of hybrid disturbance response may provide new opportunities for targeted management within ecotones.