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

Authors: Porensky, Lauren; Derner, Justin; PELLATZ, DAVID - Thunder Basin Grasslands Prairie Ecological Association

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2018
Publication Date: 8/9/2018
Citation: Porensky, L.M., Derner, J.D., Pellatz, D.W. 2018. Plant community responses to historical wildfire in a shrubland-grassland ecotone reveal hybrid disturbance response. Ecosphere. 9(8):302363. https://doi.org/10.1002/ecs2.2363.
DOI: https://doi.org/10.1002/ecs2.2363

Interpretive Summary: Wildfire is a prominent driver of ecosystem structure and function, and fire regimes vary dramatically across ecosystems. We investigated multi-decadal plant community responses to wildfire in a 7,000 km2 ecotone between mixed-grass prairie and sagebrush steppe, two of North America’s most dominant ecosystems. These two systems have dramatically different historical fire regimes and sensitivity to fire. 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 1937-2012. 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. We found that shrub 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, the understory plant community response to fire was 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 functionality may provide new opportunities for targeted management within ecotones.

Technical 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, two ecosystems with 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 1937-2012. 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 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, the understory plant community response to fire was 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 functionality may provide new opportunities for targeted management within ecotones.