Thriving in an unfriendly environment: Persistence of sagebrush in a fire-dependent landscape

Fire refugia and patchiness are important to the persistence of fire-sensitive species and may facilitate biodiversity conservation in fire-dependent landscapes. Playing the role of ecosystem engineers, large herbivores alter vegetation structure and can reduce wildfire risk. However, herbivore effects on the spatial variability of fire and the persistence of fire-sensitive species are not clear. To examine the hypothesis that large herbivores support the persistence of fire-sensitive species through the creation of fire refugia in fire-prone landscapes, we examined the response of a fire-sensitive plant, Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis [Beetle & Young]) to fire and grazing in the fire-dependent mixed-grass prairie of the northern Great Plains. We carried out a controlled burn in 2010 within pre-established exclosures that allowed differential access to wild and domestic herbivores and no record of fire in the previous 75 years due to fire suppression efforts. The experiment was set-up with a split-plot design to also examine potential changes in plots that were not burned. Canopy cover of big sagebrush was recorded before the burn in 2010 and again in 2011 with percent area burned recorded within one-month post-fire in the burned plots. Percent area burned was the greatest in ungulate exclosures (92 ± 2%) and the least in open areas (55 ± 21%) suggesting that large herbivores influenced fire behavior (e.g. reducing fire intensity and rate of spread) and likely increased fire patchiness through their alterations to the fuel bed. Regression analysis indicated that the proportion of sagebrush cover lost was significantly correlated with the proportion of area burned (R2 = 0.76, p = 0.05). No differences in the non-burn plots were observed among grazing treatments or among years. Altogether, this illustrates the potential importance of large herbivores in creating biotic-driven fire refugia for fire-sensitive species to survive within the flammable fuel matrix of fire-dependent grassland ecosystems like the mixed-grass prairie. Our findings also attest to the resiliency of the northern Great Plains to fire and herbivory and underscore the value of managing grasslands for heterogeneity with spatial and temporal variations in these historic disturbances.