Many contemporary fire regimes are considered altered relative to historical conditions, but a diversity of causes and fire effects exist. Here we examined the effects of recurrent fire on two highly interspersed sagebrush communities in the channeled scablands of the Columbia Plateau: big sagebrush (A. tridentata) occurring on silt “mounds” and scabland sagebrush (A. rigida) on rocky, thin-soiled “flats.” The study area served as a natural experiment, with three overlapping wildfires resulting in once-, twice-, and thrice-burned areas, as well as unburned, in both sagebrush communities. Study objectives were to better understand how different sagebrush communities and soils respond to increasing fire frequency and invasive annual plants (primarily Bromus spp.) by addressing the following questions: 1) Do vegetation characteristics differ significantly among number of times burned within each sagebrush community type? 2) How do vegetation responses to fire frequency compare between the two community types? 3) How does fire frequency affect soil physical, chemical, and biological properties in the big sagebrush community? We sampled vegetation in both communities, and soils in big sagebrush, and analyzed data using statistical and qualitative comparisons among fire frequencies and between sagebrush communities, respectively.
Results/Conclusions
Preliminary results indicate that while both sagebrush communities were substantially altered by fire-invasive species interactions, vegetation responses were unique between the two. Composition, cover of certain functional groups, and biomass were significantly different between burned and unburned sites for both communities. However, while scabland sagebrush persisted at low abundance after multiple fires and maintained dominance of native grasses, big sagebrush largely disappeared after one fire and had much higher increases in nonnative herbaceous cover/biomass. These differences likely arise from unique fuel loadings, species responses, and fire behavior. Soil physical, chemical, and biological properties differed among burned and unburned mounds, with significant decreases in soil carbon after burning once (only); increases in soil bulk density, pH, and NO3-N with fire frequency, as enzyme activity decreased; and significant differences in microbial activity among some unburned-burned strata. Our findings suggest that a single fire in big sagebrush communities can induce a state change, reinforced by altered plant-soil feedbacks, and initiate the “invasive grass-fire cycle.” In contrast, scabland sagebrush communities persisted in a semi-native state after multiple fires. These preliminary findings suggest using diverse strategies for fuels management and post-fire restoration in spatially heterogeneous sagebrush landscapes.