Sagebrush Alliances in the Great Basin are one of the most imperiled ecosystems in North America. Exotic plant invasions and wildfires have combined to convert these systems into those composed mostly of exotic annual grasses and forbs at a broad scale. While it is well documented that annual grass invasions are increasing wildfires, and that exotic annual grasses thrive after fire, the lasting effects that multiple fires have on plant communities are unclear. Namely, do multiple fires affect biodiversity and community composition in a cumulative fashion, or is one fire enough to initiate a lasting alternate state? We created a fire history atlas from 31 years satellite-derived fire data to design a stratified sampling scheme along a fire frequency gradient. We sampled 28 plots for plant community composition and soil characteristics. We examined fire’s effect on species richness using species accumulation curves, and calculated alpha-diversity and 3 metrics of beta-diversity. We used non-metric multidimensional scaling to examine community clustering, and PERMANOVA models to examine how climate variable around the time of the fire affected community clustering and beta diversity.
Results/Conclusions
Community clustering measures suggest that one fire pushes the system into an alternate state. Biodiversity measures indicated cumulative effects. There was no significant difference in alpha diversity per plot by fire frequency, but species accumulation show a clear step-wise progression of diversity decreasing as fire frequency increases. Beta diversity showed a significant decline after 3 fires. Non-metric multidimensional scaling showed most burned plots clustering into the one community type dominated by cheatgrass, except three thrice-burned plots. When examining plots just by exotic and native cover, there was a clear threshold effect after one fire, with native shrubs dominating the unburned plots, and exotic annual grasses dominating the burned plots. While one burn might be enough to change the general community type, successive burns still continue to influence biodiversity.