Land managers are increasingly using prescribed fire to restore fire-adapted Pacific Northwest prairie ecosystems. While many studies have examined experimental burn application in small plots, few studies have evaluated the difference between landscape-scale fire re-introduction and controlled plots. In our study, we used large (1,000 m2) nested plots on managed prairie sites in western Washington state, covering a broad range of edaphic conditions. We collected vascular plant community data over five years to compare plant communities in paired burned and control treatments at three spatial scales. We predicted that there would be differences in plant cover, species richness, diversity, evenness, and community composition between treatments. While site-specific edaphic factors can be strong influences on community composition, we predicted differences would be better explained by burn treatment than by plot location.
We found that although there was a significant fire effect, plot location explained up to 20 times more variation in plant communities than fire application. This location effect covaried with average soil percent C, N, C:N, and moisture. Many species indicating for burned or control treatments were also sensitive to gradients in these edaphic factors. Plant community differences among burn treatments were significant at small scales, but often lost at large scales. There were also significant differences in diversity and species evenness among treatments in certain years, but there were no consistent trends over time. These results suggest that prescribed fire application may be driving community differences at a small scale, but at the landscape scale changes in community composition may be more dependent on factors related to location and microsite. While fire application influenced plant communities, edaphic factors in particular may have played a more important role in determining community composition in this fire adapted landscape, suggesting generalities regarding the effect of prescribed fire application on communities are not consistent.