Fire is a critical ecological process that drives forest structure and biodiversity in the Sierra Nevada. However, fire suppression has significantly impacted natural disturbance regimes in these forests. This study compares understory diversity and resource heterogeneity under different management strategies in a mixed conifer forest at the Teakettle Experimental Forest (TEF) in order to answer two questions. 1) How does understory plant diversity respond after different numbers of prescribed burn events, and do these effects differ based on initial thinning treatment? 2) Do these treatments have differing effects on understory plant community composition?
TEF follows a full-factorial experimental design of thinning and burning treatments carried out in 2001, with a second burn treatment in 2017. Percent cover was measured yearly for all non-tree plant species in 9 - 49 subplots within each 4-ha plot, along with environmental variables including soil moisture, slope, aspect, and woody debris cover. We use a Bayesian mixed-effects model to compare alpha and beta diversity and relative abundance of different functional groups over time before and after initial and subsequent burn treatments.
Results/Conclusions
After initial treatments, combined burn-thin treatments had greater increases in α richness and evenness than control, thin-only, and burn-only treatments. However, the burn-thin treatments show decreased β diversity over time. Following a second application of fire in the burn treatments, burn-only plots show an increase in α and γ richness, but retain low evenness and β diversity, indicating that those plant communities are dominated by fewer, consistently occurring species. Different functional groups of understory plants responded differently to the treatments, with herbaceous plant cover increasing immediately after burn-thin treatments, but shrubs increasing shortly after, and persisting over time. These changes in the understory plant community may be driven by changes in environmental conditions following the treatments, as well as biotic interactions between different functional groups. These results may reflect the importance of heterogeneous patches of higher and lower intensity within burns for maintaining understory diversity.