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) to mixed conifer forest stands with active fire regimes, in order to answer two questions. 1) How do understory plant diversity and fine-scale environmental heterogeneity respond after prescribed burns, and do these effects differ based on initial thinning treatment? 2) How do the fine-scale heterogeneity and understory species diversity of control and treatment plots at the Teakettle Experimental Forest compare to newly established reference plots in active-fire forests?
TEF follows a full-factorial experimental design of thinning and burning treatments. Reference plots in Yosemite and Kings Canyon share similar conditions and have experienced 3-5 low-intensity fires within the last 60 years. Percent cover for every plant species was measured at 9 - 49 gridpoints within each 4 ha plot at TEF and each reference plot. Environmental variables (including soil moisture, slope, aspect, and woody debris cover) were measured.
Results/Conclusions
Initial results from the first year of data collection show that understory plant species richness and environmental heterogeneity in active fire forests in Kings Canyon and Yosemite vary both within and across plots, similar to observed patterns of plant diversity in burn treatments at TEF. Herbaceous plant cover and richness displayed positive relationships with soil moisture, and negative relationships with shrub cover in both active fire reference plots in Kings Canyon and Yosemite, as well as TEF. We also found that burn plots at TEF without thinning showed high understory diversity and low shrub cover compared to burn plots with initial thinning treatments after the second application of prescribed fire, where burning did not substantially remove shrub cover. This unexpected result may reflect the importance of heterogeneous patches of higher and lower intensity within burns for maintaining understory diversity.
Next steps in this study include measuring understory diversity for an additional year to capture inter-annual variability across our sites, and analyzing soil nutrients, hemispherical photographs, and overstory composition and structure at each location to see how other important plant resources (nutrient and light availability) effect impact understory diversity within the study sites.