Plant community response to thinning and repeated fire in Sierra Nevada mixed-conifer forest understories

Fire suppression in the western United States has significantly altered forest composition and structure, resulting in a higher risk of stand-replacing fire, large-scale drought mortality, and bark beetle outbreaks. Mechanical thinning and prescribed fire are common treatments designed to reduce high-severity fire risk, but few studies have tracked long-term understory plant community response to these treatments with repeated fire application that emulates historic fire regimes. We evaluate changes in understory plant diversity and composition, as well as light availability, soil moisture, and litter depth over two decades following a factorial field experiment crossing thinning and two applications of prescribed fire at the Teakettle Experimental Forest (TEF) in the southern Sierra Nevada. We compare responses in experimental fuels treatments against those in nearby old-growth, mixedconifer forests with restored low-severity fire regimes in Yosemite and Kings Canyon National Parks. We hypothesized that 1) understory plant richness, evenness, and beta diversity would increase with each burn event; 2) repeated fire after initial thinning would produce the highest understory plant diversity; 3) the second burn would reduce shrub cover, especially in the initially thinned treatments that demonstrated vigorous Ceanothus cordulatus growth after initial treatments; 4) the second burn would increase light availability and bare ground while reducing litter depth; and 5) treatments with initial thinning followed by multiple prescribed burns would show the greatest similarities to reference forests in diversity metrics, shrub cover, and environmental conditions. Although initially local (10 m2-scale) understory plant richness and diversity increased most following thinning combined with prescribed fire, this treatment did not generate understory conditions similar to those in nearby reference forests over the long term. Vigorous shrub growth resulted in low understory evenness and beta diversity over time, which a secondary burn treatment did not alter. Burning without thinning retained a more heterogeneous understory following initial treatment. Two years after the second burn treatment, this burning without thinning resulted in high understory richness and evenness similar to reference forest understories. Our results suggest management treatments may need to focus on creating heterogeneity in burn effects to limit shrub cover and foster more diverse understory communities.