UC Davis

Increases in the occurrence and extent of high severity wildfire, driven by climate change and fire suppression, pose significant threats to dry western forests that historically experienced frequent, low severity fire. In this dissertation, I examined how fire severity, time since fire, and management efforts impact forest health and recovery in Sierra Nevada yellow pine mixed conifer forests. I also assessed the efficacy of various pre-fire fuel reduction treatments in preventing future catastrophic fire. In Chapter 1, I assessed the effects of fire severity on plant diversity across a nine-year period following the 2007 Angora fire near Lake Tahoe, CA. I found a unimodal relationship between fire severity and plant alpha and gamma diversity, with high-severity areas showing progressively lower diversity as time since fire increased. Beta diversity, which measures species turnover, decreased significantly in high-severity areas, leading to floristic homogenization as fire-adapted shrubs such as Ceanothus cordulatus and Arctostaphylos patula came to dominate the landscape. These findings highlight the long-term impacts of high-severity fire on biodiversity and the role of fire severity in shaping post-fire plant communities. In Chapter 2, I evaluated the impacts of pre- and post-fire thinning on forest recovery in the same area burned by the Angora Fire. The effect of high severity fire overrode the signal of management scenario for most of our response variables, but across the rest of the fire severity spectrum, and in unburned areas, thinning significantly improved post-fire health and recovery by reducing shrub cover, increasing live tree cover, and lowering fuel loads (which also helps mitigate the effects of future fires). Areas thinned both before and after the fire had the highest tree cover and lowest fuel accumulation nine years after the fire, compared to unthinned areas which exhibited higher fuel loads and slower recovery. These findings suggest that incorporating both pre- and post-fire thinning into forest management plans is valuable for promoting forest resistance and resilience in fire-prone landscapes. In Chapter 3, I used fire behavior simulations to compare the effects of four management scenarios (no management, hand thinning and pileburning, prescribed fire only, and a natural range of variation thin followed by a broadcast burn) on fuel conditions, tree mortality, and fire behavior in one of the last remaining old-growth stands in the Lake Tahoe Basin. Our simulations showed that the most intensive management scenario, thinning to a tree density within the natural range of variation followed by a prescribed burn, was the most effective at reducing large tree mortality and maintaining forest structure post-wildfire. In contrast, areas with no management or only a prescribed fire experienced 100% tree mortality during simulated wildfire. These results suggest that proactive management, such as thinning and prescribed burns, is essential for preserving old-growth forests in fire-prone areas. This dissertation demonstrates that high-severity wildfires lead to significant biodiversity loss and floristic homogenization in Sierra Nevada yellow pine and mixed conifer forests, while forest management interventions, particularly thinning and prescribed burns, can enhance post-fire recovery and improve forest resilience. Proactive management that incorporates both pre- and post-fire thinning can reduce fuel loads, mitigate the risk of catastrophic wildfire, and support long-term ecosystem health. The findings highlight the importance of integrating fire-adapted management practices into conservation strategies to address the dual threats of climate change and altered fire regimes in Sierra Nevada forests.