Dissertation Defense: Ved Bhoot

Title: Forest Recovery, Fire-Fuel Feedbacks, and Climate Controls on Aboveground Biomass Loss from Wildfire in California

Abstract: California's natural lands are experiencing increasingly intense and large wildfires. This dissertation defense talk examines changes in California's fire-vegetation system with respect to vegetation composition shifts, fire-fuel feedbacks, and climate controls on wildfire driven aboveground biomass loss.

In my first chapter I study vegetation cover change from 1985-2022 and test if recovery patterns of two forest types have changed. I find large reductions in conifer and hardwood forests throughout the state, with shrublands and herbs taking their place. Using fire-year cohorts I show that conifer recovery at 15 years after fire has significantly declined whereas hardwood has not. These changes in the vegetation composition in conjunction with declining conifer forest recovery points to rapid compositional change under an intensifying fire regime.

In my second chapter I focus on fuel-constrained fire-risk, as burn probability and flame length, to better understand fire-fuel feedbacks and how fuels have changed fire-risk from 1995-2024. I show in this period that California has become more susceptible to burning but less intensely in regions already burned. These changes track increases in shrub fraction and decreases in tree fraction. BP and FL recovery trajectories in under one decade of forest fire show BP overshoots but FL reduces. Moreover, forest fires with extensive tree cover loss lead to an exacerbation of BP overshoot which is sustained by at least 25 years since fire, and for FL low tree cover loss leads to a sustained weak overshoot after one decade.

In my third chapter I examine the climatic mechanisms underlying wildfire driven aboveground biomass loss and changes in climate-zones exposed to wildfire from 1995-2025. Multiple linear regression analyses reveal that precipitation anomalies significantly predict AGB density loss (as a measure of fire intensity) but not summer temperature-maximum anomalies. However, the opposite is the case for predicting total burn area. For total annual AGB loss I find both the climate anomalies are significant predictors. I also show that wildfires have increasingly encroached into cooler, wetter, more biomass-dense regions of California.

Together, these findings advance mechanistic understanding of how California's fire-vegetation system is evolving and the implications for the composition, flammability, and carbon dynamics of the state's natural lands.