Megafires in the temperate conifer forests of the Sierra Nevada mountains of California from 1985 to 2020: remote sensing and field-based analysis of fire characteristics and vegetation recovery dynamics
Global burned area has seen a decrease over recent decades, but there are stark regional trends of increasing burned area, fire frequency and burn severity. As a result of fire suppression-mediated fuel accumulation and accelerating climatic changes, this trend of an intensifying fire regime is evident in the temperate mixed-conifer forests of the Sierra Nevada region of California. This thesis employs a mixed-methods approach of remote sensing, field surveys, and statistical analysis to investigate the effects of fire events of extreme size, frequency, and severity on conifer regeneration. A general overview of the study region and the historic fire regime, as well as a discussion on assessing burn severity and vegetation recovery after disturbance is provided in \textbf{Chapter 1}. \textbf{Chapter 2} examines the occurrence and effects of fires of extreme size in the Sierra Nevada ecoregion over the last four decades. We find that megafires drive annual burned area in the region and exhibit distinct spatial configurations of large, compact high-severity burn patches. Our random forest model predicts that this, combined with more extreme post-fire climatic conditions has resulted in 154,124~ha of conifer forest that burned at high severity in a megafire from 1985 to 2020 at risk of undergoing a vegetation type conversion to a more open ecosystem type with lower carbon storage capacity. In \textbf{Chapter 3} we show that areas affected by repeated burns have been increasing markedly over the last decades, albeit this increase has been proportional to the increase of total burned area. Areas burned twice during the study period exhibit a similar burn severity in the second burn as in the initial burn, with high-severity reburns only occurring in previously severely burned areas and after at least ten years fire-free interval. Areas that burned three times on the other hand showed minimal high-severity reburns, suggesting a self-limiting quality of the fuel-burn severity- vegetation recovery feedback cycle. A case study of the KNP Complex megafire in \textbf{Chapter 4} illustrates the strong effect of large stand-replacing burn patches on conifer recovery. The critical impact of a compact spatial configuration of high-severity patches on conifer regeneration is emphasized and field surveys of conifer seedlings two years post-fire confirm these findings on a species level. Giant sequoia seedlings are the only conifer species with a weak and non-significant relationship between seedling density and distance from seed source, but although these seedlings were found even far from unburned trees, mortality of mature trees was too high, and seedling density too low for a full recovery of forest structure and density without management intervention. Finally, \textbf{Chapter 5} aims to synthesise our findings on extremely large, frequent, and severe fires under a changing fire regime in the Sierra Nevada. We discuss implications for carbon resilience in temperate conifer-forests, pre- and post-fire management, as well as future research directions with regards to improvements in methodology and technology, and other ecological lenses through which extreme wildfire- vegetation interactions could be inspected.
wildfire, fire, ecology, Sierra Nevada, megafire, remote sensing, fire regime, disturbance, resilience, vegetation recovery, landscape metrics
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