Wildfires for resilience in California’s Sierra Nevada: fine-scale assessment at regional scales with airborne lidar

Abstract

Montane forest ecosystems in western North America hold significant ecological and social value. However, these valued forest ecosystems are at heightened risk in the modern era due to a warming climate and increasingly severe wildfires, droughts, and bark beetle outbreaks. In dry forest types, a history of fire exclusion, removal of Indigenous burning, and extensive logging have drastically altered forest structures and compositions over the past century, making the impacts of climate change and shifting disturbance regimes particularly severe. Fire played a vital ecological role in maintaining the resilience and adaptive capacity of dry forest ecosystems historically. As managers work to conserve dry forest landscapes in the 21st century, it is crucial to understand the contemporary role of wildfires in these ecosystems, and the extent to which wildfires themselves can be utilized as a management tool to increase resilience. In this dissertation, I leveraged the high-resolution and synoptic coverage of airborne lidar to explore interactions between contemporary wildfires and dry forest ecosystems in California’s Sierra Nevada. For chapter one, I developed a geospatial dataset delineating sites where a frequent and low- to moderate-severity fire regime has begun to reestablish in the modern era, then used lidar to compare multi-scale structural patterns between these sites and fire-excluded control sites. I found that contemporary fire-intact dry forest landscapes were characterized by distinct and heterogeneous structural patterns at the neighborhood- (1 ha) and site-level (100-1000 ha) and consistency in these patterns among sites, indicating a strong bottom-up shaping mechanism of contemporary wildfires in dry forests. For chapter two, I analyzed 35 recent wildfires spanning a gradient of biophysical conditions in the Sierra Nevada to (1) quantify the extent to which first-entry burns produced structural patterns that aligned with contemporary reference landscapes and (2) evaluate the environmental drivers of restorative fire effects. I found that moderate-severity patches – occurring primarily in close proximity to recent burns, during periods of moderate fire weather, and at higher elevations – aligned most closely with reference conditions, but that relatively large extents of first-entry burned areas will likely require additional fires or management to fully achieve restoration objectives. For chapter three, I used a bi-temporal airborne lidar dataset to evaluate the resistance of forest structures during fire and drought within a partially-restored active-fire landscape in Yosemite National Park. I found that key structural features (e.g., tall trees and small tree clumps) demonstrated high resistance during drought and wildfire, stressing the importance of reestablishing active-fire conditions to increase dry forest resilience. A key finding emerging from my dissertation is that a frequent and low- to moderate-severity fire regime remains a critical ecological process in dry forest ecosystems in the 21st century which, under moderate fire weather conditions and given strategic multi-scale management, can increase the resilience of these valued forest ecosystems in the face of changing climatic conditions and altered disturbance regimes.