Effects of drought-intensified tree mortality on wildfire behavior in Sierra Nevada forests

Abstract:

The recent prolonged drought under the warming trend has triggered massive tree die-offs in Sierra Nevada, raising concerns about its cascading effects on fire risk and behavior. The interactions of drought, tree mortality, and wildfires further threaten forest health and sustainability. However, it is not clear how tree mortality alters wildfires, and contradictory findings have been reported from very limited survey-based small-scale studies. We used a machine learning approach, extreme gradient boosting, to investigate how pre-fire tree die-offs affected the rate of fire spread and area burned, for large fires over 1000 acres since 2012 in Sierra Nevada. Tree mortality maps were derived from high resolution NAIP and Planet remote sensing imagery during 2012-2019, and daily fire behavior metrics were generated from MODIS and VIIRS active fire products. We found that topography, pre-fire dead tree density, cumulative mortality percentage, and years since last burning had relatively stronger effects on fire progression than other environmental factors. Our preliminary analysis indicated a possibility of decreasing fire spread by the increase in tree mortality within the daily burned area but the rate of this negative effect decreases with the increase of mortality. Ongoing further analysis will provide more robust findings and explain two competing effects due to changes in fuel dryness and fuel continuity resulted from tree die-offs. This study is expected to reconcile some disagreements among previous studies on the interplay between tree mortality and fire behavior, and shed insights for developing adaptive management strategies to mitigate the impact of tree death and fire risks under the future warming droughts in Sierra Nevada forests.