Insect and drought induced tree mortality dramatically increased in the Sierra Nevada during California’s recent prolonged drought, causing widespread concern that severe tree mortality would lead to increased severity of subsequent wildfires. Effects of tree mortality on wildfire severity have been studied in U.S. Pacific northwest and Rocky Mountain forests with historically low frequency and high severity fire regimes. To our knowledge, no field studies have examined the relationship between tree mortality and subsequent wildfire severity in Sierra Nevada mixed-conifer forests, where historic wildfire occurred at high frequency and low severity. To best allocate resources to the highest priority fuels reduction treatments, managers in California need information on whether severe tree mortality constitutes an increased risk of severe wildfire. In this ongoing study, we examine this relationship using field data collected within areas of patchy red phase (dead needles retained on branches) insect infestation that subsequently burned in wildfire. We aim to determine whether and under what conditions wildfire severity relates to severity of pre-fire tree mortality in Sierra mixed-conifer forests. We collected data on 50 plots in 2016 within the 2015 Rough fire and will collect data on ~100 plots on the 2016 Cedar fire during early summer 2017.
Results/Conclusions
Pre-fire tree mortality was identified as influential to our two metrics that measured fire severity in terms of the change from live to dead fuels (Relative differenced Normalized Burn Ratio [RdNBR], a remotely sensed metric, and the percent of pre-fire live tree basal area killed by fire, a field metric), but not to our physical measure of fire severity (torch percent, a field measure of the proportion of tree needles consumed by fire). Our analyses do not reveal mechanisms by which pre-fire tree mortality relates to increased fire-caused tree mortality, and our data do not indicate that increased charring or torching drives the relationship. Pre-fire tree mortality measures had much higher magnitudes of influence on fire severity metrics when ≥25% of trees or basal area were in the red phase pre-fire. While it is intuitive that lower levels of pre-fire mortality would have less influence on fire severity, further study should investigate whether there is a threshold level of bark beetle-caused mortality over which it has an important influence on fire severity, and under which it could be considered by forest managers as less of a fire severity concern.