The first field-based investigation of the relationship between pre-fire tree mortality and wildfire severity in the Sierra Nevada has found a significant positive relationship between the two. The authors of this 2021 paper published in Ecological Applications, Rebecca Wayman and Hugh Safford, analyzed pre- and post-fire conditions at 180 plots spread across areas of mixed-conifer forest burnt by the 2015 Rough Fire and the 2016 Cedar Fire in the southern Sierra Nevada. For the pre-fire conditions, trees on the plots were examined for signs of infestation by bark beetles (e.g., pitch tubes, insect galleries, and boring dust), and a pre-fire mortality status was assigned to each tree with respect to the bark beetle damage. In addition, the authors also accounted for the role of other factors, such as relative humidity and topography. To quantify the fire severity at each plot, the authors relied on three metrics: (1) the amount of tree basal area killed by the fire, (2) an estimate of the amount of biomass that was killed by the fire, and (3) an estimate of the percentage of the canopy destroyed by the fire.
Ash and smoke rise up from the 2016 Cedar Fire near Kernville, CA (www.bakersfieldnow.com).
Map of the 2016 Cedar Fire (www.wildfire.today).
For the Cedar Fire, the authors discovered that relative humidity had the greatest effect on fire severity (i.e., fire severity decreased with increasing relative humidity) and that pre-fire tree mortality had the second-most important effect on two of three measures of fire severity. For the Rough Fire, pre-fire tree mortality was the most important factor for one of the fire severity metrics and the second-most important for another (topographic position had the dominant effect). Counter-intuitively, other studies have failed to find a relationship between pre-fire tree mortality and fire severity, and the authors speculate that fire behavior may be a critical factor: high-intensity, weather-driven fires may burn through stands regardless of the condition of the individual trees, while healthy trees are less likely to burn in moderate fires. In other words, trees previously killed by bark beetle infestations and drought can increase fire severity in low- to moderate-intensity fires, but not necessarily in extreme fires.
Map of the 2015 Rough Fire, near Fresno, CA (www.wildfire.today).
Ash and smoke rise up from the 2016 Cedar Fire near Kernville, CA (www.bakersfieldnow.com).
Map of the 2016 Cedar Fire (www.wildfire.today).
For the Cedar Fire, the authors discovered that relative humidity had the greatest effect on fire severity (i.e., fire severity decreased with increasing relative humidity) and that pre-fire tree mortality had the second-most important effect on two of three measures of fire severity. For the Rough Fire, pre-fire tree mortality was the most important factor for one of the fire severity metrics and the second-most important for another (topographic position had the dominant effect). Counter-intuitively, other studies have failed to find a relationship between pre-fire tree mortality and fire severity, and the authors speculate that fire behavior may be a critical factor: high-intensity, weather-driven fires may burn through stands regardless of the condition of the individual trees, while healthy trees are less likely to burn in moderate fires. In other words, trees previously killed by bark beetle infestations and drought can increase fire severity in low- to moderate-intensity fires, but not necessarily in extreme fires.
Map of the 2015 Rough Fire, near Fresno, CA (www.wildfire.today).
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