Tree Mortality and Fuel Changes from an Extreme Drought and Bark Beetle Outbreaks in California

California is currently in the midst of a record-breaking drought and its forests are undergoing massive die-offs. This extensive tree mortality will drastically alter fuel loads, likely changing fire behavior and severity in fire-prone ecosystems and in the wildland-urban interface. Minimal information is currently available for post-bark beetle outbreak fuels data for use in fire behavior models, particularly for high levels of mortality in pinyon, ponderosa and Jeffrey pine and mixed conifer stands. Most work on bark beetle impacts to fuel loading has focused on mountain pine beetle in lodgepole pine, and there is extremely limited information for other forest types. High levels of tree mortality can rapidly alter fuel complexes, potentially resulting in increased fuel hazard which foster more intense and severe future fires. Dead pine trees in the red stage alter canopy fuel availability and may have higher ignition potential and burn more intensely. Surface fuel loading also may increase as foliage, branches, and trees decompose following successful bark beetle attacks.

In addition, recent work has yielded insights into predisposing factors to tree death; however, it is still unclear how site, stand, and individual tree factors interact to impact forest mortality. Such information is greatly needed to understand how past drought events affected tree growth and recovery and to identify possible thresholds of drought that lead to accelerated mortality.

PROJECT OBJECTIVES:

1. Determine temporal changes in fuel loading and hazard due to bark beetle and drought-associated mortality

2. Identify the edaphic site, stand, and tree growth factors relating to conifer tree mortality and monitor changes in vegetation after mortality. In 2016, we installed stand monitoring vegetation and fuel plots to determine fuel hazard in areas of recent mortality. We are monitoring these plots for several years to quantify how drought and bark beetles change fuel loading and hazard over time. We also collected cores from live and dead trees to quantify tree susceptibility and resilience from drought using tree ring analysis.

Images

Widespread tree mortality due to drought and bark beetles on the Sierra National Forest.
Widespread tree mortality due to drought and bark beetles on the Sierra National Forest.

Select Publications & Products

Reed, C. C., S. M. Hood, D. R. Cluck, and S. L. Smith. 2023. Fuels change quickly after California drought and bark beetle outbreaks with implications for potential fire behavior and emissions. Fire Ecology 19:16.

Keen, R. M., S. L. Voelker, S.-Y. S. Wang, B. J. Bentz, M. L. Goulden, C. R. Dangerfield, C. C. Reed, S. M. Hood, A. Z. Csank, T. E. Dawson, A. G. Merschel, and C. J. Still. 2022. Changes in tree drought sensitivity provided early warning signals to the California drought and forest mortality event. Global Change Biology 28:1119-1132.

Reed, C. C., & Hood, S. M. (2020). Few generalizable patterns of tree-level mortality during extreme drought and concurrent bark beetle outbreaks. Science of The Total Environment, 750, 141306.

Axelson, J. N., J. Battles, B. Bulaon, D. Cluck, S. Cousins, L. Cox, B. Estes, C. Fettig, A. Hefty, S. Hishinuma, S. Hood, S. Kocher, L. Mortenson, A. Koltunov, E. Kuskulis, A. Poloni, C. Ramirez, C. Restaino, M. Slaton, S. Smith, and C. Tubbesing. 2019. The California Tree Mortality Data Collection Network — Enhanced communication and collaboration among scientists and stakeholders California Agriculture.