Cumulative Severity of Thinned and Unthinned Forests in a Large California Wildfire

Chad T Hanson

doi:10.3390/land11030373

Abstract

Studies pertaining to fire severity in commercially thinned versus unthinned forests are based on a comparison of tree mortality between the two categories. Commercial thinning is widely conducted on public and private forestlands as a fire management approach designed to reduce fire severity and associated tree mortality. However, tree mortality from thinning itself, prior to the occurrence of the wildfire, is generally not taken into account, which leaves a potentially important source of tree loss, with its associated forest carbon loss and carbon emissions, unreported. This study investigated the “cumulative severity” of commercially thinned and unthinned forests in a large 2021 wildfire, the Antelope fire, occurring within mixed-conifer forests on public lands in northern California, USA. Using published data regarding the percent basal area mortality for each commercial thinning unit that burned in the Antelope fire, combined with percent basal area mortality due to the fire itself from post-fire satellite imagery, it was found that commercial thinning was associated with significantly higher overall tree mortality levels (cumulative severity). More research is needed, in other large forest fires, to determine whether the finding, that commercial thinning killed more trees than it prevented from being killed, is common elsewhere.

Highlights

Research regarding commercial thinning and fire severity in conifer forests of the western USA is highly variable
As fire severity in forests is fundamentally a metric pertaining to the level of tree mortality [6,7], there is reason to understand cumulative tree mortality, and “cumulative severity”, from thinning and wildfire, in order to determine whether the result of thinning is more, or fewer, live trees in the landscape
This study investigated whether cumulative fire severity, based on percent basal area mortality from commercial thinning plus percent basal area mortality from subsequent wildfire, would be different in thinned versus unthinned forests, using fire severity data from the 2021 Antelope fire in northern California in combination with pre-fire published data on tree mortality from commercial thinning in the same area, as described in the methods and results and contextualized in the discussion below

Summary

Introduction

Research regarding commercial thinning and fire severity in conifer forests of the western USA is highly variable. As fire severity in forests is fundamentally a metric pertaining to the level of tree mortality [6,7], there is reason to understand cumulative tree mortality, and “cumulative severity”, from thinning and wildfire, in order to determine whether the result of thinning is more, or fewer, live trees in the landscape. This is true in light of plans to substantially increase the pace and scale of commercial thinning of western U.S forests as a fire management and forest resilience strategy.

Methods

Results

Conclusion