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Dryad

Trees have similar growth responses to first-entry fires and reburns following long-term fire exclusion

Willson, Kevin1; Hurteau, Matthew1

Published Nov 12, 2024 on Dryad. https://doi.org/10.5061/dryad.6djh9w18z

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Nov 12, 2024 version files 300.74 MB

Abstract

Managing fire ignitions for resource benefit decreases fuel loads and reduces the risk of high-severity fires in fire-suppressed dry conifer forests. However, the reintroduction of low-severity wildfires can injure trees, which may decrease their growth after a fire. Post-fire growth responses could change from first-entry fires to reburns, as first-entry fires reduce fuel loads and the vulnerability among trees to fire effects, which may result in trees sustaining less damage during reburns. To determine whether trees had growth responses that varied from first-entry fires to reburns, we cored 87 ponderosa pine trees in the Gila Wilderness, New Mexico, USA that experienced 3-5 fires between 1950 and 2012 following long-term fire-exclusion and 67 unburned control trees from the Gila and Apache-Sitgreaves National Forests. We assessed tree growth response to fire by comparing tree-ring growth among burned and unburned trees during the growing season and the two growing seasons before and after the fire. We compared growth between burned and unburned trees using a bootstrapping procedure to calculate annual median tree-ring width index values with 95% confidence intervals. We compared post-fire growth after first-entry fires and reburns following long-term fire exclusion. Burned trees had similar growth responses following first-entry fires and reburns, with lower growth rates during the growing season and the two growing seasons after fires compared to unburned controls. Burned tree growth returned to expected rates following these immediate post-fire growth reductions. Interestingly, trees had lower growth during the year before and the year of reburns compared to the first-entry fire, reflecting greater aridity before reburns. Greater aridity may have contributed to larger-than-expected growth reductions following reburns that could explain similar growth responses to first-entry fires and reburns. Our results indicate that trees had consistent short-term growth responses to low-severity fires following long-term fire exclusion. As trees retained vigor after multiple fires, managing fires for resource benefit is an effective approach to reduce the likelihood of high-severity fires without long-term negative effects on tree growth.