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

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.