Trees in Fire-Maintained Forests Have Similar Growth Responses to Drought, but Greater Stomatal Conductance Than Trees in Fire-Excluded Forests

In the western US, increased tree density in dry conifer forests from fire exclusion has caused tree growth declines, which is being compounded by hotter multi-year droughts. The reintroduction of frequent, low-severity wildfire reduces forest density by removing fire-intolerant trees, which can reduce competition for water and improve tree growth response to drought. We assessed how lower forest density following frequent, low-severity wildfire affected tree stomatal conductance and growth response to drought by coring and measuring competition surrounding ponderosa pines (Pinus ponderosa) in the Gila and Apache-Sitgreaves National Forests, New Mexico, US that either experienced 3–5 fires following long-term fire exclusion or remained fire-suppressed. We quantified tree growth declines during (resistance) and increases after (resilience) two recent multi-year droughts, which we compared between trees in fire-maintained and fire-excluded forests. We assessed stomatal conductance among trees by sampling wood from tree rings to measure stable carbon isotopes during and after both droughts, which we used to calculate evaporative water use efficiency. Trees in fire-maintained forests had greater resistance than trees in fire-excluded forests during the first drought, but growth responses became similar once the first drought ended. Interestingly, growth responses rarely varied despite evaporative water use efficiency increasing two times faster among trees in fire-excluded forests after the first drought commenced. Post-drought growth responses varied, in part, by aspect, with trees on northerly aspects exhibiting greater resilience to drought than trees on southerly aspects. Our results indicate that while trees had density-independent growth responses to drought, trees in fire-maintained forests were less water stressed than trees in fire-excluded forests. Therefore, the reintroduction of frequent, low-severity wildfire regimes has the potential to moderate some effects of hotter droughts as climate change intensifies.