Wildfire-contingent effects of fuel treatments can promote ecological resilience in seasonally dry conifer forests

Fire suppression has made many seasonally dry conifer forests more susceptible to high-severity wildfires, which cause large changes in forest structure and function. In response, management agencies are applying fuel reduction treatments to millions of acres of forest, with the goal of moderating fire behavior by reducing tree density and understory fuel loads. However, despite their wide application, we still lack basic information about the extent to which these treatments contribute to forest restoration by increasing forest resilience to recurring wildfire events. To address this question, we established 664 plots across 12 different sites in California, USA, where wildfire burned through fuel treatments, and measured a suite of forest characteristics relating to overstory structure, understory cover, and woody plant regeneration. We tested a "wildfire-contingency" hypothesis that there should be strong interactions between treatment and fire, specifically that the direction and magnitude of fuel treatment effects on forest characteristics will depend on subsequent disturbance. This interaction hypothesis had strong support, driven largely by effects on trees: without wildfire, live-tree cover was lower in treated stands than in untreated stands, but after wildfire, it was higher in treated stands than in untreated stands. Treated stands had higher soil moisture and more shrub seedlings than untreated stands without wildfire but had greater soil moisture and fewer shrub seedlings than untreated stands after wildfire. Conversely, litter depth, litter cover, and tree seedling abundance were lower in treated stands than in untreated stands without wildfire but higher in treated stands than in untreated stands after wildfire. Ordination revealed that the magnitude of ecological change attributable to wildfire is lower in treated stands than in untreated stands. We conclude that properly implemented treatments can promote resilience to both first-entry and subsequent wildfires.