Active-fire landscapes demonstrate structural resistance to subsequent fire and drought

A key tenet of contemporary management in dry, fire-adapted forests of western North America is the reintroduction of a frequent and low- to moderate-severity fire regime. Where this fire regime has been fully or partially restored, it is critical to evaluate the degree to which these landscapes demonstrate forest structural resistance (i.e., the capacity to retain intrinsic structures through time) under novel climates and disturbances. In this study, we used overlapping airborne lidar datasets spanning active-fire landscapes in the Sierra Nevada, California, to evaluate how tree densities, clumping patterns, and height distributions changed during a decade of moderate-intensity wildfires and extreme drought. We evaluated structural resistance in landscapes affected by drought alone (drought-only) and by the combination of drought and wildfire (drought-fire) and examined how patterns of change varied across pre-disturbance conditions. We observed moderately high landscape-scale resistance of structural patterns during the extreme 2012–2016 drought, with only 5 % loss of trees > 32 m. Structural changes in the drought-fire context were more pronounced (25 % total tree loss); however, medium-sized (16–32 m) and tall (>32 m) trees as well as 1–4 and 5–9 tree clumps, demonstrated relatively high resistance at the landscape scale. Structural changes in the drought-only context were primarily associated with cooler and wetter microclimates. In the drought-fire context, structural changes occurred primarily in sites with higher pre-disturbance canopy cover (>40 %), in more exposed topographic positions, and where time since past fire was longer (16 + years). Overall, results from our study suggest that management practices that restore active-fire conditions in dry forest landscapes are likely to increase the resilience and adaptive capacity of these ecosystems.