Fire history and atmospheric drying jointly control forest reburn severity

Background Wildfire activity is increasing across western United States forests, leading to more frequent reburns, or areas that burn multiple times within short intervals (approximately two decades). Prior fire can reduce subsequent burn severity through fuel limitation, but this effect is not consistent across landscapes or climate conditions. Understanding when reburns burn less severely and when they do not is critical for predicting future fire behavior and informing management. Here, we evaluated how reburn severity varies with fire history, time since fire, and atmospheric drying across reburn events in 23 EPA Level III ecoregions in the western U.S. Results Using 755 small and large forested reburn events from 2001–2024 and hierarchical Bayesian models of modeled burn severity (Composite Burn Index, CBI, range 0–3), we found that predicted severity was about 15% higher in previously unburned areas than in reburned areas. However, this effect was structured by interactions between fuel history and atmospheric drying (vapor pressure deficit, VPD). Within reburned areas, reburn severity increased with time since fire and atmospheric drying, with both effects depending on initial fire severity. Predicted severity ranged from ~0.6 CBI under low VPD and shorter intervals to ~1.7 CBI under high VPD and longer intervals of the observed time range. Atmospheric drying had a stronger effect following low-severity initial fires, whereas time since fire had a stronger effect following higher-severity initial fires, indicating a shift from climate-mediated to fuel-mediated controls. Across ecoregions, these interactions reveal multiple pathways to elevated reburn severity, with interval effects strongest in more productive systems and weaker in drier interior regions. Conclusions Reburn severity is governed by interacting effects of fire history, time since fire, and atmospheric drying, rather than prior fire alone. While prior fire often reduces subsequent burn severity, this buffering effect weakens over time and under extreme fire weather. The moderating effect of prior fire therefore diminishes over time and is contingent on environmental conditions. Strategies that rely on recent fire, such as managed wildfire or prescribed burning, to reduce future severity may provide short-term benefits but are unlikely to remain reliable as fire weather becomes increasingly warm and dry.