Within and among years atmospheric moisture availability influences wildfire burn severity in the boreal forest

Abstract Background Atmospheric moisture can either mitigate or exacerbate burn severity depending on fuel and environmental conditions. However, changes in fuel moisture typically lag behind shifts in atmospheric moisture associated with weather or climate changes. When atmospheric moisture decreases, lagged decreases in fuel moisture can affect fire hazard in two ways: they can increase fuel flammability and wildfire severity in the short term, or alternatively, they can decrease wildfire severity over longer periods by slowing the accumulation of combustible materials. Yet, the temporal scales at which lagged moisture changes most strongly influence burn severity are not well understood. Hence, we investigated how time-lagged moisture availability at both inter- and intra-annual timescales, forest composition, and drought conditions during the burn period contribute to wildfire burn severity across 1025 fires (1986–2024) in Ontario (Canada). Specifically, we quantified the relative effects of daily vapour pressure deficit (VPD), annual climate moisture index (CMI) anomalies, fuel composition, and fire‑season weather using distributed lag generalized additive models and hierarchical partitioning to compare the contributions of moisture at intra-annual lag intervals up to 30 days before fire (VPD) and interannual lag intervals up to 5 years before fire (CMI). Results We found that intra-annual atmospheric dryness and interannual moisture availability jointly regulate burn severity, with effects varying by lag. Coniferous forest cover was the strongest predictor of both median and extreme severity. Interannual moisture availability (CMI) explained more variation than intra‑annual drying (VPD). Wetter‑than‑normal conditions 4–5 years before fire increased severity, while long‑lag dryness reduced it, likely through fuel limitations. At 1–2 year lags, dry conditions increased severity and wet conditions lowered it. When considering extreme burn severity, cumulative multi‑day VPD was more influential than individual lag effects. Conclusions These results show that burn severity is driven by short‑term flammability and multi‑year fuel legacies, underscoring the need to integrate lagged moisture dynamics into fire‑severity modelling under climate change.