The footprint of fire history on fuel structure strongly shapes fire severity in Mediterranean and Oceanic ecosystems of southern Europe

Extreme wildfires are increasingly frequent across southern Europe, yet the extent to which fire history legacies modulate ecosystem-specific fire severity responses remains poorly understood in this region. This study aimed to improve understanding of how fire history and pre-fire fuel structure jointly can modulate fire severity across contrasting Mediterranean and Oceanic ecosystems. We analyzed six large wildfires that occurred during the exceptionally extreme 2025 fire season in northwestern Spain, using the relativized burn ratio (RBR) derived from Sentinel-2 imagery as a proxy for fire severity. Fire recurrence and time since last fire (TSLF) were reconstructed over the 40-year period preceding the 2025 wildfires, and pre-fire ecosystem type and fuel structure were characterized using wall-to-wall national forest inventory data and remote sensing products. Fire severity differed markedly among ecosystem types and forest development stages, with conifer forests (Cfs) and less mature stands exhibiting the highest severity. Fire recurrence and TSLF interacted to shape non-linear, ecosystem-specific fire severity responses. Fire severity peaked at short to intermediate TSLF (< 20 years) and declined thereafter. In conifer forests, fire severity remained higher than in other ecosystem types at intermediate TSLF. Piecewise structural equation modelling (pSEM) results revealed that fire history influenced fire severity predominantly through indirect pathways mediated by changes in pre-fire fuel structure. In forests, these pathways were associated with increased fuel load and vertical fuel continuity promoted by higher fire recurrence and shorter TSLF, whereas in shrublands (Sh) fire severity was primarily mediated through surface fuel accumulation under higher recurrence and longer TSLF. Direct effects of fire history on fire severity were limited and restricted to fire recurrence in forest ecosystems. Overall, our findings highlight the central role of fire history legacies in modulating ecosystem-specific fire severity responses under extreme wildfire conditions.