Assessing fire severity using understory fire-legacy metrics in forest ecosystems

Fire severity assessment is crucial to understanding post-fire ecosystem responses and guiding management decisions. However, field-based quantification of fire severity remains methodologically heterogeneous and often ecosystem-dependent. This study evaluated the correspondence between understory fire-legacy metrics, measured either as absolute values (e.g. tip diameter -TD-) or as relative metrics (e.g tip-basal diameter ratio -TBR-), with fire severity (CBI and individual fire severity indicators) across vegetation and soil in three contrasting forest ecosystems of the northwestern Iberian Peninsula ( Pinus pinaster , Pinus sylvestris , and Quercus pyrenaica ). Using beta mixed-effects regression models, we related understory fire-legacy metrics measured in post-fire shrub remnants to fire severity indices (CBI total, vegetation CBI, soil CBI) and individual fire severity indicators (crown scorch and consumption, litter consumption, and soil alteration). Analysis of understory fire-legacy metrics revealed that relative metrics, particularly the tip-basal diameter ratio (TBR), consistently outperformed absolute size-based descriptors as indicators of fire severity across ecosystems. TBR showed stable and significant relationships with total CBI and vegetation and soil CBIs, as well as with individual fire severity indicators (percentage of crown scorched, percentage of crown consumption, percentage of litter consumed, and soil alteration). By contrast, the explanatory power of absolute metrics was highly ecosystem dependent, limiting their general applicability. Multivariate models combining multiple understory fire-legacy metrics improved explanatory performance, achieving high marginal (R 2 m = 0.71–0.97) and conditional (R 2 c = 0.85–0.98) coefficients of determination for both CBIs and individual fire severity indicators. Overall, our results demonstrate that relative understory fire-legacy metrics provide robust, transferable, and operationally efficient field-based proxies for fire severity assessment across structurally heterogeneous forest ecosystems. These metrics provide a standardized and ecologically grounded framework for field-based fire severity assessment by integrating surface fuel combustion with canopy and soil responses.