Post-fire live residuals of maritime pine plantations in Portugal: Structure, burn severity, and fire recurrence

Abstract

The drivers, characteristics and impacts of surface fire regimes in the Mediterranean Basin are poorly understood. We describe the post-fire structure of residual maritime pine (Pinus pinaster Ait.) patches in frequent-fire mountain landscapes of northern Portugal and relate it with burn severity and fire history. Live trees within each plot were measured and cumulative burn severity metrics were assessed at the tree and plot levels. Plot fire recurrence and fire intervals (mean, MFI; minimum, MinFI; and maximum, MaxFI) were calculated from a digitized fire atlas. Fire thinned from below, with a 15.5-cm diameter at breast height corresponding to 50% survival probability. Bole char and crown kill (or live crown base) heights (CKH) were correlated with live tree height; crown ratio averaged 0.38. Each patch burned 1–9 times since 1975, with a MFI range of 1.7–16years in multiple-fire plots. We found wide variation in stand height (7.8–22.5m), basal area (1.4–47.9m2ha−1) and tree density (14–2199ha−1) but single-storied structures prevailed. Higher fire recurrence was associated with lower stand density and higher percentage of fire-scarred trees, indicating cumulative thinning and cambium damage effects. Bole char height and CKH increased with longer fire intervals, i.e. with fuel accumulation and potentially higher fire intensity. MFI (accounting for 70% of the explanation), terrain aspect, and MaxFI explained 55% of plot-level variation in CKH using regression tree analysis. MFI <4.2years generated the lowest CKH. For stands with MFI ⩾4.2years, CKH increased on slopes facing east or south (drier and warmer) and was highest when MaxFI ⩾4.8years. Active crown fire was less likely in forest patches stocked at <20m2ha−1 and <200ha−1, which should decrease crown fire hazard and limit high-severity fire if combined with adequate pruning and surface fuel treatments every 3–5years. The study indicates that patches surviving a first-entry wildfire are likely to persist under a frequent fire regime, adding to the understanding of Mediterranean pines resilience to fire and providing quantitative empirical evidence useful to guide the management of fire-prone forests.