Higher potential fire intensity at the dry range margins of European mountain trees

Aim: Potential fire intensity (PFI) is among the main drivers of fire-vegetation interactions, but how it varies across species ranges is unknown. We test whether PFI and crown fire likelihood (CFL) increase preferentially towards the warm/dry range margins of mountain trees. Location: The western Alps. Methods: We analysed PFI and CFL patterns across the environmental niches of major mountain forest tree species, using field observation data of surface fuels with fire simulations. Empirical relationships between PFI and climate and vegetation structure were identified in communities sampled for surface fuels, and then used to predict PFI in national forest inventory plots. Results: Simulations indicate that autumn drought, spring precipitation, tree cover and tree basal area drove the variation in understorey fuels among communities, leading to different PFI among and within the environmental ranges of mountain tree species. PFI was consistently higher in the open-canopy, dry margins of dominant tree species. The highest PFI values were found in the sub-Mediterranean and subalpine forests in conjunction with higher CFL. Main conclusions: Mountain trees are exposed to higher PFI and CFL at their dry range margins, suggesting higher potential fire impacts at their rear distribution edges. Moist montane species (Fagus sylvatica, Abies alba, Picea abies) are exposed to surface fires of lower intensities than sub-Mediterranean (Quercus pubescens, Pinus nigra, P. sylvestris) and subalpine species (Larix decidua, P. uncinata, P. cembra) which may both experience higher likelihood of crown fires at their dry and flammable margins.