Are high-severity burns in Alpine beech forests related to eruptive fire behavior?

Abstract

Abstract In the complex Alpine topography, canyon-like terrain morphology combined with steep slopes may cause sudden changes in fire behavior, for instance, turning fast-spreading surface fires into eruptive fires. This phenomenon was observed during the vegetation dormancy in mountain beech (Fagus sylvatica L.) stands, a forest type usually spared from wildfires. The aim of this study is to understand the impact of eruptive fire behavior on the post-fire trajectories of beech trees. To this end, over the course of 3 years, we monitored the post-fire dynamics of 100 beech trees subjected to different fire intensities in February 2020, including a small-scale eruptive fire behavior associated with a steep slope (~41.4° on average) and canyon-like terrain features. Dendrometric and sylvicultural parameters and fire-related damages such as the presence of bark blackening, scars, foliage loss, crashes of branches or whole trees, and parasitic attacks were assessed. The observed medium to high fire severity was highly dependent on the position of the tree within the eruptive fire sector, whereas the relationship with dendrometric and sylvicultural tree characteristics, such as the crown position or the stem diameter, was less clear. Due to the lethal temperatures generated by the heat convection within the eruptive fire sector, the exposed bud and cambium tissues of the beech trees partially or totally failed to develop in the first post-fire season. Beech crown and stem tissue killed by exposure to convective heat fluxes rapidly dries out, loses its elasticity, and becomes brittle. Starting from the second post-fire season, this greatly increases the risk of the crashing of these crown parts and/or entire trees. A detailed understanding of the factors leading to an eruptive fire behavior would highly benefit the fire risk assessment and the detailed planning of the firefighting tactics in mountain areas, eventually increasing the safety of firefighters and preventing post-fire natural hazards, such as rockfall, erosion, surface runoff, and debris flow.