Closer-to-nature pyro-silviculture mitigates crown fire potential in dry mountain conifer forests

Dry conifer forests of the southwestern European Alps are increasingly exposed to high-severity crown fires due to prolonged droughts and foehn winds intensified by climatic change. On steep terrain, traditional fire hazard reduction treatments, such as uniform thinning, are often ineffective because they fail to disrupt the alignment of wind, slope, and flammable canopy fuels that drive crown fire propagation. This study addresses the need for effective, ecologically grounded strategies to mitigate crown fire behavior in steep, dry Scots pine forests by applying and evaluating an innovative pyro-silvicultural treatment implemented over 40 ha. Grounded in fire behavior physics under a closer-to-nature approach, the treatment increased forest structural heterogeneity and reduced crown fuel continuity through a two-order network of elliptical canopy gaps. Larger gaps were oriented along the expected fire-spread direction to disperse convective heat, while smaller transverse gaps were designed to limit crown fire sustainment along the flanks. Using the physics-based Wildland-urban interface Fire Dynamics Simulator (WFDS), post-treatment simulations showed reductions in heat release, air temperature, fire rate of spread, and crown fuel consumption. These changes indicate altered convective dynamics, fragmentation of the fire front, and a transition toward surface fire behavior, demonstrating the effectiveness of spatially explicit canopy manipulation in mitigating crown fire sustenance. Overall, the proposed pyro-silvicultural approach provides a proactive framework to enhance the resistance and resilience of dry conifer forests to crown fires on steep mountain slopes.