Quantifying the resistance of protective mixed-forest against natural hazards in the Pyrenees

In mountain areas, extreme weather events can trigger hydrogeomorphological processes (HGEs), such as torrential floods, snow avalanches, landslides or rockfalls. To mitigate the risks associated with these natural hazards, ecosystem services based risk reduction (Eco-DRR) approaches can be applied. In an Eco-DRR scheme, vegetation plays an important role, not only in reducing the probability of occurrence, but also in minimizing its impact providing natural barriers that limit the propagation of flow and energy. Understanding how the vegetation resists such events within a given forest stand is relevant for designing better forestry practices and maximizing the protective role of the forest. Here we focus on quantifying the mechanical resistance of trees subjected to HGE processes considering two potential failure modes, namely tree overturning and stem breakage. To this end, we perform pulling tests on 53 trees of two main species (Abies alba Mill. and Fagus sylvatica L.) growing on two plots (Gourzy forest – France and Arañones Forest – Spain) in the Pyrenees. We also collected structural and neighborhood characteristics of trees and forest stands and carried out dendroecological studies on selected trees. Both areas have a similar soil type (sandy soil - dolomites and calcarenites) composed of limestone, marl, clay and sandstone, and are affected by recurrent snow avalanches and rockfalls. Using a structural equation model (SEM) statistical framework, we test whether mechanical capacity is determined by either functional traits (i.e. species, tree growth, diameter and height) or structural traits (i.e. tree density, tree structure and slenderness). Our results suggest that forest competition modifies the mechanical capacity of trees through two pathways involving both functional and structural traits. Overall, functional traits condition the individual stiffness parameter of trees, whereas structural traits are mostly related to changes in elastic modulus. These results shed light on the behavior and plasticity of both species in avalanche and rockfall events, revealing better adaptations depending on certain allometric and structural traits, and providing relevant information for foresight on management strategies of forests with a protective role against natural hazards in the face of climate change.