The protective effect of forests against rockfalls across the French Alps: Influence of forest diversity

The role of forests in the mitigation of natural hazards has been repeatedly demonstrated. The protective effect of mountain forests against rockfalls has especially been pointed out because it can constitute a natural and cost-effective protection measure in many situations. However, this particular ecosystem service may substantially differ according to the structure and the composition of the forest. Until now, the rockfall protection capability has always been studied at a local scale with only few forest types. Moreover, the comparison of the protective effect of the different forest types studied remains difficult because different methods and indicators were used. For the same reasons, it is not possible to draw conclusions about the influence of biological and structural diversities on the protection capabilities of forests from former works.The aims of this study were (1) to quantitatively assess the protective effect of forests at the French Alps scale and build a classification based on the protection capability, (2) to compare the protective effect of the different forest types present in the French Alps and (3) to analyze the relations between the protective effect and the forest diversity in terms of stand structure and tree composition. For this purpose, the model Rockyfor3D was used to simulate the propagation of rocks on 3886 different forest plots spread over the whole French Alps. Quantitative indicators characterizing the protective effect of each forest plot were then calculated from the simulation results and used to perform the different analyses.Our results emphasized the importance of taking into account the length of forest in the maximum slope direction for an accurate assessment of the protective effect. Thus, the minimum length of forest to get a reduction of 99% of the rockfall hazard was chosen as indicator to compare protective effect between forests. Using this indicator, half of the French Alpine forests presented a high level of protection after a short forested slope (190m). A decreasing gradient in the protection capabilities was observed from forest types dominated by broadleaved species to those dominated by conifer species. Moreover, considering an equivalent proportion of conifers, stands dominated by shade-tolerant tree species showed better ability to reduce rockfall hazard. Finally, our study highlighted that a high biodiversity and a structural heterogeneity within the forest have a positive effect on the reduction of rockfalls hazard.