Assessing the effect of invasive tree species on rockfall risk - The case of Ailanthus altissima

The invasive tree species Ailanthus altissima increasingly occupies forests in the southern parts of the Alps. Many of these forests grow on steep slopes and protect settlements and infrastructure from natural hazards, such as rockfall. It is feared that the mechanical properties of Ailanthus are less favourable for energy reduction of falling rocks due to lower wood strength as well as higher prevalence of heart rot. Therefore, the question has arisen whether the spread of Ailanthus will substantially change rockfall risk. The objective of this study is to analyse the influence of the spread of Ailanthus trees in forest stands on their protective effect against rockfall. We first quantify the block energy reduction capacity of single Ailanthus trees based on a block-tree impact model and compare it to that of other species. Subsequently, we analyse the effect of Ailanthus on rockfall risk for different forest scenarios with a varying proportion of Ailanthus at the stand scale. The capacity of Ailanthus to reduce the block energy was quantified using a model of the block impact on a tree based on the Discrete Element Method. We then integrated the obtained results in the rockfall trajectory model Rocky For 3D. Based on rockfall simulations, we finally calculated rockfall risk for different forest scenarios representing current forest conditions and an increasing spread of Ailanthus. The energy reduction capacity of Ailanthus lies in the range of the species predominantly present in the study area, as well as other species that are typically found on rockfall slopes in the Alps. Rockfall risk for houses and roads does not increase with an increasing proportion of Ailanthus without change in the forest structure. Assuming a decrease in tree diameters with an increasing proportion of Ailanthus trees, rockfall risk, however, critically increased. Consequently, whether or not Ailanthus changes rockfall risk in the long term, strongly depends on its influence on the forest structure. To anticipate the evolution of protection forests invaded by Ailanthus, more long-term ecological data on growth and succession of Ailanthus at stand scale is required.