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

The invasive tree speciesAilanthus altissimaincreasingly occupies forests in the southern parts of the Alps. Manyof these forests grow on steep slopes and protect settlements and infrastructure from natural hazards, such asrockfall. It is feared that the mechanical properties ofAilanthusare less favourable for energy reduction of fallingrocks due to lower wood strength as well as higher prevalence of heart rot. Therefore, the question has arisenwhether the spread ofAilanthuswill substantially change rockfall risk. The objective of this study is to analysethe influence of the spread ofAilanthustrees in forest stands on their protective effect against rockfall. Wefirstquantify the block energy reduction capacity of singleAilanthustrees based on a block-tree impact model andcompare it to that of other species. Subsequently, we analyse the effect ofAilanthuson rockfall risk for differentforest scenarios with a varying proportion ofAilanthusat the stand scale. The capacity of Ailanthus to reduce theblock 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 RockyFor3D. Based on rockfall simu-lations, wefinally calculated rockfall risk for different forest scenarios representing current forest conditions andan increasing spread ofAilanthus.The energy reduction capacity ofAilanthuslies in the range of the speciespredominantly present in the study area, as well as other species that are typically found on rockfall slopes in theAlps. Rockfall risk for houses and roads does not increase with an increasing proportion ofAilanthuswithoutchange in the forest structure. Assuming a decrease in tree diameters with an increasing proportion ofAilanthustrees, rockfall risk, however, critically increased. Consequently, whether or notAilanthuschanges rockfall risk inthe long term, strongly depends on its influence on the forest structure. To anticipate the evolution of protectionforests invaded byAilanthus,more long-term ecological data on growth and succession ofAilanthusat stand scaleis required.