Resistance of Trees against Dynamic Impacts Due to Mass Movements

The role of trees and forests as mitigation measures is more and more taken into account in natural hazard engineering. This requires quantification of the capacity of individual trees to dissipate the energy released by dynamic impacts, which has previously only been based on data obtained from static tree-pulling tests or from dynamic impact tests on wood samples. We predicted that these data are not representative of the maximum amount of energy that can be dissipated by living trees during dynamic impacts. To test this prediction, we carried out rockfall experiments on a forested slope in the French Alps. To calculate the rock’s energy before and after impact, rockfalls were filmed digitally. The recordings of nine impacts causing instantaneous breakage of Abies alba Mill. trees were analyzed in detail. An exponential relationship between stem diameter at breast height (DBH) and the maximum amount of energy a tree can dissipate was highly correlated for all of our experimental data. We applied this relationship to other tree species based on published fracture energies. The relationships obtained were significantly correlated with data from other dynamic impact tests in the field. The results showed that data obtained from dynamic impact tests on wood samples underestimates the maximum amount of energy that can be dissipated by living trees during dynamic impacts. The results will help improve quantification of the protective eects of trees and forests. More accurate comparisons of the protection aorded by forests with civil engineering works will facilitate the combined use of silvicultural interventions and technical protective structures in mitigating hazardous slope processes.