Overturning resistance of large diameter Norway spruce (Picea abies (L.) Karst) on sloped conditions

Increasing knowledge on tree stability in the forest environment has two major objectives: the first is to provide detailed information to help the decision-making processes that aim to maximize the efficiency of localized silvicultural interventions; the second is to provide useful information and safety guidelines for those who operate in these forests. With this aim, the mechanical response of large diameter Norway Spruce (Picea abies (L.) Karst) trees was investigated in the north-eastern Italian Alps. The aim of the study was to define the mechanical parameters necessary to model the overturning propensity of shallowly rooted mature trees growing on steep terrain characterized by soils with limited cohesion. Twelve standing trees (mean DBH = 50.6 cm) were equipped with sensors at their base and pulled until their complete overturning. Thanks to the pulling method employed and the high precision sensors it was possible to describe the overall overturning behaviour in terms of bending moment vs. rotation curves. The pulling of two additional tree stumps also provided an example of the great contribution of the stem and crown weight to overall stability. Results show an increased resistance of the trees when compared to equivalent conditions but on flat terrain. On the contrary, the equivalent secant stiffness of the root-plate did not manifest significant variations. The obtained outcomes may be directly used as input data for management tools such as predictive models that quantify windthrow risk, protection against rockfall events or avalanches in all alpine contexts having similar characteristics to those dealt with in this study.