Modeling the effect of rockfall on forest development in a dynamic forest landscape model

Mountain forests below rocky cliffs are regularly impacted by falling blocks. Rockfall thus increases tree mortality and can locally alter the forest structure. At the same time, trees can decelerate or stop falling blocks and play an important role in protecting settlements and infrastructure from rockfall impacts. Furthermore, trees in the upper part of a slope protect trees further downslope. Considering the interaction between forest dynamics and rockfall disturbance in dynamic forest models is necessary to accurately predict the development of rockfall protection forests in the long term. In this study, we integrated the disturbing effect of rockfall on trees in the dynamic forest landscape model TreeMig through a coupling with three‐dimensional rockfall simulations and analyzed the rockfall‐forest feedback over time. We introduced an additional mortality per cell, based on the probability and severity of rockfall disturbance derived from rockfall simulations. We implemented the potential feedback effect between rockfall disturbance and forest development using a meta‐model of the rockfall simulations and analyzed the sensitivity of forest development to varying disturbances for a case study in the Swiss Alps. With increasing disturbance, the total biomass of the forest decreased, whereby differences were relatively small at the scale of the forest complex, but more pronounced at local (cell) scale. Generally, the comparison to light detection and ranging (LiDAR)‐derived forest data showed a better agreement between the modeled forest and reality when considering the rockfall disturbing effect. The coupled simulations further revealed a positive feedback effect of rockfall disturbance and forest development. Disturbance probability and severity clearly decreased with advancing forest growth, which, however, lead to an overestimated recolonization of the disturbed areas. Still, the rockfall disturbance module clearly improved the simulations of a rockfall protection forest with TreeMig. Future improvement of the model should include a better representation of soil formation and water availability in the disturbed areas and the consideration of long‐term effects of the rockfall disturbances, such as pests and diseases. The consideration of the rockfall disturbance in forest modeling is particularly relevant for small‐scale studies requiring a detailed representation of spatial differences in forest cover, as this is the case in protection forest management.