Parameterization of rockfall source areas and magnitudes with ecological recorders: When disturbances in trees serve the calibration and validation of simulation runs

On forested talus slopes which have been build up by rockfall, a strong interaction exists between the trees and the falling rocks. While the presence and density of vegetation have a profound influence on rockfall activity, the occurrence of the latter will also exert control on the presence, vitality, species composition, and age distribution of forest stands. This paper exploits the interactions between biotic (tree growth) and abiotic (rockfall) processes in a mountain forest to gather and obtain reliable input data on rockfall for the 3D process based simulation model RockyFor3D. We demonstrate that differences between the simulated and observed numbers of tree impacts can be minimized through (i) a careful definition of active source areas and (ii) a weighted distribution of block sizes as observed in the field. As a result of this field-based, optimized configuration, highly significant values can be obtained with RockyFor3D for the number of impacts per tree, so that results of the model runs can be converted with a high degree of certainty into real frequencies. The combination of the field-based dendrogeomorphic with the modeling approaches is seen as a significant advance for hazard mapping as it allows a reliable and highly-resolved spatial characterization of rockfall frequencies and a realistic representation of (past) rockfall dynamics at the slope scale.