Abstract. This article examines how digital terrain model (DTM) spatial resolution influences rockfall modelling using a probabilistic process-based model, RockyFor3D, while taking into account the effect of forest on rockfall propagation and runout area. A rockfall site in the Trenta valley, NW Slovenia, was chosen as a case study. The analysis included DTM spatial resolutions of 1 m, 2 m, 5 m, 10 m, 12.5 m and 25 m, based on lidar data. The highest spatial resolution (1 m) was used to calibrate the surface roughness coefficients of the model while also taking into account the effect of forest since it shapes the rockfall propagation and runout area. The results of the calibration runs were evaluated using goodness-of-fit indices, and the best set of parameters were further used for modelling rockfalls with and without the effect of forest for all spatial resolutions. Accuracy statistics were used to validate the modelled rockfall propagation and runout area for each spatial resolution, with/without the effect of forest. Finally, modelling outputs, such as the mean of the maximum and maximum kinetic energy, the number of block passes and forest parameters in the rockfall propagation area, were compared.