Wooden structures made of felled trees are used in forested slopes as protection structures against rockfall. A model based on the Discrete Element Method is developed to analyse their response to normal impacts of blocks. The interaction between the blocks and the stems and the stem response are explicitly integrated. After the model calibration using impact tests, a wide range of impact scenarii representative of real configurations is explored. The influence of three parameters (stem diameter, impact velocity and block diameter) is analysed. Three impact types were identified for different ratios $R_D$ between the block and stem diameters. For small $R_D$, the impact is limited to a brief contact. For intermediate $R_D$, successive contacts of similar duration are observed. After the last contact, the block is sent back with a large velocity. For large $R_D$, two contacts are observed. The second contact can be assimilated to a quasi-static loading of the stem. In addition, structure damage increases for increasing $R_D$ ranging from almost no damage for the first impact type to rupture of the stem for the third type. The largest $R_D$ associated with the first impact type is the target configuration that favours block energy decrease and limits structure damage.