To compensate for the temporary loss of protection due to the reduced forest in the felling area, a considerable part of the trees felled in rockfall protection forests are left in place. Although being a widely used technique, there is little objective information on the efficacy of these trees felled across the slope. To quantify the protective function of such structures, full scale rockfall tests on a protective structure made with trees felled across the slope were carried out. The results show significant energy losses when impacting the structure. However, the rock energy significantly increases again after the structure if the rock is not stopped. Consequently, a larger number of felled trees is required to stop all rocks and, to increase the number of stopped rocks, the structure height has to be determined in relation to the rock size which promotes, in most cases, structures made of stacked trees.