Residual Tree Damage in Thin- and Thick-Barked Broadleaved Species: Evidence from Beech, Oak, Birch, and Aspen Stands

Residual tree damage is an unavoidable consequence of thinning operations and may affect future stand quality, tree health, and timber value. Despite the increasing use of mechanized harvesting systems in broadleaved forests, limited information is available on the factors controlling damage occurrence under different stand conditions. This study aimed to quantify residual tree damage following mechanized thinning, identify the main determinants of damage occurrence, and develop a model describing damage probability in broadleaved stands. The study was carried out at six locations in five European countries representing four broadleaved tree species: European beech (Fagus sylvatica L.), oak (Quercus L.), birch (Betula L.), and European aspen (Populus tremula L.). A total of 288 sample plots were established and 4576 trees were inventoried. Thinning operations were performed using two harvesters, HSM 405H and UTC 150—6LS, both equipped with a brand new CTL 40 HW harvesting head, designed for broadleaved tree species. Residual tree damage was assessed immediately after harvesting and analyzed using logistic regression models. Overall, 271 out of 3230 residual trees were classified as with damage, corresponding to a damage rate of 8.4%. No significant differences in damage occurrence were found between the two experienced harvester operators. In contrast, bark thickness and distance from the strip road were identified as the only significant predictors of residual tree damage probability. Thin-barked stands (beech) exhibited significantly higher damage rates (11.8%) than thick-barked stands (oak, birch, and aspen; 5.7%). The developed logistic model provides a practical tool for estimating residual tree damage probability and may support harvesting planning and stand protection.