Effects of Increasing Debarking Intensity on Tree Growth, Vitality, and Cork Thickness in Cork Oak Under Dry Mediterranean Conditions: Results from a 20-Year Experimental Trial

Cork oak (Quercus suber L.) is a key Mediterranean species whose sustainable management depends on balancing cork production with tree vitality. Debarking intensity is regulated through the debarking coefficient (dcoef), which expresses debarking height relative to tree perimeter at breast height, but its long-term effects on tree growth, cork thickness, mortality and vitality remain insufficiently understood. This study evaluated the effects of increasing debarking intensity in a long-term experimental trial established in 2003 at Herdade da Contenda, southern Portugal, under dry Mediterranean conditions. Three dcoef trajectories were considered, corresponding to increasing debarking intensities relative to the legal limits established by Portuguese legislation. Cork oak trees were monitored between 2003 and 2023. Over this period, diameter growth, mature cork thickness in 2023, tree mortality and tree vitality indicators—including crown dieback symptoms and debarking-related wounds—were assessed. Full and restricted datasets were analysed to distinguish population-level responses from the response of trees that maintained the planned debarking trajectories. Linear mixed-effects models and Cochran–Armitage trend tests were used to evaluate the effects of increasing debarking intensity. Increasing debarking intensity was not associated with significant reductions in diameter growth, which was mainly correlated to tree diameter and total height variables. Some debarking intensity variables showed weak positive correlations with cork thickness. No significant differences in tree mortality were detected among the established trial dcoef trajectories. By 2023, mortality alone still did not show a significant increasing trend across debarking intensity trajectories. However, when mortality and reduced tree vitality were analysed jointly, a significant increasing trend was detected with increasing debarking intensity. The results obtained from both datasets showed no statistically significant reductions in diameter growth or cork thickness among trees that maintained the planned debarking trajectories. Nevertheless, increasing debarking intensity may increase the long-term risk of vitality loss through crown dieback symptoms and/or debarking-related wounds. These findings should be interpreted in light of the selection effect associated with the restricted dataset, as higher debarking intensities were maintained only in trees that remained sufficiently vigorous to sustain the planned trajectories. Consequently, the absence of detectable negative effects on growth or cork thickness in this subset may not reflect the response of the entire initial population. By combining full and restricted datasets with mortality and vitality assessments, this study provides a comprehensive evaluation of the long-term responses of young cork oak trees to increasing debarking intensity in dry Mediterranean conditions.