The Ecophysiological Effects of Thinning in Anatolian Black Pine [Pinus nigra J.F. Arnold ssp. pallasiana (Lamb.) Holmboe] Afforestation Areas in Çankırı Region

This study was conducted to assess the short-term effects of thinning treatments of varying intensities (control, moderate, and heavy) on selected morphological and physiological parameters in 35-year-old Anatolian black pine (Pinus nigra J.F. Arnold subsp. pallasiana) plantations located within the boundaries of the Çankırı Forest Enterprise Directorate. Morphological traits assessed included diameter and height, as well as their seasonal increment values, while physiological parameters comprised predawn and midday leaf water potential (Ψpd and Ψmd) and photosynthetic efficiency (Fv/Fm). Field measurements were carried out during the 2023 growing season. Morphological data were collected in April and October, and physiological measurements were conducted in July, August, and September. The results indicated that thinning intensity had a statistically significant effect on diameter increment, with the highest values observed under moderate thinning (A2). In contrast, height and height increment were not significantly influenced by thinning intensity over the short term. For water potential measurements, Ψpd values differed significantly among months, reflecting seasonal drought stress, whereas no statistically significant differences were detected among thinning levels. Similarly, Ψmd values did not vary significantly across either months or thinning treatments. Fv/Fm values remained within the range associated with physiologically healthy plants across all treatments, although seasonal variation among months was significant. However, no statistically significant differences were found between thinning levels in terms of Fv/Fm. The findings suggest that thinning interventions can positively affect growth-related morphological parameters (especially diameter increment) in the short term, whereas physiological responses appear to be more strongly influenced by environmental conditions. Furthermore, factors such as the plantation origin of the stand, prior thinning history, and regular spacing likely contributed to lower competition among trees from the outset, potentially limiting the magnitude of treatment effects observed in some parameters.