Assessing regional variation of individual-tree diameter increment of Crimean pine and investigating interactive effect of competition and climate on this species.

Nowadays, many forests are intensely exposed to the adverse effects of climate change such as drought. To increase the resilience of the forests against climate change, prescribing appropriate silvicultural treatments is of great importance. At this point, closely scrutinizing the simultaneous effects of competition and climate on the forests has become a very important issue. In the present study, a climate-based individual-tree diameter increment model based on the Weibull growth equation was developed using the mixed-effects framework. The data were collected from naturally established and managed Crimean pine (Pinus nigra subsp. pallasiana (Lamb.) Holmboe) stands located in three separate climate regions, ranging from southwest to north of Türkiye. The measurements were conducted in a total of 108 randomly selected sample plots from these regions. The results of the current study showed that there were significant differences in the diameter increment between climate regions, ranging from 13 to 30%. The growing season total precipitation (GSTP) and the warmest month maximum temperature (WaT) were identified as significant climate variables in explaining the variation in the diameter increment. In addition, GSTP had a stronger effect on the diameter increment than WaT. The quantitative analysis demonstrated that a 10% decline in GSTP together with a 1 °C increase in WaT resulted in a considerable reduction in the diameter increment, varying from 10 to 25% depending on the competition levels. The proposed climate-based diameter increment model may be useful for the forest managers and practitioners to determine optimal thinning cycle under various climate change scenarios.