Long-Term Effect of Thinning on Radial Growth and Intrinsic Water Use Efficiency of a Pinus koraiensis Plantation on MountGari

This study investigated the effects of thinning intensity on tree radial growth and physiological responses to drought in an approximately 43-year-old Pinus koraiensis plantation. To analyze tree-ring width, a total of sixty wood cores (5 mm) were collected from three thinning treatments applied in 2007: control (Con), light thinning (LT), and heavy thinning (HT). Drought vulnerability indices were examined across three distinct drought periods (2000–2001, 2007, and 2014–2016), and intrinsic water use efficiency (WUEi) was estimated from stable carbon isotope analysis (Con vs. HT). Thinning increased basal area increment (BAI) in both the LT and HT groups, with the strongest and most persistent response observed in the HT group. During the 2007 drought, trees in thinned plots, particularly those in the HT group, showed higher resistance and resilience than trees in the Con group. The increased indices during the 2000 and 2007 drought periods predominantly reflected immediate thinning-induced growth release. WUEi in the HT group increased relative to the Con group during both the 2007 and 2014–2016 drought periods; however, the underlying physiological mechanisms differed, with enhanced net photosynthetic capacity immediately after thinning in 2007 and a likely consistent reduction in stomatal conductance during the 2014–2016 drought. Although constrained by a non-replicated stand design, this site-specific long-term case study provides valuable insights into the multi-decadal growth and physiological trajectories of conifer plantations responding to climate stress.