Short-Term Effects of Thinning on Soil Physicochemical Properties, Microbial Characteristics, and Growth of Middle-Aged Picea koraiensis Forests in Eastern Northeast China

Picea koraiensis Nakai is a precious tree species in Northeast China with excellent traits, but research on thinning effects on its growth remains limited, especially regarding soil-thinning–growth interactions. This study focused on a 50-year-old Picea koraiensis plantation in the Mengjiagang Forest Farm, Jiamusi. Four thinning intensities were set: CK (no thinning), T1 (10%–20%), T2 (20%–30%), and T3 (40%–50%). Short-term (1–3 years) stand growth, soil properties, microbial biomass, and extracellular enzyme activities were measured, with stand volume and large-diameter timber yield estimated via self-established equations. Results showed that T3 significantly promoted average DBH (1.98 × CK) and tree height growth (1.60 × CK). T2 achieved the highest increases in stand volume (38.07 m3/ha) and large-diameter timber yield (56.02 m3/ha), exceeding other treatments by 1.20–7.12 m3/ha and 5.60–11.64 m3/ha, respectively. Stand growth indices were positively correlated with thinning intensity, soil microbial biomass carbon, and soil C/P ratio; DBH and height also correlated with soil catalase activity. Thinning intensity has a direct effect on stand growth. Meanwhile, observational data show that it is significantly correlated with changes in soil organic carbon fractions and soil extracellular enzyme activity, and these correlations may constitute potential pathways that indirectly affect stand growth. Moderate-intensity thinning (20%–30%) is recommended for scientific tending and large-diameter timber cultivation of middle-aged Picea koraiensis plantations in this region.