Knowing the effect of thinning on forest ecosystem services is an important aspect of sustainable forest management. This study analyzed the traditional thinning on tree growth and soil nutrients in the Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantations. The Chinese fir plantations were 11 years old with different initial densities in Jinji (4000 tree·ha⁻¹), and Yingde (3000 tree·ha⁻¹), Xiaolong (2000 tree·ha⁻¹) forest farms, and 20 years old in Yangmei (2000 tree·ha⁻¹) forest farm. The thinning intensity was 35% in Yangmei and Xiaolong forest farms, and 43% in Jinji and Yingde forest farms. Tree growth was measured as the increment of diameter at breast height and stand volume; soil nutrients were measured as pH, soil carbon and nitrogen contents at 0–10 cm soil. The thinning led to an increase in the diameter of trees in all study plots, with the fastest growth rate in Jinji (22.02%) forest farm. The stand volume growth rate was higher in thinning plots than in control plots, with the highest volume growth rate in Xiaolong (27.8%), due to its higher leaf area index and lower density. There was an increasing pattern of C and N contents in the higher initial density plots after thinning (Jinji and Yingde forest farms). During the extreme drought year in 2021, the thinning mitigated the changes in soil acidity and soil moisture, which indicated that thinning could also increase drought tolerance in the short term. Thinning response studies frequently focus on the long-term effect; our results demonstrate how thinning promotes tree growth in the short term.

Forest thinning is a widely used silvicultural method in forest management and has complex effects on carbon sequestration in different types of forest ecosystems. The present study examined the short-term effects of different thinning intensities on carbon storage in an 11-year-old mixed broadleaved plantation. The results partially supported that different thinning intensities have varying impacts on carbon storage in different parts of forest ecosystems. The main results were as follows: (1) The effect of thinning on promoting the growth of fast-growing tree species (Michelia macclurei Dandy and Schima superba Gardn. et Champ.) was earlier than that of slow-growing tree species (Castanopsis hystrix Miq.). (2) A greater thinning intensity conferred greater effects on promoting the tree biomass carbon growth, litter carbon storage, and understory plant diversity, in the order of 41%~50% > 31%~40% > 20%~30%, but these values were lower than those for the unthinned plots. (3) The soil carbon storage declined most in the 41%~50% thinned plots, due to the reduced carbon storage in the humus layer. (4) The 20%~30% thinning intensity promoted carbon sequestration in the short term in the mixed broadleaved plantation. The results suggested that a lower thinning intensity promoted carbon sequestration in the short term, a greater thinning intensity reduced carbon storage at first, but the negative effect on carbon storage exhibited trade-offs later by the growth of tree and understory plant biomass carbon and the accumulation of litter layer carbon.