Short-Term Effects of Thinning on Tree Growth and Soil Nutrients in the Middle-Aged Chinese Fir (Cunninghamia lanceolata (Lamb.) Hook.) Plantations

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 center dot ha(-1)), and Yingde (3000 tree center dot ha(-1)), Xiaolong (2000 tree center dot ha(-1)) forest farms, and 20 years old in Yangmei (2000 tree center dot 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.