Objective Thinning is an important forest management method to improve forest quality and optimize forest structure, which significantly influences the carbon sequestration capacity of forest ecosystems. At present, there is no consensus on the response of carbon stock at the ecosystem level in natural mixed forests to different thinning intensities. This study aimed to explore the dynamic changes in carbon stock in coniferous and broadleaved mixed forest ecosystems under different thinning intensities, providing a theoretical basis for rational selection of thinning intensities and enhancing forest carbon sequestration capacity.

Method In 2011, light thinning (intensity of 17.3%), moderate thinning (intensity of 34.7%), heavy thinning (intensity of 51.9%), and control plots were established. The carbon stocks of various components were quantified in 2021. The dynamic changes in forest carbon stocks over the past 10 years after thinning were analyzed to reveal the response patterns of forest ecosystem carbon stocks to different thinning intensities.

Result After 10 years of thinning, the carbon stock of forest ecosystem showed an upward trend with increasing thinning intensity (333.72 t/ha for light thinning, 358.48 t/ha for moderate thinning, and 386.93 t/ha for heavy thinning). There was no significant difference in forest ecosystem carbon stock among plots with different thinning intensities, but the carbon stock in tree layer of heavy thinning plot was significantly lower than that in control, while there was no significant difference between the light thinning plot and control (carbon stock in tree layer of light thinning plot was 1.09 times of control). The carbon stock of shrubs in moderate thinning plots was significantly higher than control (p < 0.05). The carbon stock of herbaceous plants was significantly lower in heavy thinning plots than control (p < 0.05). There was no significant difference in soil carbon stock under different thinning intensities, but it showed an upward trend with increase of thinning intensity, which was one of main reasons for increasing trend of ecosystem carbon stock.

Conclusion A thinning intensity of less than 20% can achieve the goal of forest structure adjustment and promote the accumulation of carbon stock in the vegetation layer of forest ecosystems. After 10 years of thinning, it has been restored to the level of control. From the perspective of ecosystem, if other carbon components such as soil are included, the thinning intensity has no significant impact on total carbon stock of the ecosystem. Therefore, it is necessary to comprehensively consider the restoration status of various components of ecosystem and select the most suitable thinning intensity for nurturing. The time span of this study is 10 years, which is relatively short compared with growth cycle of forests. Therefore, it is necessary to evaluate the changes in ecosystem carbon stock on a longer time scale in the future.