Objective Thinning is an important forest management method to improve forest quality and optimize forest structure, which has a significant impact on regulating carbon sequestration capacity of forest ecosystems. At present, there is no consensus on the response of carbon storage at the ecosystem level of natural mixed forests to different thinning intensities. The aim of this study is to explore the dynamic changes in carbon storage 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%) plots and control were established to calculate the carbon stocks of various components in the ecosystem 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 storage 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 storage among plots with different thinning intensities, but the carbon storage in tree layer of heavily logged plot was significantly lower than control, while there was no significant difference between the lightly logged plot and control (carbon storage in tree layer of lightly logged plot was 1.09 times of control). The carbon storage of shrubs in moderate thinning plots was significantly higher than control (p < 0.05). The carbon storage of herbaceous plants was significantly lower in heavily logged plots than in control (p < 0.05). There was no significant difference in soil carbon storage under different thinning intensities, but it showed an upward trend with increase of thinning intensity, which wass one of main reasons for increasing trend of ecosystem carbon storage.

Conclusion A cultivation and logging intensity of less than 20% can not only achieve the goal of forest structure adjustment, but also promote the accumulation of carbon storage in vegetation layer of forest ecosystem. 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, heavy thinning may actually promote the increase of carbon storage in 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 the growth cycle of forests. Therefore, it is necessary to evaluate the changes in ecosystem carbon storage on a longer time scale in the future.