Abstract: Thinning is a crucial silvicultural practice in forest management, the rational intensity of which plays an important role in increasing carbon sequestration capacity of forest ecosystems. However, it is not clear how different thinning intensities affect forest ecosystem carbon stocks and their fractions. We investigated Larix kaempferi plantations in the mountainous regions of eastern Liaoning Province, analyzed changes in carbon stocks and fractions with different thinning intensities (0, 10%-30%, 30%-50% and 50%-70%), and explored key factors influencing stand productivity and soil organic carbon dynamics. The results showed that tree biomass carbon stocks gradually decreased with increasing thinning intensity (from 110.89 Mg C·hm^-2 to 65.77 Mg C·hm^-2) and that herbaceous biomass carbon stocks were significantly lower at different thinning intensities than in control stand, indicating that higher thinning intensities resulted in substantial carbon loss. Compared to the control stand, different thinning intensities increased the reserved individual tree biomass C increment, but only light thinning (25%) improved the reserved stand biomass C increment, suggesting that light thinning was the optimal intensity for L. kaempferi plantations. Diffuse solar radiation was the main factor affecting reserved stand biomass increment. In 0-20 cm soil layer, particulate organic carbon (POC) stocks showed significant difference between different thinning intensities. POC stocks were positively correlated with litter biomass and soil C:N. In 20-40 cm soil layer, soil organic carbon (SOC) and POC stocks showed significant difference between different thinning intensities. SOC and POC stocks were negatively related to soil C:N and litter biomass, respectively. These results suggested that there are different mechanisms of SOC formation and stabilization in different soil layers.

Key words: Larix kaempferi, thinning, light environment, biomass carbon, soil organic carbon