This study aims to systematically evaluate the relationship between tree underplanting intensity and carbon storage in Cunninghamia lanceolata plantations, explore the effects of underplanting tree species number and diversity on carbon storage, and to provide theoretical support for optimizing the management model of Cunninghamia lanceolata plantations. Experiments with different underplanting intensities and tree species combinations were conducted to identify the optimal underplanting strategy that balances carbon sequestration and forest biomass gain.

The study selected Cunninghamia lanceolata plantation sample plots in four typical subtropical regions of Hunan Province. These plots were grouped according to different underplanting intensities (no underplanting, low-intensity underplanting, medium-intensity underplanting, and high-intensity underplanting), and biomass and carbon storage were monitored over two years. The study measured the diameter at breast height (DBH), tree height, and biomass of underplanted species, and analyzed the relationship between underplanting intensity and carbon sequestration through carbon storage calculations.

The results showed that underplanting intensity significantly affects the changes in carbon storage in Cunninghamia lanceolata plantations. Medium-intensity underplanting (20-50 trees) not only significantly enhanced the growth rate of individual tree biomass but also promoted an increase in carbon storage while maintaining a relatively high carbon stock. Specifically, the carbon storage growth rate in medium-intensity underplanting plots was significantly higher than that in plots without underplanting. Additionally, the diversity of underplanted species contributed differently to the carbon storage, with fast-growing species such as Phoebe zhennan (Minnan) and Michelia compressa (Wood oil camphor) playing a prominent role in enhancing the carbon sequestration capacity of the community.

Medium-intensity underplanting effectively promotes the increase of carbon storage in Cunninghamia lanceolata plantations and provides scientific basis for diversified forest management. Future studies should optimize underplanting schemes according to different ecological conditions and tree species characteristics, balancing carbon sequestration and biodiversity enhancement, thereby achieving a dual improvement in forest productivity and carbon storage.

To explore the growth differences of Chinese fir plantations under different tree species configurations, to comprehensively improve forest quality and comprehensive benefits, and to provide relevant references for promoting the improvement of quality and efficiency of Chinese fir plantations.

Eleven sample plots of Chinese fir plantations were established in four counties in southern Hunan, Including 9 experimental plots and 2 control plots, continuous monitoring of basic information of the plot and forest growth index factors for 3 years.

1) In Chinese fir plantations with different tree species configurations, the growth of both Chinese fir and replanted tree species has shown an increasing trend year by year, better growth in middle-aged and near mature forests, the main tree species for replanting are Phoebe bournei, Liquidambar formosana, Cyclobalanopsis glauca, Schima superba and Cinnamomum camphora; 2) The effect of different forest reserve densities on the growth of Chinese fir and replanted tree species is significant, the average annual growth of the average breast height diameter, average tree height, and average tree height of replanted tree species in the forest stand are positively correlated with the reserve density of the forest stand; 3) The average breast height diameter of mature forest stands increases the most annually, the average annual growth of tree height in the middle-aged forest stage is the highest, the average breast height diameter and average tree height of replanted tree species show a negative correlation with the age group stage of the forest stand; 4) The replanting and growth of Phoebe bournei, Liquidambar formosana, Cyclobalanopsis glauca, Schima superba and Cinnamomum camphora in Chinese fir plantations showed stable performance, but when replanted with Houpoea officinalis, broad-leaved trees species perform poorly.

The improvement of quality and efficiency of Chinese fir plantations should be based on differentiated thinning and replanting plans according to forest age and density, Select local precious tree species Should be selected for replanting according to local conditions, dynamically adjust tree species structure and forest density, to select the optimal tree species configuration mode that is conducive to improving forest quality and efficiency.