Abstract: To explore potential responses of ecosystem carbon density to changes of community structure during natural regeneration of woody plants, we analyzed the relationships between ecosystem carbon density and its components, tree species diversity, structural diversity (CV_DBH) and spatial structure parameters (mingling, aggregation, dominance, crowding) of Cunninghamia lanceolata forests with different sprouting densities (1154, 847 and 465 individuals·hm^-2) at the early stage of succession in Baishanzu National Park. The results showed that tree species diversity (species richness index and Shannon diversity index) increased with the decrease of sprouting density of C. lanceolata. Among the stand structural parameters, CV_DBH, stand density, and mingling increased with the decrease of sprouting density of C. lanceolata. The stand distribution pattern of different C. lanceolata densities was uniform, with sub-dominant stand growth status and relatively dense status. The carbon density of tree layer under high, medium, and low sprouting densities of C. lanceolata were 57.56, 56.12 and 46.54 t·hm^-2, soil carbon density were 104.35, 122.71 and 142.00 t·hm^-2, and the total carbon density of ecosystem were 164.59, 182.41 and 190.13 t·hm^-2, respectively. There was little variation in carbon density of understory layer and litter layer among different treatments. The carbon density distribution characteristics of different C. lanceolata densities were following the order of soil layer (63.4%-74.7%) > tree layer (24.5%-35.0%) > understory layer and litter layer (0.8%-2.0%). The results of variance partitioning analysis indicated that the change of tree layer carbon density was mainly influenced by stand structure diversity, soil layer carbon density was influenced by both tree species diversity and stand structure diversity, while ecosystem carbon density was mainly influenced by tree species diversity. Stand spatial structure parameters had a relatively little effect on ecosystem carbon density and its components. The sprouting density of C. lanceolata significantly affected ecosystem carbon accumulation during the conversion from C. lanceolata plantations to natural forests. A lower remaining density of C. lanceolata (about 500 individuals·hm^-2) was more conducive to forest carbon sequestration.

Key words: tree species diversity, stand structure, carbon density, natural restoration, Cunninghamia lanceolata