Plant functional traits are important characteristics for understanding the relationship between individual, community, and ecosystem functions. In the present study, we established 43 sample sets to examine 12 types of plant functional traits and corresponding carbon density information at the community scale. The objective of this work was to determine the internal relationship between the adaptive variation in functional traits and the carbon density of forest communities along an elevation gradient. The results show that at a higher altitude, the carbon to nitrogen ratio (C/N) in leaves increased linearly, whereas carbon content (C_leaf), specific root length (SRL), and height (H) initially increased and then decreased. Chlorophyll content (Chl) and fine root phosphorus content (P_root) decreased with elevation and fine root carbon content (C_root) initially increased and then flattened out with altitude. Fine root nitrogen content (N_root) initially decreased and then increased, whereas leaf nitrogen content (N_leaf), lignin content (LLC), and fine root dry matter content (RDMC) showed no significant difference with altitude. SRL, H, the content of C, P in leaf and root showed significant correlations with vegetation carbon density (VCD) and soil carbon density (SCD) in the Tianshan forest, as they have influential effects on the photosynthesis of vegetation, in which SRL and H could affect plants resource acquisition and utilization, whereas C and P would limit nutrient availability. There was a negative correlation between LLC and SCD in the Tianshan forest, as LLC would limit the decomposition rate of plant residue. VCD initially increased and then decreased, whereas SCD and total carbon density (TCD) increased gradually with altitude. Plant functional traits interact with environmental factors and forest structure functions at multi-ecological levels, and the relationship between the three components needs to be validated on a large scale.