Functional identity drives tree species richness-induced ecosystem carbon allocation in subtropical forests

Tree species diversity has a strong impact on ecosystem function and services, especially carbon (C) storage. Considering the strong linkages between above-and belowground carbon dynamics, as well as the potential impact of diversity on C allocation, the overall benefits of tree diversity for total C storage including plant and soil in forests remain unclear. Here, we evaluated the total C and its allocation between above-and belowground biomass (Shoot C: Root C ratio), as well as between plant biomass C and soil organic C (Plant C: SOC ratio), in relation to tree species richness, functional diversity and identity in a subtropical tree diversity experiment. Tree species richness increased total C, Shoot C: Root C ratio, and Plant C:SOC ratio. Meanwhile, tree communities with higher leaf thickness and leaf dry matter content, lower specific leaf area and leaf nitrogen were associated with higher Shoot C: Root C ratio and Plant C:SOC ratio. Tree communities with higher specific root length and specific root area were associated with higher total C and Plant C:SOC ratio. Our structural equational modelling revealed that greater tree species richness led to higher Shoot C: Root C ratio by increasing tree aboveground biomass C both directly and indirectly through functional identity (i.e. community-weighted mean functional trait) rather than functional diversity. Tree species richness increased Plant C: SOC ratio by increasing plant biomass C indirectly through functional identity and unchanged SOC. Our results suggest greater total C stocks in species-rich forests, primarily driven by an increase in plant biomass C, mediated by functional identity, while soil C stocks remain unchanged.