Abiotic and stand age-induced changes in tree diversity and size inequality regulate aboveground biomass and soil organic carbon stock in temperate forests of South Korea

Forest biomass and soil organic carbon are vital for the terrestrial carbon cycle. However, the mechanisms underlying the interaction between trees and site properties (e.g., tree diversity, stand age, climatic and edaphic conditions) that determine the soil organic carbon stock (SOC) remain unclear. Here, we hypothesized that tree diversity, functional identity, and stand structure regulate SOC and aboveground biomass (AGB). To clarify these relationships, we used data from 125 plots dominated by 11 major tree species in South Korea. We analyzed the effects of stand age, tree diversity, functional identity, stand structural attributes, and abiotic drivers (i.e., topographic, climatic, and edaphic variables) on AGB and SOC. We also examined the effect of AGB on the biotic and abiotic drivers of SOC. We performed model averaging and piecewise structural equation modeling to compare and quantify the effects of the drivers on AGB and SOC. Stand age, species richness, and stand structural attributes significantly affected AGB, but their effect on AGB varied depending on the dominant tree species. For SOC, climate, soil pH, and AGB directly regulate SOC. In particular, species richness was the primary biotic driver that directly controlled SOC in termperate forests of South Korea. Dominant tree species also modulated the SOC changes. Our results show that the relationships of biotic and abiotic drivers with dominant tree species are important in both AGB and SOC despite the differences in the direction and magnitude of their effects. Therefore, our study suggests that understanding the relationships of biotic drivers with environmental conditions, which determine the forest ecosystem's carbon cycle, is vital in predicting the global carbon cycle.