Species evenness declines but specific functional strategy enhances aboveground biomass across strata in subtropical-Warm-temperate forests of South Korea

The relationships amongst environmental conditions, stand age, tree diversity, and trait identity with aboveground biomass (AGB) remain highly debated in forest ecosystems, but these relationships across forest strata (i. e., overstory and understory) remain poorly assessed in subtropical - warm-temperate forests. Here, we hypothesized that environmental conditions, stand age, tree diversity and trait identity jointly control AGB, but divergent species' functional strategies regulate AGB in different forest strata of subtropical - warm-temperate forests. To do so, we analyzed datasets from 72 plots across five sites in subtropical - warm-temperate forests of Jeju Island, South Korea. We used multimodel inference tests based on multiple linear regressions models and then piecewise structural equation modeling (pSEM) to assess the relationships amongst topographic factors, stand age, tree diversity (i.e., species, phylogenetic and functional diversity), and trait identity (i.e., communityweighted mean - CWM - of a trait values) and AGB in the overstory, understory, and whole community. The pSEMs showed that lower species evenness increased AGB in overstory and understory, but CWM of tree maximum height and seed mass enhanced overstory and understory AGB, respectively. Moreover, species evenness mediated the divergent effects of stand age and topography on overstory and understory AGB, respectively. This study shows that the effect of low species evenness on AGB is ubiquitous across forest strata, but divergent species' functional strategies also contribute to shaping AGB. We argue that species' functional strategies (i.e., the mass ratio effect) across forest strata and developmental stages should be taken into consideration for sustainable forest management to achieve the biodiversity conservation and carbon sequestration that underpins human wellbeing through climate change mitigation strategies.

Automated summary

The study reveals that aboveground biomass in different forest strata is influenced by tree environmental conditions, stand age, tree diversity, and trait identification. Low species evenness and different species' functional strategies impact aboveground biomass across forest strata. Therefore, considering the functional strategies of different species is essential for sustainable forest management to achieve biodiversity conservation and carbon sequestration.