Subordinate tree species diversity explains carbon stock better than dominant tree diversity after decades of forest rewilding

Forest rewilding is expected to help support nature restoration and mitigate climate change by promoting soil carbon (C) stocks. Increases in biodiversity after decades of rewilding may affect forest soil C stocks; however, the relative contribution of subordinate and dominant tree species diversity to soil C stocks after decades of forest rewilding remains poorly understood. Here, we conducted a standardized field survey to investigate how subordinate (i.e. rarer) and dominant (i.e. more common) tree species diversity (determined based on basal area) correlate with multiple aspects of soil C stocks (i.e. total soil C stock, dissolved organic C, microbial residue C, and microbial respiration) after 31 years of the establishment of a forest plantation in a subtropical ecosystem. We found that both subordinate and dominant tree species diversity were positively correlated with soil C stock, dissolved organic C, microbial residue C, and microbial respiration. Meanwhile, tree functional traits (e.g. the proportion of N-fixing taxa) among subordinate species were positively correlated with bacterial residue C. Strikingly, subordinate tree species diversity explained a larger portion of variation in soil C stock compared with the diversity of dominant tree species. Structural equation model (SEM) further suggested that subordinate plant biodiversity influenced soil C stocks via its influence on plant traits. Synthesis and applications. Our work provides new insights on the crucial role of subordinate tree species diversity in supporting C stocks after decades-long rewilding of a subtropical forest. Therefore, the preservation of the rarer subordinate plant species is fundamental to develop sustainable forest management strategies, and for policymakers to promote climate change mitigating ecosystem services.