Linking understory functional traits to soil organic carbon storage in natural and planted subtropical forests

Soil carbon (C) provides many ecosystem services essential for mitigating climate change. Natural ecosystem regeneration and plantation forests have been suggested as important alternatives to increasing soil C sequestration. Despite being vital to the world's carbon cycle, little is known about soil C dynamics in subtropical forests. Furthermore, limited information is available on how the taxonomic and functional structure of the tree understory influences ecosystem services related to soil organic C in subtropical natural and plantation forests. To address these questions, we evaluated two land uses - native forest and plantation forest (Eucalyptus sp.) at four locations in the subtropical portion of the Atlantic Forest Biome. We determined soil C stocks, litter and roots C:N ratio, and the functional and taxonomic composition of the natural regenerating tree understory. We found that in native forests, understory taxonomic and functional composition are not associated with soil C storage. Nevertheless, litterfall was associated with higher soil C stocks in these forests. We further observed that canopy openness, basal area, and functional richness of the understory community are positively associated with soil C stocks in plantation forests. Our study highlights the complex relationship between different tree strata in plantation forests, suggesting that additional C can be sequestered in the soil pool if suitable conditions exist to develop the tree understory. We recommend that maintaining and enhancing natural regeneration is essential for providing ecosystem services related to soil organic C.