Environmental, structural and taxonomic diversity factors drive aboveground carbon stocks in a semi-deciduous tropical rainforest strata in Cameroon

Tropical forests play an important role in biodiversity conservation and ecosystem functioning. Few studies have teased apart the independent effects of biotic and abiotic factors on aboveground carbon across tree size groups and the whole tree community level in a tropical semi-deciduous rainforest. This study aims to analyze the relative and independent effects of abiotic (topography, soil fertility/texture and disturbance) and biotic factors on carbon stocks across tree size groups, as well as at the whole tree community in semi-deciduous plots, including logged plots. We used data from 30 1-ha plots and 22,064 trees distributed in a semi-deciduous tropical rainforest of Cameroon. For each plot, we quantified disturbance, topography and eleven soil conditions. Besides, we quantified three taxonomic richness and diversity Gini index. We used structural equation models to test the hypothesis that all drivers have independent, positive effects on aboveground carbon stocks across tree-sized groups and the whole tree community. The logging disturbance decreased aboveground carbon stocks for the whole tree community and trees size groups across our site except for large trees. Topographical factors (i.e., slope and elevation) increased aboveground carbon (AGC) of the whole tree community and the large trees. In contrast, soil nitrogen (Nsoil) and clay proportion increased AGC of the small stems and understorey trees groups. This study found that biotics factors have some indirect and direct effects on AGC. Taxonomic diversity through niche complementarity had a positive relationship with aboveground carbon stocks across the trees size groups and the whole tree community, while diversity Gini index had strongest relationships with aboveground carbon stocks. The significant effects of structural diversity on aboveground carbon stocks of the whole tree community, large trees, and understory trees highlight the importance of maintaining a layered structure and tall trees in the forest.