Structural complexity and large-sized trees explain shifting species richness and carbon relationship across vegetation types

1. It is prominently claimed that enhancing forest diversity would play a dual role of nature conservation and climate regulation. While the idea is intuitively appealing, studies show that species richness effects on above-ground carbon (AGC) are not always positive, but instead unpredictable especially across scales and complex terrestrial systems having large-diameter and tall-stature trees. Previous studies have further considered structural complexity and larger trees as determinants of AGC. Yet it remains unclear what drives differential diversity–AGC relationships across vegetation types. 2. Here we test whether structural complexity and large-sized trees play an influential role in explaining shifting diversity–AGC relationships across vegetation types, using a 22.3-ha sampled dataset of 124 inventory plots in woodlands, gallery forests, tree/shrub savannas and mixed plantations in West Africa. 3. Natural vegetation had greater species richness and structural complexity than mixed plantations, as expected. In addition, AGC was highest in gallery forests and mixed plantations, which is consistent with favourable environmental conditions in the former and high stocking densities and presence of fast-growing species in the latter. Significant interaction effects of species richness and vegetation on AGC revealed a vegetation-dependent species richness–AGC relationship: consistently, we found positive species richness–AGC relationship in both mixed plantations and woodlands, and nonsignificant patterns in gallery forests and tree/shrub savanna. Furthermore, there was a vegetation-dependent mediation of structural complexity in linking species richness to AGC, with stronger positive structural complexity effects where species richness–AGC relationships were positive, and stronger positive large-sized trees’ effect where species richness–AGC relationships were neutral. 4. Our study provides strong evidence of vegetation-dependent species richness– AGC relationships, which operated through differential mediation by structural complexity of the species richness and large trees’ effects. We conclude that even higher species richness in diversified ecosystems may not always relate positively with AGC, and that neutral pattern may arise possibly as a result of larger dominant individual trees imposing a slow stand dynamic flux and overruling species richness effects.