Do functional diversity and trait dominance determine carbon storage in an altered tropical landscape?

Altered landscapes play a major role in biodiversity conservation and carbon (C) storage in the tropics. There is increasing evidence that C storage potential is controlled by tree functional diversity, but underlying mechanisms are debated. We analyzed the effects of trait dominance (mass-ratio hypothesis), species diversity, and trait variation (species complementarity) on C storage in the soils and vegetation of 20 agroforestry systems (AFS) and seven forested sites in Costa Rica. AFS consisted of organic and conventional coffee farms and pastures with trees. We used the community weighted mean (CWM) to measure trait dominance, and functional divergence (FDvar) to evaluate trait variation of wood densities (WD) and maximum heights (Hmax) of woody plants at each site. Species richness, the number of woody plants per hectare, and slope of the terrain were also considered as independent variables. Soil organic carbon (SOC) increased with higher -CWMWD and with higher variability of Hmax (FDvarHmax) across land-use types. Aboveground carbon (AGC) was controlled by the number of woody plants per hectare and by species richness. Our results suggest that dominant traits as well as species complementarity play an important role in determining C storage. Diverse, multilayered AFS which incorporate trees with high WD, combined with the conservation of remnant forests, can maximize C storage in the soils and vegetation of altered tropical landscapes.