Decoupling the Complementarity Effect and the Selection Effect on the Overyielding of Fine Root Production Along a Tree Species Richness Gradient in Subtropical Forests

The mechanism whereby tree species richness and identity affect the production of fine roots (≤ 2 mm) in forests remains controversial. Complementarity effects (via resource partitioning and facilitation, CEs) and selection effects (that is, dominant of species with particular traits, SEs) are the two hypotheses to explain biodiversity effects on ecosystem functions. This study aimed to (1) examine how tree species diversity affects fine root production and (2) disentangle the complementarity effect and the selection effect on the relationship between biodiversity and fine root production. A total of 60 tree clusters with 15 combinations of diversity gradients consisting of 1–4 tree species (Pinus massoniana, Choerospondias axillaris, Cyclobalanopsis glauca and Lithocarpus glaber) were established in subtropical forests. The sequential soil core and ingrowth core methods were used in each cluster to measure fine root biomass and productivity. Fine root production increased with increase in tree species richness. The biodiversity effects on fine root production mostly resulted from CEs. In the nongrowing season, in most cases, the CE on biomass was positive and became stronger as richness increased, but the opposite situation was observed in the growing season. The strong positive and negative effects of the proportions of C. glauca and L. glaber in the tree clusters on fine root biomass, CEs and SEs, suggest the coordinated action of species diversity and identity in modulating biodiversity effects on belowground processes.