The role of different dimensions and components of diversity
We found that functional diversity showed stronger effect on forest productivity than taxonomic and phylogenetic diversity (Fig. 2), which is consistent with recent findings (Craven et al. 2018; Haoet al. 2018; Mazzochini et al. 2019; Staples et al.2019). This is somehow not surprising because functional diversity is designed to reflect the functional differences among coexisting species that may not be well captured by taxonomic and phylogenetic diversity, but are more closely related to species niches and thus ecosystem functions (Petchey & Gaston 2006). Interestingly, we also found markedly higher effect of functional diversity than taxonomic and phylogenetic diversity on biomass stability, though not for the temporal stability of productivity. Plant functional traits may affect the resistance, resilience and stability of communities through mechanisms as follows (Mori et al. 2013; Schnabel et al. 2019): communities dominated by fast-growth strategy species may have higher resilience, while communities dominated by slow-strategy species may have higher resistance. Consequently, communities with higher mixture of species strategies from fast-growth to slow-growth can exhibit both greater resistance and resilience, and thus higher ecosystem stability. However, this hypothesis can only explain our findings for biomass stability, but not for productivity stability. A recent study also reported that taxonomic and phylogenetic diversity stabilized community productivity through enhanced asynchrony of coexisting species, while functional diversity revealed a weak effect (Craven et al . 2018). Biomass and productivity are distinctive types of ecosystem functions (stock and rate, respectively), which have been shown to be affected by diversity differently in both manipulation experiments and natural forests (Caspersen & Pacala 2001). Our results are consistent them and suggest that the mechanisms how different diversity dimensions affect forest stability is different between carbon stock and carbon assimilation rate, which still needs further studies to examine why, because such knowledge is critical for sustainable forest managements with different management purposes (Craven et al. 2018).
As for different diversity components, we found that richness was generally the strongest predictor of biomass and productivity and their stability, but evenness and dispersion also played a role (Fig. 3). Richness is an indicator of the quantity of species number, functional space, and phylogenetic distance among species. The higher the richness, the more spatial and temporal niches are occupied by coexisting species in the community (Laliberté & Legendre 2010), which can increase the complementary and asynchrony in space and time and thus increased ecosystem productivity and stability. As for evenness and dispersion, some studies have shown that they can also significantly improve ecosystem functions and stability (Zhang et al. 2012; Potter & Woodall 2014). The relative weaker effect of evenness and dispersion than richness observed in our study may suggest that: the volume of niche space (i.e. amount of resources) occupied by a community is more important for ecosystem functions and stability, while how the niche space is filled by coexisting species (as reflected by evenness and dispersion, see Mason et al . 2005) plays a second role.