Diversity is the main driver of ecosystem stability
In contrast to biomass and productivity, we found that the relative importance of diversity is much higher than stand factors for ecosystem stability at different grain sizes (Table 3 and Fig. 1). In more-diverse communities, the growth and development of coexisting species are often asynchronous in time and space, which allows the community to stabilize ecosystem functions under environmental perturbations (Loreau & de Mazancourt 2013; Morinet al. 2014). The asynchrony of coexisting species is mainly determined by the species-specific attributes (such as functional traits, evolutionary history, etc.), and these differences in species attributes can be reflected by diversity indices (e.g. taxonomic, phylogenetic and functional diversity) (Mori et al.2013; del Río et al. 2017). This may be why diversity metrics were powerful in explaining ecosystem stability in our analysis. Hector et al. (2010) proposed the over-yielding effect hypothesis, which suggests that diversity may also stabilize ecosystem functions under environmental disturbances by increasing community biomass or productivity. However, the hypothesis was not well supported, because we found that the influence of diversity on biomass and productivity was weak, and thus the over-yielding effect seem not to be the main driver for ecosystem stability. Recent studies have also shown that the relationship between diversity and ecosystem stability is mainly shaped by the asynchrony of species’ response to environmental fluctuation and interactions among species, instead of the over-yielding effect (del Ríoet al. 2017).
It is also suggested that diversity can increase ecosystem stability through species facilitation, which was found to reduce mortality caused by climate events such as drought (Fichtner et al. 2017; Hutchison et al. 2018; Schnabel et al. 2019). Under disturbances or environmental changes, the asynchrony, facilitation and species interactions may be the main drivers of ecosystem stability, and thus the difference in attributes among coexisting species (i.e. diversity indices) may be more important than stand factors. Recent studies have reported that taxonomic diversity explained forest stability better than structural diversity (Schnabel et al. 2019), which also provided some supports to our finding that stand factors showed weak effect on stability. However, analyses on the relative effect of stand factors vs. diversity on ecosystem stability are still limited till now (Mazzochiniet al. 2019; Wang et al. 2019), and the mechanisms why stand factors are so weak predictors of ecosystem stability across grain sizes is still largely unknown (considering the dominant effect of stand factors on biomass and productivity per se , this is surprising). More studies are needed to test our findings and to better understand the underlying mechanisms.