Stand factors are more important than diversity in affecting forest biomass and productivity
Our results revealed the clearly dominant effect of DBHmax on forest biomass and productivity across all grain sizes (Table 3 and Fig. 1). According to the metabolic theory of ecology, the metabolic rate of an organism scales as a power function of it’s body mass (Brown et al. 2004), which means that larger trees accumulate organic matter (i.e. productivity) much faster than small ones, and thus further lead to unproportionally higher biomass. Consistent with this theory, increasing studies have shown that DBHmax or maximum tree height are good surrogates of plot biomass (Wang et al. 2013; Wu et al. 2015b; Bastinet al. 2018; Lutz et al. 2018), and that large trees are predominant contributors to forest productivity (Xu et al. 2019).
We found that diversity has a weak effect on biomass and a medium effect on productivity (Fig. 1), which provided some supports to the complementary effect hypothesis. However, the effect of diversity is clearly weaker than that of stand factors, which is consistent with recent findings (Ouyang et al. 2019; Staples et al. 2019). These increasing evidences suggest that previous BEF studies may have put too much emphasis on the effects of diversity (van der Plas 2019) compared with stand factors. Forest ecosystems have complex community structures, and many studies have found that stand structure attributes are strong predictors for community biomass or productivity (Wu et al. 2015b; Fotis et al. 2018; Gough et al. 2019). Further, studies have reported that the species-poor planted forests can have biomass and productivity that were comparable to (or even higher than) natural forests with higher diversity (Guo & Ren 2014), which also suggests that diversity may not be the main driver of forest biomass and productivity compared with stand factors.