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.