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.