Changes of the relative importance of diversity vs. stand
factors with grain size
The models in Table 3 were based on (sub)plots that differed markedly in
sample size, consequently we randomly extracted a same
number of subplots for a better
comparison across grain sizes (see Methods). Figure 1 again showed that,
for forest biomass and productivity, stand factors revealed a markedly
stronger effect than diversity indices at each grain size (especially
for biomass), no matter measured by the pure or total effect. This
confirms our conjecture that stand factors are dominant in driving
forest biomass and productivity. However, we also noted that the total
and pure effects of diversity on biomass both showed an increase from
the 400 to 1200 m2 grain size (for the 2500
m2 results, see below). For productivity, the total
effect of diversity increased with grain size while the pure effect
decreased, suggesting that the joint effect between diversity and stand
factors increased with lager plot size. We thus concluded that the
biodiversity effect on forest biomass and productivity might increase
with grain size.
For the stability of biomass and productivity, however, the total and
pure effects of diversity were markedly higher than that of stand
factors across grain sizes, confirming that diversity is dominant in
affecting ecosystem stability. However, the total and pure effects of
diversity and stand factors on the two stability metrics did not
revealed a consistent pattern across grain sizes.
For comparison with the above results based on random sampling of
subplots, we also reported the variation partitioning analyses based on
models in Table 3 (Fig. S3). We found that while the basic results were
similar (stand factors were important for biomass and productivity,
while diversity was dominant for stability), there were also notable
differences. For biomass and productivity, diversity occasionally showed
higher explanatory power in Fig. S3 compared with that in Fig. 1, while
at other grain sizes diversity was excluded from the models in Fig. S3.
Meanwhile, the explanatory power of diversity from 400 to 1200
m2 in Fig. S3 were ca. 23% and 20% for biomass and
productivity, respectively, which were clearly lower than that in Fig. 1
(ca. 40% and 37%, respectively). These differences suggest that the
results based on random sampling of plots may be more robust (see
Discussions for details). Consequently, the results of the 2500
m2 (which was not based on random sampling) should be
viewed with caution, and was listed in Fig. 1 for reference only, and
not reported in subsequent analyses.
The relative
importance of different diversity components and dimensions
The results based on random sampling
of the subplots showed that (Figure 2), functional diversity (including
the effect of richness, evenness and divergence) seemed to be more
important for biomass and productivity than phylogenetic and taxonomic
diversity, and the importance of functional diversity increases markedly
with larger grain size. Thus the increase in the explanatory power of
diversity in Figures 1A and 1B may be mainly caused by functional
diversity. For biomass stability, functional diversity is also clearly
more important than phylogenetic and taxonomic diversity, while the
three dimensions of diversity showed similar importance on productivity
stability. Meanwhile, the relative importance of three diversity
dimensions did not reveal a consistent trend across grain sizes, which
may be why the effect of diversity did not show a clear pattern with
plot size in Figures 1C and 1D.
As for the three components of diversity (Figure 3), the importance of
richness (mean importance of species, functional and phylogenetic
richness) on ecosystem functions and stability were generally higher
than that of evenness and divergence. However, for biomass and
productivity, the importance of divergence increase with grain size.
While for the two stability measures, the change of importance value
with grain size differed among three diversity components.