4)Stand and soil factors
We calculated stand total basal area (TBA), maximum breast diameter
(DBHmax) and stand density for all plots (or subplots).
TBA was excluded from final data analyses, because of its collinearity
with DBHmax. Unlike TBA which is correlated with both
DBH and density, DBHmax and stand density were selected
for statistical analyses because they reflects distinctive stand
structural attributes, and previous studies have also shown that they
may have important effects on biomass and productivity (Lutz et
al. 2018; Ouyang et al. 2019).
In preliminary analyses, we have also used the ten soil variables (see
“The dataset” section) as environmental factors. However, the soil
variables were not significantly correlated with the four response
variables in most cases, and were rarely retained in the final
multivariate models. This is not surprising because the ten plots were
located within a relatively small region and thus not differed much in
site condition (http: //www.cnern.org.cn/). Consequently, in the final
data analyses we excluded the soil variables to focus on the relative
importance of stand factors vs. diversity on ecosystem function and
stability.