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.