3) Functional diversity
Previous studies have shown that maximum tree height and wood density were key functional traits to predict forest biomass and productivity (Ruiz-Benito et al. 2014; Woodall et al. 2014; Wu et al. 2015b). Consequently, we calculate functional diversity based on these two traits. The maximum tree height data for each species was obtained from the ”Flora of China” (Editorial Committee of Flora of China, 2004). The wood density data were retrieved from the global wood density database (http://datadryad.org/repo/handle/10255/dryad.235) and China Woody Plant Wood Density Database (Zhang et al. 2011). To represent the three diversity components, we calculated functional richness (FRic), functional evenness (FEve), and functional divergence (FDiv) for each (sub)plot. FRic is the amount of niche space filled by species in the community, while FEve is the evenness of abundance distribution in filled niche space, and FDiv is the degree to which abundance distribution in niche space maximizes divergence in functional characters within the community (Mason et al. 2005). All functional diversity indices were calculated with the R package “FD” (Laliberté & Legendre 2010).