Increasing species richness promotes tree biomass stock in mixed species stands
Plot-level aboveground biomass increased with increasing species richness, as per our expectation and in line with several previous studies at local (Con et al., 2013; S. Li et al., 2018; Liu et al., 2018; S. Mensah, Veldtman, Assogbadjo, et al., 2016; Wang, Lei, Ma, Kneeshaw, & Peng, 2011), regional (Ouyang et al., 2019; Paquette & Messier, 2011; Ruiz-Benito et al., 2014) and global scales (Cavanaugh et al., 2014; Liang et al., 2016). None of other three taxonomic diversity metrics (Shannon diversity, Pielou evenness and Simpson index) explained variation of the biomass in these stands, suggesting that species richness was the only studied taxonomic diversity metric predictor of AGB. Increasing plot level species richness may drive difference in AGB through increasing stocking capacity (higher density), or co-existence of functionally different species increasing resource partitioning (Mittelbach et al., 2001; Waide et al., 1999), for example, several canopy strata optimising use of vertical growing space and light (S. Mensah, du Toit, et al., 2018; Morin et al., 2011; Yachi & Loreau, 2007), in line with the niche complementarity hypothesis. These functionally different species may also enable plant–plant interactions such as facilitation, whereby some species could improve growing conditions (e.g. enhancing soil fertility through fixing nitrogen) for the productivity of other species (Erskine et al., 2006; S. Mensah, Veldtman, Assogbadjo, et al., 2016). However, increasing species richness may also enhance biomass production through highly performing dominant species; i.e. by increasing the likelihood of their presence in the ecosystem or by increasing the chances of having dominant species traits that naturally favored biomass production (Fotis et al., 2018; Lin et al., 2016; S. Mensah, Veldtman, Du Toit, et al., 2016; Wasof et al., 2018). Although our dataset reflect a local scale variation of species richness and AGB, it must be noted that the result adds to the general evidence that species richness has a positive effect on forest biomass.