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.