Relationship between plant richness and ecosystem functioning
In accordance with our hypothesis, aboveground biomass and SOC were
positively associated with species richness in this study. This is
consistent with previous biodiversity and ecosystem function (BEF) work
(e.g. Ma et al. 2010; Maestre et al. 2012; Pasari et
al. 2013; Wagg et al. 2014; Jing et al. 2015). Though
there is still debate about the mechanisms by which biodiversity impacts
ecosystem function (Eisenhauer et al. 2019), many studies have
demonstrated that biodiversity is a major determinant of ecosystem
functioning (Isbell et al. 2011; Tilman et al. 2014;
Isbell et al. 2017; Seabloom et al. 2017). Moreover, there
have been several studies showing that positive biodiversity effects on
ecosystem functioning are particularly strong when multiple ecosystem
functions are considered (Isbell et al. 2011; Maestre et
al. 2012; Pasari et al. 2013; Jing et al. 2015; Van Der
Plas et al. 2016; Fanin et al. 2018; Hautier et al.2018; Meyer et al. 2018). Our study, across an extensive aridity
gradient, complements the growing BEF literature by demonstrating strong
plant diversity effects on both plant biomass production and SOC (Table
1, Figs. 3 and 4). Although the present study was not designed to
identify the underlying mechanisms, based on previous work we speculate
that positive effects of species richness on ecosystem functions may be
mediated by complementarity in the resource use and increased plant
productivity, and cascading effects on multiple organisms and ecosystem
processes (Pringle et al. 2007; Maestre et al. 2012;
Tilman et al. 2014). Notably, in this study, plant species
richness had greater effect on SOC than on aboveground biomass,
underlining the significant role of plant diversity for soil carbon
storage in grassland (Lange et al. 2015).
In addition, the positive relationships between plant species richness
and aboveground biomass as well as SOC were stronger at more arid sites
(Fig. 3). This suggests that plant diversity has stronger beneficial
effects on ecosystem functioning in dry regions, supporting the
hypothesis that biodiversity may be particularly important to maintain
ecosystem functions under stressful environmental conditions (Callawayet al. 2002; Ratcliffe et al. 2017; Baert et al.2018). In the more humid regions, the weaker effects of species richness
on ecosystem functioning may result from reduced complementarity effects
(Wright et al. 2017) and increased functional redundancy at high
diversity (Craven et al. 2016). These results have significant
implications for land management in a changing climate, as they indicate
that the preservation of plant diversity is particularly important to
maintain ecosystem functioning and services in arid environments.