Aboveground facilitation and not complementary resource use cause overyielding among grasses in Tibetan alpine ecosystems

Partitioning the effects of plant diversity on ecosystem functions at different trophic levels

“…This idea is further supported by the different effects of the context drivers on the various diversity effects. Differences in the balance of effects also led to different net effects of diversity: as in many studies, we found positive net effects for biomass largely due to positive complementarity (e.g., Mahaut et al, 2019; Spehn et al, 2005; van Ruijven & Berendse, 2003; Yin et al, 2018). We also found negative effects of diversity on herbivory, in line with the idea that diverse communities are better protected against herbivores (Barnes et al, 2020), which implies reduced energy transfer to higher trophic levels and reduced secondary productivity in diverse plant communities.…”

“…Adding nitrogen to an ecosystem, through aerial deposition or in the form of fertilizer (Battye et al, 2017), would remove the benefits of resource partitioning and should thus reduce complementarity effects for biomass, as has been observed for nutrients in general (Craven et al, 2016) and nitrogen specifically (Roscher et al, 2016, Yin et al, 2018, but see Wacker et al, 2009). Further, high nutrient levels often favor a few species that are well adapted to productive environments, which would be reflected in increased inter‐ and potentially also intraspecific selection effects for biomass, which has also been previously observed for nutrient addition in general (Siebenkaes et al, 2016) and nitrogen specifically (Roscher et al, 2016, but see Yin et al, 2018). We thus hypothesize that nitrogen enrichment reduces complementarity effects (H16; Table 1) and increases selection effects (H17; Table 1) for biomass production.…”

“…For example, legumes increase complementarity effects for biomass, because their symbiotic rhizobacteria help to fix atmospheric nitrogen, which is otherwise inaccessible to plants (e.g., Marquard et al, 2009). Adding nitrogen to an ecosystem, through aerial deposition or in the form of fertilizer (Battye et al, 2017), would remove the benefits of resource partitioning and should thus reduce complementarity effects for biomass, as has been observed for nutrients in general (Craven et al, 2016) and nitrogen specifically (Roscher et al, 2016, Yin et al, 2018, but see Wacker et al, 2009). Further, high nutrient levels often favor a few species that are well adapted to productive environments, which would be reflected in increased inter‐ and potentially also intraspecific selection effects for biomass, which has also been previously observed for nutrient addition in general (Siebenkaes et al, 2016) and nitrogen specifically (Roscher et al, 2016, but see Yin et al, 2018).…”