The stress-gradient hypothesis predicts in increase in the importance of positive plant-plant interactions with increasing environment severity. However, recent works indicated that the effect disappeared with reduction in precipitation, which challenged the hypothesis. The disappearance was attributed to that the symbiotic nitrogen fixation is sensitive to water stress and thus the drought stress retard the facilitation effect.

Scientists from Ecological Stoichiometry Research Group in IAE synthesized a global dataset from 66 studies and evaluated how shrubs affected soil organic carbon and nitrogen in grasslands along a precipitation gradient.

The results show that, at the global scale, leguminous shrub-mediated effects on soil nutrients increased with precipitation, while no relationship was found for non-leguminous shrub.

Since meta-analyses do not explain the effect of shrub species variation, research developed on a single leguminous genus Caragana.

The results showed that with the increases of drought stress along a regional precipitation gradient (147-342mm) the facilitation effect to the surrounding soil C pool and grass growth) was weakened, supporting the hypothesis that the mutual facilitation effect would collapse in stressful environment.

The results implicate that precipitation and the shrub functional traits (N-fixers vs. non-fixers) deeply affects the biogeochemical cycling of soil carbon and nitrogen and plant-plant interactions.

The results were published in Functional Ecology entitled as "Facilitation by leguminous shrubs increases along a precipitation gradient".

This work was supportedby Strategic Priority Research Program of the Chinese Academy of Sciences.

　　Fig 1. Response ratio (LnRR) induced by shrub encroachment on neighbor plant biomass, soil carbon (C), and nitrogen (N) pool for Caragana stenophylla (circle) or C. microphylla (triangle) (Image by ZHANG Haiyang).