The Stress Gradient Hypothesis (SGH) proposes that competition prevails in undisturbed and productive environments, and shifts to facilitation in disturbed or stressful environments. Yet the environmental condition where facilitation or competition prevails is highly debated.
Liancourt et al. investigate variation in plant interactions, by analysing how Caragana versicolor species and associated community perceive and respond to the ambient level of stress, over the species’ entire elevation range in the arid Trans-Himalayas. The authors propose a double-sided interpretation of SGH: the Stress Gradient Hypothesis that predicts where facilitation should prevail at the community level and, on the other side of the coin, the Strain Gradient Hypothesis that predicts where facilitation should prevail within the niche of a particular species.
Liancourt, P., Le Bagousse-Pinguet, Y., Rixen, C., & Dolezal, J. (2017). SGH: stress or strain gradient hypothesis? Insights from an elevation gradient on the roof of the world. Annals of Botany, 120(1), 29–38. https://doi.org/10.1093/aob/mcx037