The changes in plant–plant interactions along environmental gradients have been a focus of recent ecological research. It has been suggested that both above- and below-ground competition and their interplay vary along gradients, but few studies have investigated this idea, and in most cases, the role of facilitation has not been considered, despite its importance in high stress environments. Here we used two-layer ‘zone-of-influence’ models to simulate the effects of facilitation, size-asymmetry of competition, abiotic stress, resource availability and the balance of root–shoot growth on shoot and root interactions and their interplay along an environmental gradient. In the absence of facilitation, shoot and total competition became weaker, while root competition and the interplay between shoot and root competition were unchanged under increasing stress when root competition was completely symmetric. In contrast, shoot, root, total interactions and the interplay between shoot and root interactions were all negative, and they increased with increasing stress when root competition was size-symmetric. When facilitation was included in the models, net effects of shoot, root, total interactions and the interplay of root–shoot interactions were very different from those without facilitation, and many were positive under highly stressful conditions. The type of stress (non-resource or resource) did not significantly influence the simulation results. Our study provides an alternative interpretation of the interplay between above- and below-ground plant–plant interactions across an environmental gradient.