Fig. 1. The change of plant–plant interactions under stress.

a How interaction intensity–density relationships should shift along a stress gradient and how the shift may affect the balance between facilitation and competition. Under low density (left of the intersection between the two highest stress levels), interactions become less negative with increasing stress but are not most positive for the highest stress level, whereas under high density, the shift in interaction intensity is continuous and follows the predictions of the stress-gradient hypothesis. b Same model as panel a but displaying how plant–plant interactions at differential densities change along a stress gradient. This shows that the SGH applies only to relatively high densities.