• climate change;
  • competition;
  • environmental stress;
  • facilitation;
  • foundation species;
  • Pinus edulis;
  • plant–plant interactions;
  • removal experiment


  • • 
    Theoretical and empirical research has supported the hypothesis that plant–plant interactions change from competition to facilitation with increasing abiotic stress. However, the consistency of such changes has been questioned in arid and semiarid ecosystems.
  • • 
    During a drought in the semiarid south-western USA, we used observations and a field experiment to examine the interactions between juveniles of a foundation tree (Pinyon pine, Pinus edulis) and a common shrub (Apache plume, Fallugia paradoxa) in replicated areas of high and low stress.
  • • 
    The presence of F. paradoxa reduced P. edulis performance at low-stress sites, but had the opposite effect at high-stress sites. However, the intensity of the interactions depended on temporal variation in climate and age of P. edulis. Both above- and below-ground factors contributed to competition, while only above-ground factors contributed to facilitation.
  • • 
    These results support the hypothesis that interactions can change from competition to facilitation as abiotic stress increases in semiarid environments. A shift from competition to facilitation may be important for the recovery of P. edulis and other foundation species that have experienced large-scale mortality during recent droughts.