These papers are a contribution to the long-standing question : what is a plant communtiy ? I was just about to start my Ph.D. thesis when my dear colleague Richard Michalet and I met Ray Callaway from the University of Montana, USA. This fortuitous meeting with Ray held in the Caucasus mountains near Kazbegui (!) and certainly changed my life as a scientist. We have been embarked in a global-scale analysis of of plant-plant interactions in mountain grasslands testing the hypothesis of a shift from competition (negative interactions) to facilitation (positive interactions) when environmental conditions for plant life get harsher. We came out to the conclusion that in stressful environments a plant community is an integrated level of organisation challenging the individualistic’ paradigm of plant community organization. I am very grateful to all the "alpine pals" for the great time we have had working on facilitation. Callaway & al. (2002) for a momentum of this research.