No evidence of facilitation collapse in the Tibetan plateau

QuestionsThe relative importance of facilitation under different environmental conditions has raised some recent controversy centred on predictions of the stress gradient hypothesis (SGH) for its apparent lack of predictive power under very stressful conditions. This criticism, however, is not widely shared nor fully supported by empirical data. We addressed whether changes in interaction intensity in plots under severe environmental stress determine an increase in facilitation and whether facilitation would expand the realized niche of some plant species. LocationFour sites differing in elevation and environmental conditions along a N-S transect in the Central Tibetan Plateau, Qinghai Province, China. MethodsWe assessed plant interaction intensity between the cushion plant, Androsace tapete, and species growing inside its canopy. We recorded species and number of individuals in cushions and in gaps, and mass of the two most frequent species in each site. ResultsInteractions ranged from competition to facilitation, depending on environmental severity. Net interaction balance followed predictions of the SGH, from competition on the relatively more mesic sites to facilitation in the relatively more stressful sites. There were no hints of facilitation collapse even though our harsher sites posed extreme conditions for plant survival. The main limiting factor seems to be temperature, as interaction intensity responded most to this factor, rather than to elevation or rainfall. ConclusionsFacilitation expanded the realized niche of some species, countering adverse environmental conditions. We found no collapse of facilitation under the most stressful conditions, even though some sites were at the edge of the distribution range for the cushion species.