Unexpected mechanisms sustain the stress gradient hypothesis in a tropical alpine environment

Questions: Does facilitation among plants increase with elevation in a humid tropical alpine system in which climatic and ecological conditions differ from other alpine environments? Whatmechanisms are involved in the interactions? Location: Volcano Antisana, Eastern Cordillera of the Ecuadorian Andes ( 00 28' S, 78 09' W). Methods: We selected the cushion-forming Azorella aretioides as a potential nurse plant along an altitudinal gradient ( 4400, 4550 and 4700 m) in the high tropical Andes. We quantified its effects on other plants at species and community levels by comparing the product vegetation cover 9 number of individuals of every vascular species found inside and outside 265 cushions, using the relative interaction index. We inferred potential mechanisms behind the interactions through analysis of microclimate, soil moisture and soil nutrient measurements inside and outside cushions. Results: Predictions of the stress gradient hypothesis ( SGH) were corroborated at community level, with transition from competitive or neutral effects of A. aretioides at 4400 and 4550 m to facilitative effects at 4700 m. Strong speciesspecific effects were observed along the altitudinal gradient, with a substantial effect of local habitat disturbance on the outcome of plant-plant interactions. Surprisingly, cushions lowered air and soil temperatures and air humidity, which reduced at higher elevations. Facilitation appeared to be caused by higher soilmoisture and nutrient content beneath cushions. Conclusions: Our data extend the framework of the SGH by corroborating it for the first time in a tropical alpine system. However, the mechanisms underlying plant-plant interactions differed fromthose generally reported fromalpine environments, with facilitation varying according to resource-mediated stress ( nutrients). It remains to be tested whether this is specific to tropical alpine systems.