Refining the stress-gradient hypothesis for competition and facilitation in plant communities.pdf

Journal of Ecology 2009, 97 , 199–205 doi: 10.1111/j.1365-2745.2008.01476.x ? 2009 The Authors. Journal compilation ? 2009 British Ecological Society Blackwell Publishing LtdFUTURE DIRECTIONS No. 5 Refining the stress-gradient hypothesis Refining the stress-gradient hypothesis for competition and facilitation in plant communities Fernando T. Maestre 1 *, Ragan M. Callaway 2 , Fernando Valladares 1,3 and Christopher J. Lortie 4 1 área de Biodiversidad y Conservación, Departamento de Biología y Geología, Universidad Rey Juan Carlos, ESCET, C/ Tulipán s/n, 28933 Móstoles, Spain; 2 Division of Biological Sciences, University of Montana, Missoula, MT 59812, USA; 3 Instituto de Recursos Naturales, Centro de Ciencias Medioambientales, CSIC, Serrano 115, 28006 Madrid, Spain; and 4 Biology Department, York University, Toronto, ON M3J 1P3, Canada Summary 1. The stress-gradient hypothesis (SGH) predicts that the frequency of facilitative and competitive interactions will vary inversely across abiotic stress gradients, with facilitation being more common in conditions of high abiotic stress relative to more benign abiotic conditions. With notable exceptions, most tests of the SGH have studied the interaction between a single pair or a few pairs of species, and thus have evaluated shifts in the magnitude and direction of pair-wise interactions along stress gradients, rather than shifts in the general frequency of interactions. 2. The SGH has been supported by numerous studies in many ecosystems, has provided a crucial foundation for studying the interplay between facilitation and competition in plant communities, and has a high heuristic value. However, recent empirical research indicates that factors like the variation among species and the nature of the stress gradient studied add complexity not considered in the SGH, creating an opportunity to extend the SGH’s general conceptual framework. 3. We suggest t