1 3 Oecologia DOI 10.1007/s00442-015-3285-7 COMMUNITY ECOLOGY - ORIGINAL RESEARCH Does extreme environmental severity promote plant facilitation? An experimental field test in a subtropical coastal dune Camila T. Castanho 1,2 · Alexandre A. Oliveira 1 · Paulo Inácio K. L. Prado 1 Received: 22 July 2014 / Accepted: 24 February 2015 © Springer-Verlag Berlin Heidelberg 2015 consistent across distinct life stages of the target species. However, the sign of the net interaction depended on the life stage of the target species. Our results provide empiri- cal evidence that the role of facilitation tends to wane, lead- ing to neutral or even negative net interactions between species as stress reaches its maximum, as predicted by the recent refinements of the SGH. Keywords Competition · Ontogenetic shift · Plant interactions · Restinga · Stress gradient hypothesis Introduction The stress gradient hypothesis (SGH) is a conceptual model of how the balance between competition and facili- tation among plants can shift along environmental gradi- ents (Bertness and Callaway 1994; Brooker and Callaghan 1998; Michalet et al. 2006; Maestre et al. 2009; Holmgren and Scheffer 2010; Kikvidze et al. 2011). Early proposals of the SGH predicted that the frequency and the impor- tance of facilitation should increase monotonically with increasing environmental severity (Bertness and Calla- way 1994; Brooker and Callaghan 1998). The rationale is that, as physical conditions that restrict resource acquisi- tion become more severe, the effects of the amelioration of stress by a neighbour on a target plant’s performance over- compensates for the effects of their competitive interaction (Callaway 2007). Several empirical tests and a recent meta- analysis have supported the predictions of early formula- tions of the SGH (Callaway et al. 2002; Armas et al. 2011; He et al. 2013). However, other studies have demonstrated that facilitation may not increase along the entire gradi- ent; it decreases in extremely severe environments (Forey et al. 2010; de Bello et al. 2011; Le Bagousse-Pinguet et al. Abstract The stress gradient hypothesis (SGH) pos- tulates how the balance between plant competition and facilitation shifts along environmental gradients. Early formulations of the SGH predicted that facilitation should increase monotonically with stress. However, a recent theo- retical refinement of the SGH postulates stronger facilita- tion under moderate stress, followed by a decreasing role of facilitation in the most severe environments. We conducted field experiments along the most severe part of a coastal dune gradient in southeast Brazil to test the effect of stress on the intensity and importance of the net interactions between two tree species. First, we compared the perfor- mance of distinct life stages of Ternstroemia brasiliensis in the presence and absence of Guapira opposita adults along a beach-to-inland gradient, a gradient of environmental severity. To test the effect of one stress factor in particular, we also manipulated water availability, a limiting resource due to the sandy soils. At the most severe part of the coastal gradient (i.e. closest to the seashore), both intensity and importance of the interaction between G. opposita and T. brasiliensis were negatively related to stress, with a pattern Communicated by Amy Freestone. Electronic supplementary material The online version of this article (doi:10.1007/s00442-015-3285-7) contains supplementary material, which is available to authorized users. * Camila T. Castanho ctcastanho@gmail.com 1 Graduate Program in Ecology, Universidade de São Paulo, Rua do Matão 314, Travessa 14, São Paulo, SP 05508-900, Brazil 2 Present Address: Departamento de Ciências Biológicas, Universidade Federal de São Paulo, Rua Prof. Artur Riedel, 275, Diadema, SP 09972-270, Brazil