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Does extreme environmental severity promote plant facilitation? An experimental field test in a subtropical coastal dune

Abstract

The stress gradient hypothesis (SGH) postulates 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 theoretical refinement of the SGH postulates stronger facilitation 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 performance 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 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 empirical evidence that the role of facilitation tends to wane, leading to neutral or even negative net interactions between species as stress reaches its maximum, as predicted by the recent refinements of the SGH.

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Acknowledgments

We would like to thank Fabiano Neves, Selmo Bernardo and Edison Nascimento for their assistance in the field, and Glauco Machado, Gislene Ganade and Leonardo L. Wedekin for helpful comments on an earlier version of the manuscript. We also acknowledge the financial support provided by PETROBRAS, the PhD scholarship provided by Coordination for the Improvement of Higher Education Personnel (CAPES), and a post-doctoral fellowship provided by São Paulo Research Foundation (FAPESP; process no 2012/09794-7) to C.T.C., as well as the grant provided by CNPq to P.I.K.L.P. The experiments performed comply with the current Brazilian laws (permission SMA 000.541/2008).

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Correspondence to Camila T. Castanho.

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Communicated by Amy Freestone.

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Castanho, C.T., Oliveira, A.A. & Prado, P.I.K.L. Does extreme environmental severity promote plant facilitation? An experimental field test in a subtropical coastal dune. Oecologia 178, 855–866 (2015). https://doi.org/10.1007/s00442-015-3285-7

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Keywords

  • Competition
  • Ontogenetic shift
  • Plant interactions
  • Restinga
  • Stress gradient hypothesis