Castanho Camila T, Oliveira Alexandre A, Prado Paulo Inácio K L
Graduate Program in Ecology, Universidade de São Paulo, Rua do Matão 314, Travessa 14, São Paulo, SP, 05508-900, Brazil,
Oecologia. 2015 Jul;178(3):855-66. doi: 10.1007/s00442-015-3285-7. Epub 2015 Mar 12.
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.
胁迫梯度假说（SGH）假定了植物竞争与促进作用之间的平衡如何沿着环境梯度发生变化。SGH的早期表述预测，促进作用应随着胁迫程度单调增加。然而，最近对SGH的理论完善假定，在中等胁迫条件下促进作用更强，随后在最严峻的环境中促进作用的作用逐渐减弱。我们在巴西东南部沿海沙丘梯度最严峻的部分进行了田间实验，以测试胁迫对两种树种之间净相互作用的强度和重要性的影响。首先，我们沿着从海滩到内陆的梯度（即环境严峻程度的梯度），比较了巴西厚皮香不同生命阶段在有和没有对叶瓜皮树成年个体存在时的表现。为了特别测试一种胁迫因素的影响，我们还控制了水分供应，由于土壤沙质化，水分是一种限制资源。在沿海梯度最严峻的部分（即最靠近海岸的地方），对叶瓜皮树和巴西厚皮香之间相互作用的强度和重要性均与胁迫呈负相关，且在目标物种的不同生命阶段呈现出一致的模式。然而，净相互作用的符号取决于目标物种的生命阶段。我们的结果提供了经验证据，表明正如SGH最近的完善所预测的那样，随着胁迫达到最大值，促进作用的作用趋于减弱，导致物种之间的净相互作用变为中性甚至负性。
