Data from: Cushion plant morphology controls biogenic capability and facilitation effects of Silene acaulis along an elevation gradient

The stress-gradient hypothesis (SGH) predicts that the balance of plant–plant interactions shifts along abiotic environmental gradients, with facilitation becoming more frequent under stressful conditions. However, recent studies have challenged this perspective, reporting that positive interactions are, in some cases, more common at the intermediate level of environmental severity gradients. Here, we test whether and how neighbour effects by Silene acaulis cushions vary along a 700 m wide altitudinal transect, in relation to cushion morphological traits and environmental severity. Field measurements along the gradient, within and outside cushions, included (i) species richness and cover of coexisting vascular plants; (ii) cushion morphology; (iii) above- and below-ground microclimate; and (iv) soil quality. We used the relative interaction index to decouple neighbour trait effects and environmental severity effects on plant diversity at different elevations. The ability of the cushion plant to facilitate heterospecifics shifts considerably along the elevation gradient, being greatest at the intermediate level. On the other hand, Silene morphological traits steadily change along the gradient, from lax, soft and flat-shaped cushion habits at low elevation to tightly knit and dome-shaped habits at high elevation. Cushion morphological changes are associated with mitigating effects on microclimate, indicating that cushions effectively act as a heat-trap at medium and high elevations, while at low elevations the soft and flat cushions avoid excessive heat accumulation by tight coupling with the surrounding atmosphere. At the upper end of the gradient, cushion cespitose–pulvinate compactness and high stem density appear to be critical traits in modulating the net effect of plant–plant interaction, since the space available for hosting other vascular species is considerably reduced. In conclusion, this work provides a mechanistic link between plant morphological traits, associated biogenic microclimate changes and variation in net plant–plant interactions along the explored severity gradient. Our findings support an alternative conceptual model to SGH, with plant facilitation collapsing at the upper extreme of the abiotic stress gradient.

胁迫梯度假说（Stress-Gradient Hypothesis, SGH）预测，植物间相互作用的平衡会沿非生物环境梯度发生偏移，在胁迫程度更高的生境中，促进作用的发生频率会显著提升。但近期的多项研究对这一观点提出了质疑，报道称在部分案例中，植物间正相互作用在环境胁迫梯度的中间水平更为普遍。
本研究以垫状蝇子草（Silene acaulis）的垫状体为研究对象，旨在检验其邻体效应是否沿700米宽的海拔样带发生变化，并探讨该效应与垫状体形态性状及环境胁迫程度的关联。
沿该海拔梯度，我们在垫状体内外开展了多项野外测量，内容包括：（1）共存维管植物的物种丰富度与盖度；（2）垫状体形态特征；（3）地上与地下微气候；（4）土壤质量。
我们采用相对相互作用指数（Relative Interaction Index），以分离不同海拔梯度下，邻体性状效应与环境胁迫效应对植物多样性的影响。
垫状蝇子草对异种植物的促进能力沿海拔梯度发生了显著变化，在海拔中间水平达到峰值。
与此同时，垫状蝇子草的形态性状沿海拔梯度呈线性变化：低海拔区域为疏松、柔软且扁平状的垫状生长型，高海拔区域则转变为紧密簇生、呈穹顶状的生长型。
垫状体的形态变化与其对微气候的缓冲效应密切相关，这表明在中高海拔区域，垫状体可有效充当热量陷阱；而在低海拔区域，柔软扁平的垫状体通过与周围大气的紧密耦合，避免了过度的热量积累。
在海拔梯度的上端，垫状体的丛生-垫状紧凑度与高茎密度似乎是调控植物间相互作用净效应的关键性状，因为此时可供其他维管植物定植的空间已大幅减少。
综上，本研究为沿所探究的胁迫梯度，植物形态性状、相关生物成因微气候变化与植物间相互作用净效应的变异之间建立了机制性关联。我们的研究结果支持一种不同于胁迫梯度假说的替代概念模型：在非生物胁迫梯度的极端上限处，植物间的促进作用会完全消失。