Journal Article

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

Saracino, Antonio, Bonanomi, Giuliano, Stinca, Adriano, Chirico, Giovanni Battista, Incerti, Guido, Ciaschetti, Giampiero

2016

Overview

Summary 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. Lay Summary

• Publisher: Wiley,Wiley Subscription Services, Inc
• Subject: abiotic stress / altitude / Community ecology / competition / Cushions / Environmental effects / Environmental gradient / heat / Microclimate / Morphology / plant community / Plant diversity / Plant morphology / Plants / plant–plant interaction / relative interaction index / severity‐interaction relationships / Silene / Silene acaulis / soil fertility / Soil properties / Soil quality / species diversity / Species richness / stem density / stress‐gradient hypothesis / vascular plants