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

Abstract

1. 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. 2. 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 belowground 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. 3. 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. 4. 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 plantplant interaction, since the space available for hosting other vascular species is considerably reduced. 5. 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.

Local trampling disturbance effects on alpine plant populations and communities: Negative implications for climate change vulnerability

“…Overall, we found that species diversity and richness within and next to cushions is lower compared to control areas, indicating net competitive interactions between cushions and other plant species (Figure ). Such negative or neutral interactions have been documented in other studies as well (e.g., de Bello et al., ; Dvorsky et al., ; Bowman & Swatling‐Holcomb, ), but surprised us given that Silene has been shown to increase species percent cover and richness (Bonanomi et al., ). Although disturbance reduces both species richness and diversity, it has no effect on species interactions, as measured by RII.…”

“…The cushion plant Silene has been found to host an increasing number of species at higher elevations (Antonsson, Björk, & Molau, ), while also demonstrating greatest facilitative effects on other species at the center of its elevational range (Bonanomi et al., ) as well as in abiotically stressful environments (Kjaer, Olsen, & Klanderud, ). We therefore expected cushion plants to first, host higher species diversity and richness compared to control areas, and second, maintain this higher diversity in areas where disturbance exerts negative effects.…”

The balance of canopy and soil effects determines intraspecific differences in foundation species’ effects on associated plants

“…Several studies in alpine systems have documented contrasting effects of different phenotypes of foundation species on subordinate species (Al Hayek, Maalouf, Touzard, & Michalet, ; Al Hayek, Touzard, Le Bagousse‐Pinguet, & Michalet, ; Al Hayek et al., ; Bonanomi et al., ; Michalet, Maalouf, & Hayek, ; Michalet et al., ). However, the mechanisms underlying these effects are still poorly understood.…”

“…(), Bonanomi et al. (), and Jiang et al. () stressed the importance of above‐ground traits—in particular canopy compactness—to explain the negative effects of tight phenotypes as compared to loose ones on associated species, although Schöb et al.…”

Fine nurse variations explain discrepancies in the stress‐interaction relationship in alpine regions

“…However, the variability of shapes in the stress–interaction relationship observed in the field remains detrimental to the generalization of the SGH (reviewed by Soliveres et al ). One flagship example of these discrepancies is the facilitation collapse or the stabilization of facilitative effects by nurses observed at the extremity of aridity gradients (Michalet et al ) but also possibly at the extremity of alpine gradients (Dvorsky et al , Bonanomi et al ).…”