Shift from facilitative to neutral interactions by the cushion plant Silene acaulis along a primary succession gradient

Abstract

Background:The stress-gradient hypothesis predicts a shift from facilitative to competitive plant interactions with decreasing abiotic stress. This has been supported by studies along elevation and temperature gradients, but also challenged by the hypothesis of a facilitation collapse at extremely harsh sites. Although facilitation is known to be important in primary succession, few studies have examined these hypotheses along primary succession gradients. Aim:To examine whether there is a relationship between the presence of the circumpolar cushion plant Silene acaulis and other species, and if so, whether there is a shift between positive and negative interactions along a primary succession gradient in a glacier foreland.Location: Finse, southern Norway. Methods:We examined the performance of the common alpine forb Bistorta vivipara, species richness of vascular plants, bryophytes and lichens, and the number of seedlings and fertile vascular plants in S. acaulis cushions, and control plots without S. acaulis, along a succession gradient with increasing distance from a glacier front, and thus decreasing abiotic stress. To examine if S. acaulis cushions modify the abiotic environment, we recorded soil temperature, moisture, organic content and pH in cushions and control plots. Results:Bistorta vivipara performed better, as shown by bigger leaves in S. acaulis cushions compared to control plots in the harshest part of the gradient close to the glacier. There were few differences in B. vivipara performance between cushion and control plots in the more benign environment further away from the glacier. This suggests a shift from facilitative to mainly neutral interactions by S. acaulis on the performance of B. vivipara with decreasing abiotic stress. A trend, although not significant, of higher vascular species richness and fertility inside S. acaulis cushions along the whole gradient, suggests that S. acaulis also facilitates community-level species richness. The causal mechanism of this facilitation is likely that the cushions buffer extreme temperatures. Conclusions:Our results support the stress-gradient hypothesis for the relationship between the cushion plant S. acaulis and the performance of a single species along a primary succession gradient in a glacier foreland. S. acaulis also tended to increase vascular plant species richness and fertility regardless of stress level along the | 43 KJAER et al. Journal of Vegetation Science

Cushion and shrub ecosystem engineers contribute differently to diversity and functions in alpine ecosystems

“…In fact, under relatively better environmental conditions (e.g., soil nutrient and water contents), species are less constrained by physical stresses and tend to compete for resources (Dohn et al., , and references therein). These findings are consistent with previous findings that in relatively benign conditions, where most species are tolerant of the local environment, species’ net interactions may be neutral or even competitive (Callaway et al., ; Kjaer et al., ; Michalet et al., ; Pugnaire & Luque, ), and hence support the stress‐gradient hypothesis (Bertness & Callaway, ).…”

“…Cushion and shrub species both modified the micro‐environments, but the magnitudes of their effects differed and varied along the environmental stress gradient (Table ; Figures ), suggesting that they do indeed act as ecosystem engineers (Jones et al., , ), thereby offering suitable micro‐sites for less tolerant species and leading to increases in species diversity (Anthelme et al., , ; Cavieres et al., ; Chen, Yang et al., ; Hupp et al., ; Kjaer, Olsen, & Klanderud, ; Yang et al., ). We found that cushions in the higher community support significantly higher species richness and abundance than equal surrounding areas (Figures ; Appendix ), indicating that cushions in such severe conditions can facilitate other species, possibly by offering suitable microhabitats.…”

Successional trends and processes on a glacial foreland in Southern Iceland studied by repeated species counts

“…Some of these studies have suggested that species competition increases in later stages of succession only when VC approaches near maximum coverage of the foreland (Elven and Ryvarden 1975;Connell and Slatyer 1977;Jones and Henry 2003). Kjaer et al (2018) concluded specifically that species competition in later stages reflects the relative decrease in abiotic stress that dominates the early successional processes in proglacial environments. The observations presented herein are consistent with the conclusion that competition between bryophytes and vascular species (i.e., forbs and low shrubs) increased as VC increased on surfaces that are both older and at a greater distance from the ice front.…”

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

“…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.…”

“…In undisturbed areas, species richness peaked at middle elevations instead of declining linearly with elevation. We suspect this is due to high levels of biotic competition at low elevations (Supporting Information Table A6 in Appendix S1) and high levels of abiotic stress at upper elevations, as well as The cushion plant Silene has been found to host an increasing number of species at higher elevations (Antonsson, Björk, & Molau, 2009), while also demonstrating greatest facilitative effects on other species at the center of its elevational range (Bonanomi et al, 2015) as well as in abiotically stressful environments (Kjaer, Olsen, & Klanderud, 2017). 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.…”