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

BackgroundThe 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. AimTo 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. LocationFinse, southern Norway. MethodsWe 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. ResultsBistorta 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. ConclusionsOur 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 gradient, suggesting facilitation at the community level. We found no collapse of facilitation at the most stressful end of the gradient in this alpine glacier foreland.