Responses of the Arctic-alpine cushion plant Silene acaulis to long-term experimental warming

Silene acaulis is a widely distributed facilitator plant that enhances Arctic and alpine plant community diversity. However, cold-adapted species like S. acaulis may be particularly vulnerable to high temperatures. We investigated the effects of experimental warming on S. acaulis performance and its interactions with other species, based an 18-year open-top chamber (OTC) experiment in alpine Finse, Norway. In the summer of 2019, we recorded air and soil temperatures, cushion size, height, colour, fruit production, and the number of vascular plants, bryophytes, and lichens growing within S. acaulis cushions in both warmed (OTC) and control plots. Warmed plots had significantly higher mean temperatures and experienced more frequent and intense extreme heat events. Warming did not significantly affect the probability of fruit production; however, larger cushions produced more fruits. Cushion health was negatively impacted by warming, with the probability of discoloration or damage more than doubling in warmed plots (52%) compared to controls (23.7%). The number of lichens and bryophytes inside S. acaulis cushions was significantly lower in warmed plots, while the number of vascular plants was unaffected by warming. The decrease in cushion health may result from prolonged exposure to extreme heat events in OTCs during warm summers. The reduced abundance of lichens and bryophytes in warmed cushions could reflect either a decline in facilitative effects by S. acaulis due to long-term warming or a lower thermal tolerance of these groups compared to vascular plants. Our findings highlight the importance of further research into the long-term effects of extreme maximum temperatures on Arctic and alpine specialists like S. acaulis, as very high temperatures may have stronger impacts than increases in average temperature alone