Population dynamics and life history of the rare arctic-alpine plant Sagina nivalis (Caryophyllaceae) on the Ben Lawers range, Scotland, UK

The flora of Scotland’s mountains is of international significance as it contains arctic-alpine species at the edge of their global range. Such rear edge populations deserve high priority investigation due to their unique local adaptations, important conservation value and because montane plants may be particularly sensitive to environmental change. Sagina nivalis is one of the rarest and least known arctic-alpines in Britain and the vast majority of plants occur within the Ben Lawers SSSI. Ten permanent plots were surveyed here from 1982-1994 by counting the number of plants and tagging and measuring individuals throughout the duration of their lives. The total number of plants present at each colony across the whole site was also recorded in 1996 and 2014. Extreme fluctuations in numbers occurred within periods of very few years and were associated with the fragile and dynamic nature of the montane habitat. Seedling mortality was very high, but age had no effect on survival rates once plants were one year old. Most were small, only lived two years and produced none or few flowers, although those that survived long enough generally reached their maximum size and flowering potential when they were five years old. The number of plants present across the whole of Ben Lawers halved between 1996 and 2014, with half of all colonies extinct and a further third in significant decline. The greatest threats to S. nivalis in Scotland were identified as habitat damage due to natural processes, sheep activity and climate change. A reduction in snow cover may cause an interaction between these factors and compromise the long-term survival of rear edge populations of this species, other rare arctic-alpines in Scotland and their associated contribution to biodiversity.