The Arenaria ciliata species complex (Caryophyllaceae) comprises a group of 3 species with overlapping morphological and ploidy identities. The group has a wide but fragmented distribution across Europe, extending from high habitats in the mountains of central Europe and Iberia to sea-level habitats in the arctic. Because of their cold habitat distribution, they are vulnerable to climate change. This distribution also indicates there are likely to be many local ecotypes across the fragmented distribution. The aim of this project was to better understand the biology of Arenaria ciliata species complex, in terms of whether and how these species respond to various environments, measured for 5 ecotypes from Ireland, the Swiss Alps, Northern Norway, and Svalbard in the Arctic. This study demonstrated wide variation in ploidy and ecophysiology among ecotypes of A. ciliata species complex. Ploidy analysis reveals that among analysed taxa various ploidy levels were recorded. Distribution of ploidy variation of A. ciliata species across Europe in the present study indicates that polyploidisation has occurred on multiple separate occasions in isolated populations, but only in Alpine habitats. Biometric analysis showed that local Irish ecotypes of both A. ciliata and A. norvegica retain the ability to grow and survive through freezing habitat conditions. However, A. norvegica shows clear divergence from A. ciliata in terms of growth and phenology. Chlorophyll fluorescence analysis illustrated that cold stress tolerance capacity in each ecotype is more closely associated with the climate of the home population site than with evolutionary lineage, showing that cold stress tolerance capacity is greatest in the ecotypes of A. bernensis (Swiss Alps) and A. norvegica (Norway). These analyses showed also there is a clear improvement in cold and heat stress tolerance between juvenile and adult plants of A. ciliata and A. norvegica from Ireland. Finally, with regard to likely future climate change stresses, adult plants of all analysed ecotypes displayed very strong heat stress tolerance