| Summary: | Global average temperature is predicted to increase from 0.3 - 4.8ºC by 2100, resulting in higher soil temperatures. My objective was to determine 1) how native Icelandic plant species, Ranunculus acris and Thymus praecox ssp. arcticus may respond to soil warming in three geothermal sites, 2) if these responses affect plant fitness, and 3) whether responses are site specific. Geothermal areas provide natural temperature gradients within small geographic areas where soil temperature can be isolated from other variables. Each site has soil temperatures ranging from 9ºC to 49ºC, but they differ in elevation and the time since warming started. Plant phenology, fitness and functional traits were recorded for both species at each site. Linear mixed models were used to describe the relationship between plant traits and soil temperature, revealing that warming responses were species and site specific. At the low elevation/short-term warmed site, temperature affected fitness and height of both species. At the low elevation/long-term warmed site, temperature affected R. acris fitness, size and height, and T. praecox phenology. In the high elevation/long-term, warmed site (Hen) temperature affected T. praecox phenology and size. Temperature effects on fitness were partly mediated by the effect of temperature on two plant traits, height and size, where resource allocation to reproduction lead to a decrease in vegetative growth and vice versa. This study highlights the complexity of plant responses to warming in subarctic environments and encourages research into whole-ecosystem responses to warming using geothermal systems.
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