Global warming is significantly increasing surface temperatures, and this increase is most pronounced towards the Arctic. High-latitude plant populations are spatially limited to shift their ranges, and should cope with warming in situ by means of phenotypic plasticity and/or adaptive evolution. Flowering phenology is a crucial trait in these extremely seasonal environments with narrow time windows for reproduction. Natural geothermal soil warming gradients provide a unique setting for studying long-term phenological responses to temperature, avoiding the confounding factors occurring in larger-scale (e.g. latitudinal) gradients, and allowing to evaluate both plastic and evolutionary responses. Here, we study trait expression and phenotypic selection on flowering phenology in the sub-arctic plant Cerastium fontanum in the Hengill geothermal area (Iceland), and we examine if temperature-driven differences in selection on phenology have resulted in genetic differentiation using a common garden experiment. We hypothesize that: 1) local soil warming leads to an earlier phenology, 2) phenotypic selection for early flowering is stronger in colder sites with shorter growing seasons, and 3) as a consequence of selection, differences in phenology in a common environment are related to soil temperature at the plant origin in a counter-gradient fashion, with plants originating from colder sites flowering earlier.
Results/Conclusions
Phenology was related to soil temperature across the geothermal soil warming gradient, with plants in locally warmer sites flowering earlier than plants at colder sites. Preliminary phenotypic selection analyses did not provide support for an interactive effect of phenology and soil temperature on fitness, and instead showed that fitness increased with early flowering, independent of temperature. We believe that this might result from environmental covariation, i.e. that both early flowering and fitness are correlated with favorable environmental conditions. Plants in the common garden responded to spring warming in a counter-gradient fashion: plants originating from warmer sites flowered relatively later than those originating from colder sites. We suggest that this is because plants from colder sites have been selected to compensate for the shorter growing season by starting development at lower temperatures, while plants from warmer sites are released from the restriction posed by a short growing season and can delay flowering until temperatures are higher. We demonstrate that soil temperature influences trait expression and provide evidence for the existence of genetically-determined variation in flowering phenology. These results illustrate how phenological responses to global warming might combine short-term plastic responses and long-term evolutionary responses that can act in opposite directions.