Small-scale adaptation to geothermal soil heating in a perennial herb revealed by combining crosses and transplantations.

Identifying environmental factors associated with local adaptation and traits under selection is key to linking evolutionary processes to the environment. While reciprocal transplantation studies and provenance experiments often have demonstrated adaptation at relatively large spatial scales, adaptation can also occur at very small spatial scales. Combining a crossing experiment with field transplantations, we investigated whether Cerastium fontanum has adapted to geothermally induced small-scale soil temperature differences. Offspring representing a wide range of parental soil temperatures were transplanted across the same temperature range, and traits and fitness components were measured over 2 yr. We evaluated the relationship between plant performance and soil temperatures, the degree of adaptation to their source thermal environment, and the dependence of adaptation on flowering time. Survival, flowering incidence, and overall fitness were lower in warmer soils. However, adaptation to temperature was asymmetric; while all plants performed well at colder sites, individuals from colder origins performed poorly at warmer sites. Flowering time and fitness varied in relation to soil temperature, as well as the difference between planting and source thermal environments. Our findings indicate that small-scale variation in soil temperature underlies fine-scale adaptation and provides important knowledge to understand evolutionary effects of microclimatic variation.