Ongoing local adaptation to climate change in yarrow ( Achillea millefolium L.)

Climate change threatens biodiversity as populations can persist if they migrate or adapt to the rapidly changing conditions of the Anthropocene. However, little is known about the persistence of plant populations under the long-term trends of increasing temperature and drought. We explore whether historical populations of yarrow ( Achillea millefolium L.) surveyed in the 1920s have persisted locally or undergone local extinction after 100 years. We resurveyed historical sites spanning a broad climatic gradient (from 1 m to 3,200 m a.s.l.), examined to what extent plant growth signature (height) has changed over time, and analyzed metabolic diversity. Nine out of ten populations persisted locally and showed phenotypic and metabolic differentiation. The only population potentially extirpated is that of the hottest and driest site. Populations from warm sites in coastal and mountain regions had grown taller than 100 years ago. In contrast, populations from dry sites in lowlands and foothills became shorter. Furthermore, we document differentiation in metabolic diversity involving plant defenses and signaling. Ongoing local adaptation is constrained by changing both temperature and precipitation as well as by biotic interactions. Preserving locally adapted populations and metabolic diversity is key for conservation efforts.