Can species adapt to drought using multiple strategies? Lessons from the California poppy.

Plants can escape drought by completing life cycles early, tolerate drought by increasing physiological limits, or avoid drought stress by obtaining or using water more efficiently. It remains unclear whether strategies vary within species across their distributional ranges due to trade-offs, and whether species can exhibit plasticity in multiple traits simultaneously. We grew 19 populations of Eschscholzia californica collected along an aridity gradient in a glasshouse with high or low water, then applied a terminal drought treatment, and measured the responses of growth and functional traits. We found clinal variation in drought adaptation strategies; populations from arid sites exhibited escape phenotypes, while populations from mesic areas exhibited avoidance phenotypes. In response to low water, plants displayed plasticity in traits associated with both avoidance and tolerance strategies, and this plasticity was expressed consistently across populations. By contrast, specific root length (SRL) displayed clinal variation in plasticity; more arid sites had higher SRL (longer/thinner roots), and SRL increased the most in response to low water in the populations from arid sites. Our experiment demonstrates that frameworks developed to predict interspecific variation in drought adaptation strategies can also operate intraspecifically, with implications for wildflower conservation in the face of increasingly frequent droughts.