Differences in drought avoidance rather than differences in the fast versus slow growth spectrum explain distributions of two <i>Asclepias</i> species

Understanding factors that determine a species' geographical range is crucial for predicting climate-induced range shifts. Two milkweed species, Asclepias syriaca and Asclepias speciosa, have overlapping ranges along a moisture gradient in North America and are primary food sources for endangered monarch caterpillars. With decreasing moisture, long-lived species often exhibit slower growth and greater drought tolerance, while many annual species exhibit faster growth strategies. Using this fast-slow framework, we assessed whether traits of these two sister species differ along a fast-slow growth continuum and could explain their distributions. We measured leaf and root functional traits in common gardens and greenhouse experiments. In key measures indicative of drought tolerance (e.g., growth, transpiration, and water potentials), the species were nearly identical. Contrary to expectations, A. speciosa did not exhibit greater drought tolerance, raising the question of how it survives in the more arid west. A reciprocal transplant study showed selection against A. syriaca in the western garden and that A. speciosa was better able to avoid seedling mortality. Focusing on seedling establishment, we found that A. speciosa exhibited faster deep-root development and a narrow leaf phenotype associated with slower wilting and delayed drought-induced mortality. Rather than differences on the fast-slow growth spectrum, our results indicate that A. speciosa avoids drought through faster deep-root growth and a slower wilting phenotype. Our study suggests that A. syriaca's range is limited by its drought tolerance, while A. speciosa employs a number of drought avoidance strategies to survive in more arid environments.