Physiological and morphological growth and hydraulic measures in Asclepias syriaca and A. speciosa under drought

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.