Common milkweed seeds exhibit latitudinal clines more consistent with adaptation to growing season length than temperature

Abstract

Overwintering monarch (Danaus plexippus) populations have declined since the 1990s. In response, restoration of milkweeds, including Asclepias syriaca (common milkweed), an important host plant in their breeding grounds, has become increasingly common. However, latitudinal variation in milkweed populations suggests the possibility of regional adaptation and the potential for seed provenance to affect restoration success. Using seeds from 20 populations throughout the range of A. syriaca, we tested whether seed mass, germination success, and germination time in the greenhouse demonstrate geographic clines consistent with available evidence for this species from other studies. In addition, we tested for patterns in germination traits consistent with adaptation to spring thermal conditions by planting seeds from 10 populations in growth chambers simulating Minnesota and Kentucky spring temperatures. Even after accounting for seed mass, seeds from higher latitudes germinated faster on average under all conditions. Elevated temperatures accelerated germination time and leaf development time; however, we did not detect geographic patterns in leaf development time, indicating that the processes underlying the latitudinal cline in germination time may be unique to the germination stage. In the thermal adaptation study, high‐latitude populations produced larger seeds and seeds that germinated at a higher rate; however, neither latitudinal trend was observed in the geographic clines study, even though individual seed mass predicted germination success. High‐latitude populations express more favorable germination traits in every setting measured, perhaps due to reduced dormancy. Consequently, we conclude that latitudinal clines are more consistent with adaptation to growing season length than to spring temperatures.