Common milkweed, Asclepias syriaca, exhibits latitudinal clines in germination and early growth traits.

Background/Question/Methods

Restoration of milkweed plants, including Asclepias syriaca, has increased in response to population declines of a key herbivore, the monarch butterfly, Danaus plexippus. Evidence for local adaptation in A. syriaca would indicate that local seed sourcing would improve restoration success. Using seeds from 20 populations throughout the range of common milkweed, we tested for geographic clines in germination and early growth traits in a common garden. Additionally, we tested for local adaptation in germination traits to spring thermal conditions by planting seeds from 10 populations in growth chambers simulating Minnesota and Kentucky spring temperatures.

Results/Conclusions

Even after accounting for seed mass, germination time declined with source population latitude in all growing conditions, a pattern inconsistent with local adaptation to spring temperature. 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 this developmental stage. Preliminary analyses of field data indicate that source population latitude is positively associated with early season growth but negatively associated with end of season height. We conclude that latitudinal clines suggest plant phenology is adapted to local growing conditions, but recommendations to use locally sourced seed would be better supported by direct evidence for local adaptation.