Latitudinal Variation in Phenology and Reproduction Among Seed Sources of <i>Asclepias syriaca</i> L. (Common Milkweed)

The value of ecological restoration is contingent on the appropriate sourcing of plant materials to establish a plant community and restore ecological function. Long-standing recommendations to source plants from close to the restoration site are now being complemented by strategies aimed at maximizing adaptation to future climates and evolutionary potential, such as including a greater variety of seed sources. However, intraspecific variation in phenology among seed sources may have direct effects on trophic interactions, including insect pollination. To investigate the effect of seed source on plant phenology, pollination, and reproduction, we compared three populations of Asclepias syriaca L. (common milkweed) collected along a 750 km latitudinal gradient in the Midwest. U.S. in a centrally located common garden grown for three years. Persistent phenological and phenotypic differences were found across multiple growing seasons. Plants from northern sources flowered earlier, were shorter at maturity, and demonstrated considerably less interannual variation in phenology than those from southern sources. Later flowering phenology, observed in plants from southern sources, was associated with increased floral visitation, increased number of viable fruits, and fewer aborted fruits than those from northern sources. These results suggest that plants from southern sources may perform better than locally-sourced plants or those from northern sources under restoration settings. In addition, populations may demonstrate decreased fruit production under climate change, with implications for population persistence and colonization of new sites. However, the use of assisted gene flow through the introduction of seed from southern populations could introduce adaptive phenological traits and expedite adaptation to changing climates. Additionally, these results suggest that the mixing of seed sources for restoration and habitat creation could greatly expand flowering windows with implications for pollinator conservation.