Many species demonstrate latitudinal clines consistent with adaptation to climate. However, temperature and growing season length both covary with latitude, and adaptations to each generate different predictions for germination and early growth. If plants face a tradeoff between cold tolerance and growth rate, then we would expect germination timing to demonstrate patterns consistent with local adaptation to temperature. Alternatively, if plants demonstrate adaptation to growing season length, we would expect consistent clines, with seeds from high-latitude populations emerging earlier regardless of temperature, rather than local adaptation. Here, we report on an investigation of local adaptation to thermal spring conditions in common milkweed, Asclepias syriaca. We used seeds from 10 different populations collected from across the range of common milkweed and spanning latitudes from 37-46°N. Seeds were cold stratified for either 4 or 10 weeks and then germinated in growth chambers simulating either Minnesota or Kentucky spring temperatures.
Results/Conclusions
We observed consistent patterns across chambers: seeds from northern populations displayed higher germination fractions and earlier germination. A longer period of cold stratification advanced germination time in general, but surprisingly, the effect of cold stratification length tended to be greater for seeds from lower latitudes. Together, these data suggest that common milkweed displays adaptations to growing season length, at least in the early life history stages. Our results suggest that milkweed populations might be more likely to evolve in response to the effects of climate change on growing season length rather than temperature. Although seeds sourced from northern latitudes are likely to germinate earlier and at higher fractions, further research is necessary to test whether seeds from northern populations incur long-term costs when planted in more southern field restoration sites.