In the face of rapid anthropogenic climate change, species persistence may depend on geographic range shifts, thus, we need a better understanding of when and how range shifts occur. Here, we investigated the recent northward expansion into the Northeast US of Magnolia tripetala, an understory tree native to Southeast US forests. Numerous adventive populations occur in the Northeast, up to 300-400 km beyond the historical range boundary in southern Pennsylvania.
We evaluated whether rapid range shift in M. tripetala was facilitated by the presence of horticultural trees serving as seed sources and determined the age of naturalized populations in using tree cores. Next, because horticultural trees apparently produce flowers, and successfully set fruit, at higher latitudes than where the species is presently known to naturalize, seed survival/germination may limit naturalization beyond the range edge. Thus, we tested climate limitations to seed survival and germination. Seeds collected in Connecticut were exposed to winter conditions at four sites along an elevation gradient in Massachusetts spanning elevations of known populations (Smith College, 43m) to those higher than any known naturalized population at Petticoat Hill (213m), Bryant Homestead (518m), and Notchview Reservation (675m). These seeds (800 overwintered at each site) were sown into a common greenhouse in late April.
Despite a long horticultural history of M. tripetala in the Northeast, naturalized populations traced to the mid- to late-20th century. Moreover, there was a significant correlation between the age of naturalized trees and latitude. Near the native range edge, trees established in the mid-20th century, those further north in the late 20thcentury. Most if not all naturalized populations were associated with nearby horticultural specimens. An exploratory study with a small subset of seeds overwintered from mid-October to mid-December indicate low viability from tetrazolium assays (~33%) and germination tests (9%).
Although many species are not migrating at rates rapid enough to track climate change, M. tripetala is among a set of horticultural species which might present an unusual exception. Our data suggest M. tripetalacircumvented dispersal limitation through seed sources from horticultural specimens. The age of Northeastern naturalized populations suggests that the timing of naturalizations was climate-limited.
Ibutton data indicate there were more sub-zero (°C) nights at higher sites used in our overwintering experiment, particularly in March. The size of the experiment should permit us to detect differences in survival/germination of seeds among sites, in spite of low seed viability.