Anthropogenic climate change is already causing many vagile species to shift their distributions poleward. However, some taxonomic groups, such as plants, have showed less evidence of such range shifts, likely due to dispersal limitations, raising concerns of migration lags and increased extinction risks. Some researchers have even proposed intentionally translocating threatened species to keep pace with rapid climate change, a practice referred to as ‘assisted colonization’ or ‘managed relocation’. Although this controversial approach has not been formally implemented on a large scale, increasing evidence suggests that unplanned introductions of native species via escape from cultivation might result in similar outcomes.
In this study we investigated the naturalization and recent northward expansion into the Northeast US of Magnolia tripetala, an understory tree species native to deciduous forests of the Southeast US. Numerous adventive populations have been documented in the Northeast US in recent decades, up to 300-400 km beyond the species’ historical range boundary in southern Pennsylvania. Through population surveys, historical research, tree coring, and species distribution modeling we have examined the sources and age structure of these new populations, and explored their relationship to shifting environmental factors.
Results/Conclusions
In field surveys of adventive M. tripetala populations along a latitudinal gradient extending north of the species’ historical native range, we found strong evidence of nearby horticultural specimens as likely seed sources. Further, despite a long horticultural history of M. tripetala in the Northeast, all naturalized populations traced to the mid- to late-20th century, with a significant correlation between population age and latitude. Specifically, tree core data suggested that adventive populations in warmer locations, near the native range edge, established in the mid-20th century, while those further north only established and spread in the late 20th century.
Given the long-term presence of horticultural specimens in the Northeast, and the sequential emergence of adventive populations along the latitudinal gradient, this suggests the timing of population expansions closely tracked environmental changes, such as climate warming, that might have rendered local environments in the north suitable for recruitment of this southern species. In contrast, native populations at the historical northern range edge showed signs of local expansion in the mid- 20th century, but northward movement via long distance dispersal has not been detected. Horticultural escape, rather than natural dispersal, has allowed M. tripetala to rapidly colonize newly suitable northern regions.