Assessing the potential for urban trees to facilitate forest tree migration in the eastern United States

Climate change is expected to lead to migration of tree species, i.e., in the northern hemisphere, as temperatures increase, tree species are expected to move northward in search of the temperatures and growing season duration to which they are adapted. A critical question is whether this can occur quickly enough for the species to survive. This study looked at the composition of northern urban forests (Syracuse, Boston, Hartford, New York, and Minneapolis) to find species that were present in large numbers north of their native range with the objective of determining whether urban trees could serve as seed sources along a possible northward migration. The answer was generally no. Most urban forests were composed of ornamental, nonnative species; less competitive understory trees; and species within their native range. In addition, the density of urban trees is not very high and likely insufficient to serve as a viable seed source. Finally, urban trees have been selected for traits that may make them less viable in forest settings. There is also concern that changes in climate may favor some nonnative species, leading to greater levels of invasiveness.