Climate-induced tree migration in southern Ontario: Pathways and source populations

Background/Question/Methods Under the current rapid climate change, trees are of particular interest among plants because of their relatively low migration capabilities. Although increasing landscape connectivity is an important strategy from a climate change perspective, as yet little research has identified where such increases might best take place. Current tree distributions are often fragmented by natural and human barriers, making such considerations non-trivial. Such barriers might block certain potential tree migration routes; they also might make certain source populations of critical importance in facilitating future migration. To identify future migration routes and source populations, we analyzed migrations of 127 tree species in the fragmented landscape of southern Ontario, Canada, for six climate change scenarios. We focused on two migration calculation methods: a “buffer” approach in which trees could migrate only through forest and land within 1 or 2 km of forests and a “cost-path” approach in which migration through forest was less costly than through other land cover types. For both, migration routes were based on “back-cast” migration paths; i.e., the shortest paths between presumptive future and current range locations as determined by climate-envelope models.

Results/Conclusions The results showed that species migration paths tended to be more concentrated on forested areas, in common with all migration models. However, the two models showed different relationships between forest cover and migration path concentration: the buffer method showed a marked increase in path concentration below 25-30% forest cover, whereas the cost-path method showed more-or-less linear increase in path concentration with decreasing forest cover. We found support for the importance of existing corridors such as the Oak Ridges Moraine and the Niagara Escapement in facilitating future migration, suggesting the current efforts to facilitate large-scale connectivity such as the Adirondack-to-Algonquin initiative, will prove useful in increasing future migration. Source populations showed few differences between the two models, with existing forest cover and connectivity, critical in determining their locations. Compared to straight line (crowfly) migration, fragmentation increased migration rates by a factor of 1.6 on average. In general, our results were relatively robust in that important migration routes and source populations tended to be similar among climate change scenarios and among calculation methods, which should make conservation efforts easier.