Most readers are probably familiar with some of the implications of climate change: sea level rise; more frequent extreme weather events; habitat loss for arctic species. Other implications are equally important to understand and reach into many realms of ecology (as well as other disciplines), but are not popular topics covered in the media.

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Effects of projected greenhouse gas emissions on species migration. Speed at which animals and plants can migrate represented by white rectangles. Speed at which climate is expected to change are horizontal dotted lines. RCPs (Representative Concentration Pathways) indicate levels of potential greenhouse gas emissions, 8.5 being the highest-emissions scenario, and 2.6 the lowest. If RCP lines are above speeds of animal/plant movement, that group is not expected to migrate quickly enough to track climate change. Source

Think about some of the various ways plants and animals can respond to a changing climate. Some organisms are flexible, and their physiology and behaviors can change to allow survival under the new conditions. Some organisms may need to adapt by developing new characteristics to survive. Other, less fortunate organisms may go extinct. Mobile animals might be able to pack up and move to different regions, “tracking” climate over time to make sure they remain in areas that contain just the right climate for their survival (figure on the left). In the latter case, we would expect the distributions of these species to shift either northward in latitude or upward in elevation.

What about the less mobile creatures? Their fates are perhaps more uncertain. In particular, individual trees can do little to survive when climate or precipitation patterns shift in a way that no longer suits their needs. But some researchers believe that some tree species may be able to migrate fast enough to match changes in climate. Before you start imagining trees lifting up their skirts and tromping up a mountainside (image below), let me explain.

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Illustration of an Ent (a fictional race of tree-like beings found in J.R.R. Tolkien’s Lord of the Rings).

Think about how trees maintain their populations: they make seeds, disperse them (either via birds, wind, gravity, etc.), and seeds that are lucky enough to land in a good location can germinate and grow into new trees. If seeds can get dispersed far enough away from their parents, they may, by chance, end up higher up a mountain, or higher in latitude. These areas were historically cooler, and therefore not good for survival; but perhaps with a warming climate these areas are now better suited for tree growth. Theoretically, those seeds that end up in the “new” locations may germinate and grow, while seeds landing in areas that are becoming warmer may not be able to withstand the conditions  for very long. After a while, you can imagine how a species may eventually become less common in southerly areas of their distribution, while extending further northward at the northern ends of their distribution. This is tree migration.

But does this actually happen? Scientists have looked to warming events in Earth’s geological past by examining fossils and pollen deposits in old peatlands and lake sediments to try to answer this question. It appears that trees were able to migrate to track the last warming event, which occurred just after the last glacial period (about 12,000 years ago). Different species migrated at different rates, to some degree based on how far their seeds could disperse. But this prehistoric migration occurred much more slowly than the rate of warming that is currently taking place. Can trees – particularly those whose seeds cannot disperse very far – migrate fast enough to keep up with contemporary warming trends?

Although there are different answers to this question, one recent study using a massive dataset composed of 92 tree species and tens of thousands of plots across the eastern U.S. said … not really. Well, at least most species are not migrating. While only 20% seemed to be moving north, almost 60% of the species’ distributions have contracted – that is, decreased on both the northern and southern fronts. These results imply that most tree species in the eastern U.S. may be suffering from climate change rather than adapting or migrating.

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Jacques-Cartier National Park, Quebec. Higher latitudes are expected to see more dramatic changes in climate.

What can we do to combat tree contraction? Some land managers have suggested “assisted migration,” wherein people strategically plant trees in areas that may be ideal for tree establishment. It’s unclear whether this effort would be economically feasible and worthwhile. Regardless, it will be interesting and important to keep track of our tree populations as the climate warms.

Peer edited by Diamond Ebanks & Annelise Gorensek

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This article was co-published on the TIBBS Bioscience Blog.

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