American Trees are Shifting West

For 86 common species, northwest seems to be best. But why?

Aspen Forest
Aspens are one of the American tree species moving northwest. trevorklatko - Flickr/Creative Commons

The United States has a long, fraught history of westward movement. But what happens when the movers are not people, but plants?

Now, reports Robinson Meyer of The Atlantic, it seems that American trees are doing the same thing. No, they’re not sentient—and they don't care about Manifest Destiny. But the abundance of common species of American trees has shifted westward in recent years.

In a new study in the journal Science Advances, researchers used two national tree censuses—one inventory taken between 1980 and 1995, the other finished in 2015. Of 86 species tracked in both inventories, including the shortleaf pine, black ash, sassafras and silver maples, 73 percent shifted westward, and 62 percent shifted northward.

In this case, “shift” means expand its growth into a new area as other trees die off elsewhere. “The research team compared a tree population to a line of people stretching from Atlanta to Indianapolis,” explains Meyer. “Even if everyone in the line stood still, if you added new people to the end of the line in Indiana and asked others in Georgia to leave, then the center of the line would move nonetheless.”

But why are they moving to begin with? The answer is unsatisfying: Researchers aren’t really sure. Climate change does seem to be a factor. Scientists have long predicted that plants will migrate in response to warming temperatures. However, it’s hard to tease out why plants that have moved did so.

Climate can impact trees in different ways, inflicting drought or whipping up wind, but the U.S. Forest Service notes that insects, diseases and changing wildfire patterns can indirectly affect their range, too. Though the researchers hypothesize that young trees adapt to climate change more readily than their older counterparts, there’s a lot of variability at play. They suggest that everything from how areas manage fires to the presence of invasive species might spur trees to move—and any of those variables may or may not be caused by climate change. Increasing rainfall in the West, and decreasing moisture in the East, seems to be driving the movement, but it’s not clear how much that contributes to the shift.

However, another clue points to climate as the underlying cause. The trees’ northward shift lines up with scientists’ long-term predictions. Since plants only thrive in a certain, relatively narrow band of temperatures, wisdom has it that they’ll move to find those cool temperatures over time. And as the global temperature warms, those kinds of shifts can be seen in animals like fish and birds.

But for the trees, it remains unclear if climate change is really driving them to blaze a westward trail. With the Earth warming faster than ever before, however, scientists may have answers before long.

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