Climate Change is Only 20% of Why Trees Are Moving

The Moving Trees
A 30-year study shows that certain tree species are moving westward or northward, and only part of the issue is climate change.
by Jeff Cox

Literature has several examples of a forest on the move. It was prophesized that Macbeth would die when Birnam Wood came to his castle at Dunsinane. As his opponents’ army advanced through the forest, its soldiers were instructed to cut a tree branch and carry it before them to hide behind. From Macbeth’s vantage point, it looked like Birnam Wood itself was moving up on him.

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Vintage engraving from 1877 showing a scene from William Shakespeare’s play “The Tragedy of Macbeth.”

There’s also the destruction that Treebeard and his fellow Ents visited upon Isengard in The Lord of the Rings. But in real life, forests don’t move. Or do they?

Forest scientist Songlin Fei and his team from Purdue, North Carolina State, the USDA’s Forest Service and the University of Tennessee have analyzed an abundance of data over the last 3 decades for 86 tree species in mixed forests across the eastern United States. What they found raised their eyebrows.

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This eastern forest includes hardwood trees like maples, birch, beech and oaks, as well as conifers. A 30-year study shows that the majority of eastern hardwood species are shifting their population centers westward, while conifers are moving north, possibly disrupting forest ecologies.

According to their study, published in Science Advances for May 17, 2017, deciduous, flowering tree species are moving westward, while conifers are moving northward. Of course, individual trees aren’t moving, but if you draw a circle around the range of a given species and put a pin in its geographical center, over the past 30 years that center pin will have moved significantly westward for deciduous trees and northward and to higher elevations for cold-adapted trees like conifers.

About 75 percent of the common hardwood species of eastern forests have shifted their population centers west since 1980, and about half of all species in the mixed forests have moved northward.

Why? Is it climate change? The researchers attribute about 20 percent of the shift to that, at least for the deciduous species. They hypothesize that it’s not so much change in temperature that directly causes more saplings to grow on the westward edge of the species’ range, but rather an increase in rainfall and soil moisture caused by climate change. The data shows that the Northeast has gotten a little more rain since 1980 than it did in the preceding 100 years, while the Southeast has gotten significantly less. But directly west, places like Kansas and Oklahoma have received a lot more rain.

If only 20 percent of this movement is attributable to climate change and the consequent shifting of rainfall patterns, what might be some of the remaining 80 percent of forces driving the phenomenon? It could be that new populations of insects and blights might be attacking forests from the east. Changing patterns of open fields and forested lands as people move might be a cause. Fewer or more frequent forest fires could contribute, as well as local conservation efforts. But it looks like the most reliable driver of the shift west and north is indeed climate change.

This article is excerpted from the November/December 2017 issue of Horticulture. You can read the full column in the issue. Science columnist Jeff Cox writes from his home in northern California.

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