US Trees Are Heading Westward to Chase Changing Rainfall Due to Climate Change

The trees are on the move.

Warmer, drier conditions in much of the eastern United States has resulted in dozens of America’s most common tree species shifting their ranges over the last three decades, scientists report in a new study published in the journal Science Advances. Pines and other evergreens in the Southeast are growing farther north than they did in the years before 1980, while oaks, maples, and other Northeastern hardwoods are now growing more to the west of their traditional territories.

The geographic center of some species’ ranges has been moving more than a mile a year, said Songlin Fei, an ecologist at Purdue University who led the study.  While previous studies have shown trees shifting northward and toward higher elevations in response to warming temperatures, Fei’s research suggests that the shift may have more to do with changes in rainfall.

“The Southeast has had a dramatic reduction in precipitation,” Fei said. “The western portion of the study area has more moisture available compared to the historical average.”

So while Florida still gets more rain than Indiana, new growth is moving rapidly toward the Midwest as a result of that change, with more drought-tolerant trees leading the way.

Of the 86 species included in the study, scientists saw westward shifts in 73 percent of them, while 62 percent shifted northward. The migration is far more rapid than has been seen in historical data: The center of some species’ ranges moved as much as 30 kilometers (19 miles) per decade.

Trees are always shifting in response to their environment. But the pace Fei and his colleagues clocked over the past 30 years is far more rapid than has been seen in historical records. It’s comparable to speeds estimated to have occurred as the glaciers retreated at the end of the last ice age, he said.

Climate change isn’t the only factor in that process – American woodlands are also under pressure from pests and development, Fei noted, and new plantings from conservation programs also affect their ranges. But climate change accounts for about 20 percent of the changes.

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Fei and his colleagues compiled their results from reams of US Forest Service records — an annual count from more than 100,000 locations around the country.

“These are boots on the ground,” he said. “This is something I think is really unique about this study. This is empirical data — people on the ground, counting the trees, not modeling or estimating how many trees we have or how many we’ll have in the future.”

“I think one very important thing from our study was that we’d better pay close attention to precipitation patterns and moisture change,” Fei added. “Changing moisture variability has a much stronger near-term impact on the ecosystem that we’re looking at than temperature.”

Climate scientists have a good track record of modeling future temperatures, and the warming now under way is expected to produce more extreme weather events — but according to Fei, models of future precipitation trends are far less developed.

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