New study asks why some American forests are moving West

Back in the 1800s, American settlers from the growing cities in the East moved westward to find open land for farming, hunting, and other resources. For many, the open plains in the West meant new opportunities and a chance to make their fortunes in greener pastures, shaping the landscape and infrastructure of the United States for generations to come.

But now, a different kind of westward migration is underway, as Eastern forests in the US have showed surprising signs of shifting westward over the past few decades as well.

A recent study analyzed the movement of 86 tree species found across the eastern United States. Part of the shift northward was expected as an effect of climate change; as temperatures rise, many organisms move to cooler territories. But the trees' move westward was unexpected, and harder to explain – but climate change could still be partially responsible, according to Todd Lookingbill, a landscape ecologist and chair of the department of geography and the environment at the University of Richmond.

"[This study] is consistent with several other recent reports that indicate not all species range shifts in response to climate change will follow conventional predictions up mountains and towards more northerly latitudes," Dr. Lookingbill tells the Christian Science Monitor via email. "Especially at local scales, shifts are occurring downslope, towards the coast, or laterally in mountains. The findings here indicate larger-scale, regional shifts from east to west. They highlight the important role that changes in precipitation are already having on tree distributions."

According to Lookingbill, who was not involved in the study, tree populations will move in response to changes in available resources. When water becomes less available, for example, new saplings grow in areas that get more rain – and shifting rain patterns from climate change means that the West is becoming wetter while the East is becoming drier.

According to the study, which was published in the journal Science Advances, 73 percent of tree species examined in the study have experienced a westward shift in the past three decades, while only 62 percent experienced a shift poleward. Almost none moved south or east.

"Different species are responding to climate change differently," Songlin Fei, one of the authors of the study, told The Atlantic. "Most of the broad-leaf species – deciduous trees – are following moisture moving westward. The evergreen trees – the needle species – are primarily moving northward."

There are exceptions, of course: The eastern pine has moved more than 80 miles west, while the eastern cottonwood has shifted 77 miles north, but the overall trends are clear.

"I think it's important not to lose track of the finding that hardly any shifts were reported to the south," says Lookingbill. "These are not random processes going willy-nilly in all directions."

Climate change has begun to shift weather patterns in the US, likely affecting the envelope in which these trees are able to live. Areas in the West are now wetter and more accommodating than they were 30 years ago, while eastern portions of the area that was examined have gotten drier. But while it's tempting to blame the shift entirely on climate change, there are other factors at play as well, according to Villanova University biology professor Adam Langley, who was not involved in the study.

"Introduced tree pests, such as the emerald ash borer and hemlock woolly adelgid, are clearly a huge deal in changing forest composition. One could imagine trees population centers moving west as individuals on the eastern front die from diseases, which are commonly introduced at the coast," he tells the Monitor via email.

Fires and other factors could also contribute to the shift. But it's also hard to tell how significant the move actually is, since data prior to this study is difficult to compare with the team's results.

"This dataset is very unique, and compared to the lifespan of a tree, very recent," says Dr. Langley. "It took incredible foresight by the Forest Service to establish these long-term research plots decades ago.... The historical methods don’t allow for a very precise estimate of migration speed of population centers and would not be comparable."

Still, the team behind the study argues that at least 20 percent of the change in population area is driven by changes in precipitation patterns, which is at least partially influenced by human-caused climate change.

Regardless of the causes, the westward shift of Eastern forests could carry negative implications for ecosystems that depend on certain types of trees staying put. Trees perform a number of important ecological functions, including providing shelter, food sources, and even water filtration for other plants and animals. But as some tree species head west while others move north, this ecological balance could be thrown off – though scientists aren't sure how environmentally destructive it might be.

"The question here is: Does it matter for the ecosystem if we replace one kind of tree with another?" asks Langley. "This question has received a lot of attention from ecologists, but we don't have a clear answer. I, for one, will be disappointed if our ashes and oaks are replaced by sweetgums and loblolly pines. I imagine our ecosystems will feel the same way."

This article contains material from the Associated Press.