North American trees are moving west, and scientists can’t explain why.
With seasonal temperatures breaking records almost every year, tree species are expected to adapt to the changes by slowly shifting their population centers northward and to higher elevations. But a recent survey shows the trend toward westward movement is even stronger than expected — in some cases, species have shifted their ranges west by as much as 73 percent.
The research for this groundbreaking survey came from the U.S. Forest Service’s Forest Inventory and Analysis Program, an annual census that has recorded data on the health, species mix, and density of forests in the United States since the 1930s. Recently published in Science Advances, the survey revealed that roughly three-quarters of the 86 tree species surveyed have shifted their population centers westward since 1980.
Obviously, individual trees aren’t uprooting themselves and moving elsewhere. The survey instead provides insight into general population trends as seedlings continue to take root in increasingly westward terrains, while some of the older stands of the species in the far eastern regions of the continent are slowly dying out. In this way, the center of a species’ range can gradually shift over time.
Though scientists aren’t sure what’s causing this change, the publishers of the study hypothesize that it’s connected to moisture levels. Rainfall totals across the United States have altered, causing regions such as the Southeast to experience significantly less rain annually, while the Great Plains is getting far more than its historical average. For this reason, most angiosperms, including oaks, maples, and dogwoods, are gravitating toward the increasingly watery plains, while the majority of gymnosperms, such as conifers, are sticking to the previously predicted northern migration.
Rainfall levels might be just one factor in this westward march. Wildfire frequency, new ranges for pests and diseases, and changes in land use may all factor into the shift as well. More data is needed, but for now, the research team believes that at least 20 percent of the shift can be attributed to changing rainfall patterns.
No matter the cause, this survey is highlighting some troubling trends for the future of North American forests. If angiosperms and gymnosperms continue to move in different directions, the mixed-species forest systems that other plant and animal species depend on for survival may start to break up.
For now, scientists agree that this study has raised more questions than it answers. The survey is based entirely on empirical data, meaning it isn’t making predictions about future movement. Whether the trend of westward travel will continue remains to be seen, but scientists plan to continue collecting data.
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