American Trees Are Moving West, and No One Knows Why

As the consequences of climate change strike across the United States, ecologists have a guiding principle about how they think plants will respond. Cold-adapted plants will survive if they move “up”—that is, as they move further north (away from the tropics) and higher in elevation (away from the warm ground).

A new survey of how tree populations have shifted over the past three decades finds that this effect is already in action. But there’s a twist: Even more than moving poleward, trees are moving west.

About three-quarters of tree species common to eastern American forests—including white oaks, sugar maples, and American hollies—have shifted their population center west since 1980. More than half of the species studied also moved northward during the same period.

The results are fascinating in part because they don’t immediately make sense. But there is a hypothesis: While climate change has elevated temperatures across the eastern United States, it has significantly altered rainfall totals. The northeast has gotten a little more rain since 1980 than it did during the proceeding century, while the southeast has gotten much less rain. The Great Plains, especially in Oklahoma and Kansas, get much more than historically normal.

There are a patchwork of other forces which could cause tree populations to shift west, though. Changes in land use, wildfire frequency, and the arrival of pests and blights could be shifting the population. So might the success of conservation efforts.

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