For years, scientists have expected mountain trees to move upslope in search of cooler conditions. But a new global study found nearly one-third of tree species are moving downhill instead.
That matters because where trees grow can affect how forests burn and how much water they soak up before it flows downstream to rural communities, cities and farms.
So why are some trees moving in one direction while others head the other way?
The answer may lie in how trees move water.
Tim Rademacher, a researcher at the University of Vermont and a co-author of the study, says some species can move water quickly and efficiently, helping them grow. But that efficiency comes with a cost.
“That also makes them vulnerable to drought,” Rademacher said. “And there's kind of a trade-off, so a tree cannot be both; a tree cannot be really efficient and safe.”
That may help explain why some species are quicker to track cooler conditions uphill, while others can better tolerate warming at lower elevations and expand downhill.
The researchers looked at tree-ring records and climate data from tens of thousands of trees, along with observations of how more than 100 mountain tree species have shifted their ranges. They found that how trees move and manage water was a better predictor of which direction they moved than the rate of warming alone.
In the West, Douglas fir and ponderosa pine are among the species moving higher. Whitebark pine trees, already found near mountaintops, are moving down.
That difference could make some species more difficult to conserve. Trees moving uphill can eventually run out of room as they approach mountaintops, where there's less habitat available. Other species may have more room to expand at lower elevations.
“Knowing which species are moving up, where there's less space, versus which species are moving down, where there's more space is really important when it comes to managing our expectations for future ecosystems and also trying to set conservation efforts,” Rademacher said.
The study doesn't predict exactly what any one Western forest will look like in the future, but, Rademacher said, it gives scientists and land managers a better way to anticipate which species may be most vulnerable, and which may be better equipped to adapt.
This story was produced by the Mountain West News Bureau, a collaboration between Boise State Public Radio, Wyoming Public Media, Nevada Public Radio, KUNR in Nevada, KUNC in Northern Colorado, KANW in New Mexico, Colorado Public Radio and KJZZ in Arizona as well as NPR, with support from affiliate newsrooms across the region. Funding for the Mountain West News Bureau is provided in part by the Corporation for Public Broadcasting and Eric and Wendy Schmidt.
