High on a Vermont mountain, a forest that has stood for centuries is quietly reshuffling. Some trees are climbing higher up the slope. Others are inching downward. And scientists now understand why — and what it means for the future of forests everywhere.
A major new study published in Nature Climate Change tracked more than 121,000 trees across 3,057 forest sites on mountains around the world. The researchers, working across universities including the University of Vermont and Peking University, wanted to know why some tree species move uphill as the planet warms while others head in the opposite direction.
The answer, it turns out, comes down to water.
Trees move water from their roots to their leaves through narrow tubes called xylem. The researchers found that species with highly efficient water transport systems tend to be more sensitive to climate warming — they feel the heat more acutely and shift upslope faster, chasing cooler conditions. Meanwhile, species built to survive drought show more staying power. These tough trees often expand downslope into drier areas where fewer species can survive.
This finding upends a common assumption that all mountain forests will simply march uphill as temperatures rise. "Species are not responding in the same way," said Hongyan Liu of Peking University, the study's corresponding author. "A species' hydraulic strategy helps explain whether it tracks warming to higher elevations or persists and expands into drier lower elevations."
Tim Rademacher, director of UVM's Proctor Maple Research Center and a co-author of the study, said the research gives forest managers something they have long needed: a roadmap for predicting tomorrow's forests. "Understanding how trees balance growth and drought resistance helps explain their responses to climate change," he said.
The study also flags a serious challenge ahead. As drought-sensitive species crowd into shrinking high-elevation refuges, competition intensifies and extinction risks grow. But the researchers say their work offers a practical tool for conservation. By identifying which hydraulic traits make species vulnerable, land managers can target their efforts where they matter most.
Mountain regions cover roughly one-quarter of Earth's land surface and shelter enormous biodiversity. They also store carbon and regulate water supplies for billions of people. Knowing which forests are most at risk — and why — is the first step toward protecting them. And now, scientists have a much clearer picture of how to do exactly that.