A new study published in Nature Climate Change reveals a more complex reality: some tree species are moving uphill, while others are shifting downhill, and the key to understanding these contrasting responses lies in how trees manage water.
In one of the largest analyses of its kind, an international team of researchers integrated tree-ring records from more than 121,000 trees representing 45 species, climate-growth data from 3,057 forest sites worldwide, hydraulic trait information, and global observations of elevational range shifts across 102 mountain tree species. Their findings show that a tree species' hydraulic traits, physiological characteristics that regulate water transport, drought tolerance, and growth, are powerful predictors of how forests reorganize under climate change.
A co-author of the paper, Tim Rademacher, Director of UVM’s Proctor Maple Research Center, describes the significance of their findings: “This research explains why some tree species move uphill with climate warming while others do not, improving our ability to forecast future forest distributions and identify conservation priorities. Understanding how trees balance growth and drought resistance helps explain their responses to climate change and offers a roadmap for predicting tomorrow's forests.”
The researchers discovered that species with highly efficient water transport systems tend to be more sensitive to climate warming and shift upslope more rapidly. In contrast, species with traits that provide stronger protection against drought often show greater resilience to warming and are more likely to expand downslope. These opposing responses help explain why mountain forests are changing in unexpected ways across the globe.
Importantly, the study found that the rate of local warming alone did not explain why some species move faster or farther than others. Instead, differences in species' physiological characteristics played a much larger role in determining both the speed and direction of range shifts.
The findings also raise concerns about the future of biodiversity in mountain ecosystems. Species that move upslope in response to warming encounter progressively less available habitat as mountain land area shrinks toward the summit. As a result, many climate-sensitive species could become concentrated in increasingly crowded high-elevation refuges, potentially intensifying competition and raising extinction risks.
"Climate warming is reshaping mountain forests around the world, but species are not responding in the same way," said corresponding author Hongyan Liu of Peking University. "We found that a species' hydraulic strategy helps explain whether it tracks warming to higher elevations or persists and expands into drier lower elevations. Our results suggest that climate refuges are not a universal solution. Many drought-sensitive species may ultimately be confined to smaller and smaller areas at high elevations, while drought-tolerant species expand into lower elevations. Understanding these physiological differences is essential for predicting future forest change.”
Beyond advancing ecological theory, the study offers a practical tool for conservation and forest management. By identifying hydraulic traits as a fundamental mechanism linking climate change to species redistribution, the research provides a more accurate framework for forecasting which species are most vulnerable and where future conservation efforts should be focused.
Mountain regions cover roughly one-quarter of Earth's land surface and support exceptionally high biodiversity while providing vital ecosystem services, including carbon storage and water regulation. As climate change accelerates, the authors argue that conservation planning must move beyond simple assumptions that all species will shift uphill and instead account for the physiological traits that determine species' capacity to adapt.
