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, says, "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."

Water strategy drives direction

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.

Warming alone falls short

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.