As the climate warms, some mountain trees are moving higher up, while others are spreading downhill. Scientists say something inside the trees may explain why. And that split could reshape watersheds and wildfire risk. 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.