As global temperatures rise, scientists have largely assumed that mountain tree species will move to higher elevations in search of cooler conditions. The trees growing on the highest peaks of the Green Mountains are mountain forests and were the focus of this study. A surprising new study reveals why some mountain tree species are actually moving downhill, even as climate change drives others to higher, cooler elevations. These contrasting responses help explain why mountain forests are changing in unexpected ways around the world. Species moving to higher elevations in response to global warming are finding less and less suitable habitat as the available mountain area becomes smaller towards the summit.