Global warming is leaving trees behind, according to a new study in Science. An analysis of forest species in six French mountain ranges (the western Alps, northern Pyrenees, Massif Central, western Jura, Vosges and the Corsican range) shows that more than two thirds of them moved at least 60 feet (18.5 meters) higher on the mountainsides per decade during the 20th century.

"Among 171 species, most are shifting upwards to recover temperature conditions that are optimum," says ecologist and lead study author Jonathan Lenoir of AgroParisTech in Nancy, France. "Climate change has already imposed a significant effect in a wide range of plant species not restricted to sensitive ecosystems."

Previous research has shown that plants at the highest elevations on mountains (and in the polar regions) have been shifting to adjust to global warming. But this is the first confirmation that entire ecosystems in lower, more temperate regions are moving as well.

"Species are not just moving at the extremes of their ranges," says ecologist and co-author Pablo Marquet of the Pontificia Universidad Católica de Chile in Santiago. "What we show is that they are moving everywhere."

In an effort to gauge the effect of climate change on ordinary plant life, researchers measured where the best growing conditions on the mountains were for species of trees, grasses, herbs, ferns and mosses. They discovered that those for 118 of the studied species—from the herblike three-horned bedstraw (Galium rotundifolium) to whitebeam trees (Sorbus aria)—migrated to higher elevations as temperatures warmed.

The researchers found that grasses, herbs and other short-lived species that had been through many generations shifted the most in search of perfect temperatures, whereas long-lived trees stayed largely in place. According to the authors, this is changing the composition of the forest—mixing formerly low-altitude grasses with high-altitude trees—which could potentially affect the entire ecosystem, particularly the animals that rely on specific plants to survive.

Plant ecologist Gian-Reto Walther of the University of Bayreuth in Germany says it is unclear what this finding bodes for the broader ecosystem.

Lenoir notes, for instance, that even though tree species did not show much sign of movement over the past century, that climate change may affect the next generation. "Even if adult tree species suffer from hard conditions, they are still present so you can't see any changes," he says. "But seedlings perhaps don't appear in lower elevations that are too warm."

But long-lived and slow-moving trees may ultimately be able to catch up with their smaller, faster counterparts. "As long as the older trees are not so stressed that they do not produce many viable seeds, [and] the dispersal mechanism—for example, wind, birds, mammals—is present, and the habitat where the seed lands has the appropriate soil, nutrients and temperature," says biologist Terry Root of Stanford University, who was not involved in the study, "then the trees will be able to shift."