More than two-thirds of the 171 species studied in the mountains of Western Europe had climbed an average of 95 feet per decade in recent years.
"This is the first time that it has been shown that climate change has already had a significant effect on plant species, over a wide range of temperatures, during the past century," Jonathan Lenoir, an ecologist at the university AgroParisTech in France and the lead author of the study, said in a statement.
Previous research had shown that species in extreme environments (high on mountaintops and in the polar regions) have been moving to adjust to rising temperatures, but this study is the first to find species in more moderate climates shifting as well.
The researchers analyzed information from two long-term databases of plant species in the area. They compared the distribution of plant species in the mountains between 1905 and 1985 to their distribution between 1986 and 2006.
"The most important result of our study is that among our 171 species, most are shifting upward," Lenoir told the Associated Press.
Individual plants, of course, can't move up a mountain. But over generations a species can gradually drift higher -- as seeds from the plants are blown both up and down the mountain, the ones that land in a more favorable climate are more likely to take root and grow.
Some species climb faster than others. Trees and shrubs, which have a long life cycle, didn't show significant elevation shifts in the study, whereas short-lived species like grasses and herbs did. That's a problem, according to the researchers, because when species move at different speeds it disrupts vital connections between plant species that depend on one another to survive.
University of Vienna ecologist George Grabher, who studies Alpine plants , said he was surprised by the magnitude of the shift: "These are forest plants, and forests are very resistant to migration," he told Science.