Global warming has plants on the move.

In response to Earth's warming temperatures, plants in the Alps are seeking higher ground, according to a study comparing today's flora to vegetation earlier in the century

"Our data indicate that even small differences of temperature . . . can induce [plant] migration up a mountain," Austrian researcher Georg Grabherr and his colleagues at the University of Vienna write in a letter in the June 9 Nature.

In 1992, the researchers counted the number of species and estimated the abundance of primarily flowering plants above 3,000 meters on 26 mountains in western Austria and eastern Switzerland. They compared their data to the same figures from the early and middle 1900s.

Species richness on these mountains has increased during the past few decades, thanks in part to the steady upward climb of vegetation, Grabherr says. Not only do former residents of the nival zone, the area above about 2,000 meters, move up, but former alpine plants ascend to the nival region.

"We were very surprised by that," he says. Historical documents showed no alpine plants above the nival zone.

Nine common species have ascended the mountains at the rate of 1 to 4 meters per decade -- somewhat slower than the investigators had expected. The mean annual temperature has increased 0.7[degrees]C since the early 1900s; because mountain air cools about 0.5[degrees]C every 100 m, the vegetation should have moved 8 to 10 m per decade. Why it didn't is unclear.

If global temperatures continue to increase, some mountain plants may have nowhere to go, the researchers point out. Global warming "may cause disastrous extinctions in these environments," Grabherr and his colleagues warn.