BEIJING, March 12 (Xinhuanet) -- Forests of spruce trees are invading the Arctic tundra because of global warming and evicting and endangering species that live there quicker than scientists thought, a new study was quoted as saying Monday by news reports.

    Tundra is land area where tree growth is inhibited by low temperatures and a short growing season. In the Arctic, the tundra is dominated by permafrost, a layer of permanently frozen subsoil.

    Lichens, grasses and mosses are the only vegetation that can grow in such frigid conditions. Forests of spruce trees and shrubs neighbor these tundra areas, and the boundary where they meet is called the treeline.

    The permafrost thaws in summer and the tundra becomes covered in bogs and lakes, offering a unique habitat for plants. But global warming has lengthened the summer warming season and promoted tree growth, causing the treeline to encroach on the tundra.

    Researchers reconstructed a 300-year history of tree density and treeline position by looking at tree rings. The results show trees can invade the tundra faster than previously thought.

    "The conventional thinking on treeline dynamics has been that advances are very slow because conditions are so harsh at these high latitudes and altitudes," said Ryan Danby of the University of Alberta. "But what our data indicates is that there was an upslope surge of trees in response to warmer temperatures. It's like it waited until conditions were right and then it decided to get up and run, not just walk."

    While in many places the idea of more trees is a good one, this Arctic takeover endangers species like caribou and sheep that thrive in the tundra, as well as the native people who depend on these species for their survival.

    The details of the study are published in the March issue of the Journal of Ecology. Danby plans to continue his research as a part of the United Nation's International Polar Year research effort.

    (Agencies)