Canadian researchers predict that climate change will be good for trees, but bad for tundra. They've studied tree lines--the upper limit of arboreal growth on mountains--over the past 300 years in the Yukon, and the trees are getting the better end of the deal.
During the 20th century, the tree line moved upslope by as much as 250 feet in elevation. That bodes poorly for tundra plants and for animals like caribou and bighorn sheep that eat them.
The complexities of arctic forest and tundra ecology are being studied in greater depth during the current International Polar Year. Meanwhile, if you're wondering what you can do for the tundra, your personal IPY project could be to adopt a caribou.