ANCHORAGE - Recent studies show plant life in Alaska's northern forests is declining, while the tundra is seeing accelerated growth triggered by rising temperatures and concentrations of carbon dioxide.
The vast boreal forests that stretch from Alaska's Interior into northeastern Canada, appear to be drying out as the air warms, said Scott Goetz, a senior scientist at Woods Hole Research Center in Massachusetts. Insect outbreaks and lack of nutrients may also be speeding the decline.
But the tundra of Alaska and northern Canada had been "greening" dramatically as the Arctic warms, with more plant growth and longer growing seasons, according to Goetz's study that analyzed thousands of satellite images taken over two decades.
The results surprised scientists conducting the survey.
"Everyone was assuming that these forests were going to continue to green, and it turns out that there may be other factors that are causing unexpected results," Goetz said.
The report by Goetz and three other researchers was published online in the Proceedings of the National Academy of Sciences.
A separate study, based on tundra near Nome, found plant growth may be speeding the warming trend and a thickening of flora on the tundra.
Bushes peeking above the snow trap more of the late winter sun and accelerate heating by as much as 70 percent, said Alaska snow researcher Matthew Sturm.
Sturm's study of shrubs and snow on the Seward Peninsula was published in the Journal of Geophysical Research.
The two studies reached similar conclusions about tundra growth using different tools: One spied on a continent with weather satellites, while the other intercepted sunlight a few feet above the ground in the hills northeast of Nome.
Both techniques are used in a new research area that monitors vegetation for evidence of climate change in the Arctic, said Sturm, of the U.S. Army Cold Regions Research and Engineering Laboratory at Fort Wainwright.
Global air temperature has risen at least 1 degree Fahrenheit over the past century, and many scientists agree the rise of human-produced greenhouse gases such as carbon dioxide is a major cause.
Average temperatures in the Arctic have risen more than 3 degrees in a half century. The regions near the poles have revealed disintegrating permafrost, melting glaciers and a reduction in summer sea ice.
Rising temperatures cause problems for Alaska's Interior spruce forests as shown in studies by University of Alaska Fairbanks ecologist Glenn Juday. Changing climate would shift the zones where certain plants can grow, Juday and his co-authors found. Shrubs and trees might advance north into tundra, but existing forests would face stress from hot, insect-ridden summers.
Alaskans "are probably already aware that they're right at the heart of the changes that are occurring," Goetz said. "These satellite observations confirm what people are seeing with their own eyes."