Glenn Juday stands next to a white spruce that sprouted from seed two years after Britain ceased hostilities against the colonies in the Revolutionary War, the last time fire swept through the Bonanza Creek Experimental Forest.
The 221-year-old trees are the monarchs of the northern boreal forest, rising 110 feet, and they have no business surviving on 11 inches of annual rainfall.
"That's just ridiculously low," said Juday, a forest ecologist.
What makes the forest possible are cool temperatures. At least they used to be.
A favorable combination of snow melt and rainfall that gives trees moisture just when they need it most has been disrupted by warming in recent decades -- more frost-free days, more 70-degree days, less heat loss at night, a potentially lethal combination to trees of the northern boreal forest. While climate warming has been most obvious in Alaska's glaciers and pack ice, it's also threatening to reshape the ecosystem that covers most of America's largest state.
Warming may be behind a proliferation of insects that have attacked trees in unprecedented numbers. It's a suspect in forest fires that burned a record 6.6 million acres in 2004. And drought brought on by warming threatens the hardwoods that stand next to the dominant species, the white and black spruce.
Juday said that if warming continues to accelerate, insects, fire and drought will change Alaska's forest within decades.
"It's not wild talk to claim that, well, maybe it will get too dry for the trees to grow here," Juday said. "It can and does happen."
James Kruse, a U.S. Forest Service entymologist in the Alaska Regional Forest Health Program in Fairbanks, said Alaska has been an environment of extremes, going back to the Ice Age.
"When things happen up here, it tends to happen big," he said.
He said he may be "a little more optimistic" than Juday and that change is part of the natural process.
He acknowledges that tree species may disappear or even be replaced by grasses, if models follow most dire trends. But he does not pretend to guess how bad it's going to get, he said.
"There's a lot of ifs in there," he said.
Juday, a professor of forestry science at the University of Alaska Fairbanks, grew up in Indiana but has studied Alaska's forests for 29 years, much of the time at Bonanza Creek, one of 26 sites established by the National Science Foundation for long-term ecological research.
"It's much less of an intact forest than it used to be and it's got the decline processes under way that are going to do it in," Juday said. "A few of the trees will survive, and they'll become really old trees, but it's been hit pretty hard."
He was skeptical of claims of global warming but observations in the forest proved difficult to attribute to other causes. He was swayed in part after disproving what every northern gardener knows to be true: That plants grow better if it's warm. For white spruce and some other trees in Interior Alaska, it's just the opposite.
Tree ring studies indicated the most growth when temperatures are cool. Warm years stunt growth.
"It was such a surprising result, we tested it in a lot of ways," Juday said.
The reason is physical. Plants need carbon dioxide to grow. Tree open up stomata, the microscopic openings in a plant's epidermis, to absorb carbon dioxide. However, stomata also release moisture. In warm years, trees limit opening stomata to avoid losing moisture, sacrificing growth.
"It's more important for the plant to not die of drought than it is to get a little more carbon dioxide," Juday said.
The relationship between tree growth and temperature, recorded for more than a century at nearby Fairbanks, did not vary with annual rainfall or other factors.
In May through August, daily high temperatures have not changed much. However, there's been what Juday calls a "huge upswing" in daily lows -- about 5.5 degrees.
"That's entirely consistent with the mechanism that a greenhouse gas warming would operate," he said. "It operates to dampen heat loss. It doesn't add more heat so you get higher highs, but it dampens heat loss so you get higher lows."
Greenhouse gases such as carbon dioxide occur naturally and retain heat that otherwise would be reflected out of the Earth's atmosphere. Conservation groups say that burning carbon-based fuels in cars and factories accelerates the effect by putting tons of greenhouse gases into the atmosphere, leading to dangerous warming that shows up most obviously near the poles.
Global warming, Juday said, also is the most consistent explanation for a longer frost-free season.
"On average, early in the 20th century, at the Fairbanks station, we'd get 80 days frost free," Juday said. "On average, early in the 21st century, we're getting 120 days frost free."
The subtraction of moisture from higher lows and a longer growing season is like a vise squeezing the boreal forest, and all signs point to global warming as the hand turning the lever, Juday said.
"It's a very consistent picture," Juday said. "We just keep getting more and more details like that that give us confidence that, yes, that's what we're seeing."
Warm temperatures also are the chief suspect in landscape insect outbreaks. Larch sawflies have decimated 80 percent of Alaska's mature larch. Spruce bark beetles have killed white spruce on millions of acres and spruce budworms are moving farther north than before. Amber-marked birch leaf miner and aspen leaf miner have attacked hardwoods.
Drought may be a bigger danger to hardwoods. Juday studied six sites with high prime commercial logging potential within 100 miles of Fairbanks. At five, warm temperatures had retarded tree growth. One site was particularly sensitive. He estimates another 1.8 degrees of warming in mean monthly temperature during June and July would have started die-back in trees.
"So the short answer is, we're getting very close to what should kill the trees outright," he said.
Warm, drying weather in spring has extended recent fire seasons, as did drought in late summer.
The changing conditions make it difficult to answer simple but fundamentally important questions, such as how a forest will regenerate after a fire, Juday said.
One hint may be found in Alaska's neighbor to the east, Canada's Yukon Territory.
Ted Hogg, a Canadian Forest Service research scientist, and Ross Wein of the University of Alberta, studied about 30 miles of forest valley that in 1958 burned along the Alaska Highway west of Whitehorse. The forest had mixed trees dominated by white spruce and receives just under 12 inches of rain annually.
In 48 years, the burn area has become grassy parkland with a sparse cover of aspen -- the kind of landscape likely to be found in the warm, dry foothills of Alberta, Hogg said.
The researchers concluded that drought stress has contributed to the slow growth, and that forests will be vulnerable if the climate becomes drier.Associated Press via Casper Star Tribune
