As Alaska warms, white spruce trees are withering and dying throughout the state's Interior region, scientists have found from years of field studies. But that species of tree, for decades prized as the most commercially valuable in the Interior forest, is thriving on the edge of the tundra in Western Alaska.
The findings, from field work spread over a decade along the Yukon, Kuskokwim and Tanana rivers, are detailed in a study published online in the journal Forest Ecology and Management.
In Interior Alaska, where white spruce has long been one of the dominant forest species, the trees are struggling, with "markedly lower growth" than at any time since the 19th century, the study found. If the trend to warmer and drier summers continues, "widespread tree death will be unavoidable on warmer lowland Interior sites, where persistence of white spruce is unlikely," the study said.
The picture is the opposite in the area far to the west, the flat terrain near the mouths of the Yukon and Kuskokwim.
"What we can say for sure right now is the boreal forest species -- the trees -- are growing best, growing fastest and healthiest way out west," said study leader Glenn Juday, professor of forest ecology at UAF. "That's the new heart, the climatically optimal zone of the boreal forest," he said.
The study provides confirmation that a large "biome shift" is underway in Alaska, he said.
Field work was conducted over several years between 2002 and 2012. Juday and his fellow researchers examined and took core samples from 540 trees from the Bethel region to an area upstream from Fort Yukon.
The trees in the study are growing in flood plains, away from areas used for commercial harvest and naturally protected from wildfire. In optimum conditions, they can thrive for hundreds of years; several in the study were over 200 years old.
The researchers found several signs of grim conditions for white spruce in Interior Alaska.
In some places, they found stands of several dead white spruce trees, Juday said. Beetle infestations are so fierce that the researchers, when coring trees, often encountered "spongy" centers, the result of insect-caused rotting, he said. Poor condition of Interior white spruce was also evident from the sparse needles -- "a healthy tree will retain its needles" -- and, especially, the height of the mature trees, he said.
Warmer and drier conditions are behind the trees' stress and making them more vulnerable to the insects. Juday said. But even without the insects or wildfire, conditions are "right at the threshold" for white spruce survival, he said. "It looks like it's just too darn hot and dry for the trees," he said.
If the boreal forest disappears in the Interior, what replaces it? "The best answer is, we don't know," Juday said. One possible outcome would be that the area converts to aspen parkland like that found in the Canadian province of Alberta, he said.