USFS researcher: Warming may shift climate areas north

Warming in the climate of coastal Alaska over the next century could shift the growing range of many common tree species northward, a U.S. Forest Service researcher said in a lecture at University of Alaska Southeast Tuesday night.


Research Forest Pathologist Paul Hennon said growing conditions may become more favorable north and westward of the current distribution for trees like Western hemlock and Sitka spruce as temperatures rise.

Hennon was also speaking to viewers in Anchorage and other parts of the state, and one person in Arizona, via videoconference.

“When planet scientists and ecologists look at this … the place they would look for a problem in the range of Sitka spruce would be at the south end, because it would be from warming, drought issues, things like that,” said Hennon, indicating a map of the Sitka spruce’s range, which runs from northern California to southern Alaska. “There might be part of the range … where even with the climate shift, based on where we are, the climate’s still suitable for the tree, so the tree’s not really in trouble. And then there might be yet another part of the range that offers actually new habitat, new possibilities and opportunities for the tree, because the climate limitations in that area got lifted.”

Hennon referred to data from the Scenario Network for Alaska and Arctic Planning at University of Alaska Fairbanks showing an expected rise in average monthly temperatures in Juneau, Sitka and Ketchikan, along with a very modest expected increase in monthly precipitation.

“One way to think about a future climate is to look a little bit farther south and see what kind of climate they have now,” Hennon said. “So if you’re thinking about a future Juneau climate, you might think about the current Ketchikan climate.”

Even as the “climate envelope” in which a species could grow shifts, Hennon said, trees are not likely to expand rapidly to any area where growing conditions are favorable, at least not without human assistance. He said a particularly fast migration for a tree species might be 30 miles in a century.

“These climate envelopes are probably shifting much faster than the trees can respond,” said Hennon.

That is because the life cycle of most trees is quite long, with many reproducing very slowly.

For example, Hennon said, the yellow-cedar has yet to grow into the Juneau area in much force despite making steady progress spreading northeast from the Alaska Panhandle’s southern coast since the end of the last Ice Age, about 12,000 years ago.

The yellow-cedar is actually an example of a tree whose expected future range is more complicated to predict, according to Hennon. He said some 500,000 acres of land in Southeast Alaska have exhibited significant die-offs of yellow-cedars, with research suggesting the trees thrive in snowier conditions.

Because much of Southeast Alaska’s winter temperatures hover around the “threshold” of freezing, Hennon said, climate warming is projected to affect the region dramatically in terms of how many days it will see on which temperatures drop below 32 degrees Fahrenheit. That means more rain and less snow during the winter — conditions that may favor trees like the Sitka spruce, but which yellow-cedars abhor.

Hennon also noted the limitations of climate envelope modeling. The modeling does not take into account “biotic factors” like diseases, which he studies at the Forest Service’s Pacific Northwest Research Station in Juneau, or soil conditions, he said.

“It’s not all about climate,” Hennon said. “The soils will always play a role in where the trees go.”

Even as trees like the Sitka spruce and Western hemlock thrive with climate change, Hennon said, some of their common insects and parasites may end up “winners” as well, with their distribution range expanding together with their host plants. He named the spruce tip weevil, which lives in British Columbia south of the international boundary with Alaska, as an example.

“We would expect to see that insect coming up into Alaska, because it’s right on our doorstep,” said Hennon.

Hennon’s lecture Tuesday was part of the Anchorage Master Gardeners Program, which is offered through the UAF Cooperative Extension Service.

“Because we’re having Paul talk, we opened it up to the community,” explained Darren Snyder, Juneau Master Gardeners instructor, who sat in on the lecture.

Snyder said most of the half-dozen or so attendees in Juneau were graduates of his Master Gardeners program.

• Contact reporter Mark D. Miller at 523-2279 or at


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