Wildfires and warming could transform Alaska’s forests, making leafy trees dominant over evergreens, study says

Trees that burned in the McKinley wildfire are still standing next to the Parks Highway on Tuesday, Aug. 20, 2019. (Photo by Nat Herz/Alaska Public Media)

Wildfires have burned across Southcentral Alaska all summer, and they could be transforming Alaska’s forests for the long term, according to a new scientific paper published last week.

If wildfire frequency and temperature rise in Alaska like the paper’s authors expect, their climate model says broadleaf trees like birch and aspen will become dominant, taking over from evergreens like spruce, which are better adapted to cold weather and scarce nutrients.

“We will see a longer and larger phase of deciduous forest – they grow bigger, they last longer, and they actually can outcompete the spruce for longer,” said Jim Randerson, an earth science professor at University of California Irvine and one of the paper’s authors. “That’s what leads to this forest transition.”

Such a change could affect the pace of climate change itself, according to the authors, since leafy trees pump more water into the air, which has the potential to boost the greenhouse gas effect. It could also affect Alaskans’ experiences in the forest, like when they hunt or hike.

Ted Spraker, chair of the Alaska Board of Game and a longtime hunter, said thick spruce forests are not known as an easy place to find moose.

“I’ve hunted on the Kenai for a long time, and hunting in dense cover is really difficult,” he said. “Moose are really hard to see, they’re very cagey — they easily can get away from you.”

After a fire burns in the Alaska boreal forest, there’s usually a specific chain of events that follows, which ecologists refer to as “succession.” First, it’s the leafy trees that come back. But the same reason those trees can get into those areas first is the reason that they don’t always stick around.

“Deciduous trees sort of live fast and die young,” said Bill Riley, another of the study’s co-authors and a senior scientist at Lawrence Berkeley National Laboratory in California.

The deciduous trees — the leafy ones — are good at absorbing the large amounts of nutrients that are available after a fire. But eventually, once those nutrients start to become more scarce, the evergreens start to do better, because they’re more efficient — they don’t shed their leaves every year, and they’re more tolerant of shade.

That means the leafy trees that grow right after a fire eventually get replaced by evergreens, like spruce. But the new paper, published last week in a journal called Nature Plants, predicts that global warming and more frequent wildfires could change that, in part by warming up soils and making nutrients more available.

“In the future climate, with the warming, there will be more nitrogen,” Riley said. “That benefits the deciduous plants really even more, and they grow taller and outcompete for light in the long run.”

The paper’s results depend on an array of assumptions, like how much success humans have in reducing emissions, and how wildfire frequency increases in the future.

If the authors’ assumptions turn out to be true, the paper predicts that leafy species will actually become the dominant trees in Alaska boreal forests by about 2060, taking over from evergreens. That point will come sooner with more frequent or intense wildfires, the paper says.

The general trends predicted by the paper correspond with on-the-ground observations scientists have been making in Alaska’s boreal forests, said Glenn Juday, a forest ecologist at University of Alaska Fairbanks.

“Does it violate any observations of reality as it’s unfolding that we are seeing? The answer is no, not at all,” Juday said. “It’s very much in accord with the big theme: Fires burn and are carried on the landscape by the conifers. And if they are so severe and so large and so frequent, they interfere with the ability of the conifers to recruit, outgrow and dominate the succeeding stands. We’re definitely seeing that.”

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