Scientists say Tongass trees could bounce back from budworm infestation

infested trees
Trees can survive blackheaded budworm infestations, but the thinning of the canopy affects the ecosystem in many ways that are not fully understood. (Courtesy Gordy Williams)

An insect infestation responsible for defoliating thousands of acres of the Tongass National Forest is abating.

Scientists with the U.S. Forest Service believe that the blackheaded budworm, whose numbers surged over the past three years, is now in decline.

And while it’s not clear how much lasting damage was done by the insect, there’s a good chance that some parts of the forest may emerge from the infestation better off.

Gordy Williams, who worked for years in the Alaska Department of Fish and Game, said he could see widespread budworm damage while he was riding the state ferry LeConte from his home on Killisnoo Island in Angoon to Juneau in mid-July.

“You know, I’m looking at Chichagof and Baranof (islands),” he said by phone during the trip. “There are some pretty big impacts on the east side of these islands.”

Those impacts are acre upon acre of defoliated hemlock trees, wide swaths of brown striping the otherwise endless green of Southeast Alaska. The trees’ needles have been consumed by tiny, voracious caterpillars who are fueling their eventual transformation into the budworm moth.

Williams understands that budworm and its partner-in-crime, the hemlock sawfly, have a role in the forest. But this latest event he considers extreme. The Forest Service estimates 685,000 acres were defoliated by insects in the last three years.

“It’s a natural cycle, but when it does get ramped up like this, it does have a pretty, pretty significant effect on the ecosystem,” Williams said. “So what our curiosity is at this point, and our concern is: What are the impacts of this radically thinned-out forest canopy in so many areas? You know, that’s what provides winter cover for deer and other animals — and is it going to impact stream temperatures, and that kind of thing?”

This part of Chatham Strait is notorious for winter storms – huge sou’easters that blow right up the channel between Admiralty and Baranof islands, and can make this ordinarily pleasant ferry ride a bit of a stomach-churner. Hemlock sawfly stressed these trees in 2018 and 2019. The blackheaded budworm infestation followed in 2020.

Forest Service entomologist Liz Graham described it as a one-two punch to the forest, putting it on the ropes. The weather may have finished the job.

“It definitely seems to be on some more extreme sites, too,” Graham said, “the ones that are really heavily exposed. And so I do think that it’s a little bit more like a compounding impact where there was heavy defoliation, and then maybe on top of that a big windstorm or ice storm, and that really kind of stripped the last of it. And so I do think that that’s why we have seen some of those areas with really more dense mortality – that there’s been more than one event there.”

Graham said that, depending on the area, up to half of the hemlock trees may have died. Although this sounds like a high toll, Graham’s  colleague, silviculturist Molly Simonson, says on a forest-wide scale the damage is limited. Most areas are unaffected. And some forest die-off is not necessarily a bad thing.

“Trees do die,” Simonson said, “whether it’s clusters of them during a particular event, or whether it’s just individually over the course of that forest’s development. But you know, it contributes to nutrient cycling within the ecosystem. And there’s always going to be other trees in the understory waiting to take over that space. There’s regeneration underneath those dominant trees that are just waiting to take over. And will capitalize on that.”

The last major blackheaded budworm infestation in the Tongass was in the 1950s, and good data are hard to come by. Liz Graham says tree ring studies could help her identify the timing of the budworm cycle, but humans are throwing new variables in the mix. Climate change – or specifically, the number of frost-free days – could play a role in outbreaks. But warmer weather can also disadvantage budworms. 

“The budworm population actually extends all throughout the Pacific Northwest,” Graham said. “And so the outbreaks that we’ve been experiencing here have really just been happening in Southeast Alaska and haven’t extended to British Columbia. And so, based on some of the research we’ve been looking at, it might be actually too warm down there. So it could be that we’re in this perfect little climate window right now for budworm outbreaks.”

Although the outbreak in Southeast Alaska is subsiding, there are some areas where budworms are peaking – notably Juneau and Haines. Picture a slow-moving budworm tsunami that began on Prince of Wales Island, and traveled north. Defoliation is not certain death, however; trees that were stripped near Gordy Williams’ home on Killisnoo Island are sending out new buds this year, as are many along the route of the LeConte as it steams up Chatham Strait. 

“And we’ll just have to see how many of those trees can come back and how long it takes,” said Williams.

Robert Woolsey is the news director at KCAW in Sitka.

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