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Northland trees this summer produced more seeds, fewer leaves in response to 2021 drought

Foresters are also reporting few major outbreaks of tent caterpillars this season.

aspen trees missing leaves
Many aspen or poplar trees in the Northland, like these in Grand Rapids, sprouted few leaves this summer after producing a bountiful crop of fluffy white seed. The trees may be reacting to last summer's drought, DNR forest health specialists say.
Contributed / Eric Otto, Minnesota DNR
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DULUTH — At first it looked as if the invasion had begun: the dreaded onslaught of forest tent caterpillars eating their way across Northland forests, a plague that hasn’t hit hard for 20 years.

Up and down highways 2 and 53 this summer, from Duluth north, you could see the groups of bare trees, with just bits of green where a dense canopy of leaves should be.

Upon further review, however, it turns out that the problem causing many Northland trees to have small or missing leaves this summer was the 2021 intense drought, not insects.

The drought, in some parts of the region the worst since the Dust Bowl era of the 1930s, caused trees to recoil in stress. Eric Otto, forest health specialist for the Minnesota Department of Natural Resources based in Grand Rapids, said the response from many trees was to produce a massive crop of seeds — the blizzard of white fluff from aspen trees you saw in early June — to make sure the species survived.

But creating all of those seeds robbed some trees of the energy they needed to make more and bigger leaves, which left many aspen and maple trees looking half-dead, or half-eaten, with very thin canopies of leaves this summer.

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“We think it’s a survival mechanism that the trees develop to propagate the species. They shift the resources they have into survival mode in case the tree dies,’’ Otto said. “The response to the drought was to make lots of seeds this spring, especially aspen and red maple. They were just everywhere.”

Otto said some individual trees may see leaf growth yet this summer, but that many trees will see minimal crowns all season.

maple tree missing leaves
In addition to aspen trees, many maple trees across the Northland, like this one in Grand Rapids, appear to be responding to last year's severe drought by producing more seeds this spring and fewer leaves this summer. Forest health experts say most of the trees should survive even with their minimal leaf canopies.
Contributed / Eric Otto, Minnesota DNR

“I’ve seen some maples already losing their leaves,’’ Otto said, again, in response to last year’s drought and not anything happening this summer.

Otto said most of the stressed trees with minimal canopies will likely survive, providing another major stressor doesn’t come along, like disease or an insect invasion or another drought in 2023.

Otto said this year’s tree seed crop was the largest he had ever seen for aspen and maples, as did other forest veterans. (Otto noted that the cottony aspen seed is also quite flammable: Its abundance on a forest floor in Nisswa caused a wildfire to quickly spread.) All species in the populus genus produce the fluff-like seed; quaking aspen, big-toothed aspen, balsam poplar and cottonwood have all contributed to the noticeable seed crop. Quaking aspen is the most common tree in Northland forests.

Forest tent caterpillar update: Not yet

No news continues to be good news on the forest tent caterpillar front with foresters reporting few major outbreaks again this summer.

Caterpillars
A poplar leaf is slowly devoured by a troop of half-inch-long Eastern tent caterpillars on a tree along Jean Duluth Road in Lakewood Township.
File / Duluth News Tribune

The native, cyclical caterpillars defoliated only 9,120 acres in 2021, considered a tiny outbreak by historical standards. The last even moderate outbreak was in 2013, when about 1 million acres in Minnesota saw some defoliation, although most of that was very light, Otto said.

The caterpillars were thought to be on a roughly 10-year cycle that had been increasing markedly each decade in the late 20th century. They peaked in 2001, when billions of the creepy crawlers defoliated more than 7.7 million acres of northern Minnesota forest, the most in recorded history, and then another 7.3 million acres in 2002.

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The scourge that haunted outdoor summer activities two decades ago has been mostly unseen in recent years and no one is sure why.

In those years, the caterpillars laid vast swaths of forest bare, ruined orchards and wiggled up the sides of homes and across roads and driveways, eating and defecating — becoming so gross that many people canceled outdoor plans like graduation parties, weddings and camping trips.

As they eat, the caterpillars turn a pale blue-green shade and more than double in size. Eventually, they get full and go to sleep it off, wrapping themselves in cocoons to metamorphose into little buff-brown moths. The short-lived moths usually hatch in late June or early July, mate and lay eggs to start the cycle again.

By 2006, just four years after the largest peak, forest tent caterpillar numbers had crashed, as they always have after peak years, to just 1,900 acres of defoliation. They slowly increased until 1 million acres were munched in 2013, the lowest peak in recent decades, and then dropped quickly again.

Based on 2013 as the most recent peak, Otto said he expects another gradual increase to a peak sometime between 2023 and 2029. It’s not clear how big that next peak might be. So far, outbreaks have been small, mostly in Aitkin, southern St. Louis and Itasca counties near Grand Rapids, he noted.

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“We really don’t know why they haven’t come back more since those early 2000 outbreaks,’’ Otto said.

Val Cervenka, forest health resources consultant for the DNR in St. Paul, said forestry crews will begin flying aerial reconnaissance missions soon, as they do every summer, to see where in Minnesota trees are missing leaves and why, comparing acreage of defoliation from year to year. The survey could help identify other areas where drought stress occurred, she noted, as well as forest tent caterpillar defoliation.

“We haven’t been able to start our aerial survey, which will tell us more,’’ Cervenka said Thursday. “We hope to be able to start flying next week.”

Otto said they may have to “ground-proof’’ some areas because lack of leaves from drought stress may look like caterpillar defoliation from the air. Their report will be available later this summer.

John Myers reports on the outdoors, natural resources and the environment for the Duluth News Tribune. You can reach him at jmyers@duluthnews.com.
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