Forest Lessons

Seven things you might not know about forest threats and tree survival

“Do you know what that is?” asked Lee Frelich, pointing down at a Frisbee-sized, disturbed swirl of leaves.

The retired University of Minnesota professor, and one of the world’s top forest ecologists, told the group of hikers that a European earthworm (the species we’re all familiar with) occupied that spot and was gradually eating and pulling leaves into holes it bores into the ground. 

He would go on to point out centuries-long skirmishes between rival tree species, subterranean battles over food and nutrients, and undergrowth that has vanished in plain sight along the tour hosted by the Door County Climate Change Coalition in late August.

What else surprised those along for the hike with Frelich on the Sentinel Trail on the plateau near Eagle Bluff in Peninsula State Park?

1. There’s a turf war on the forest floor. Less than a mile from the Eagle Bluff tower, Frelich stopped where several mature maples and beech were standing together near a group of hemlocks.

“In a lot of places on the Door Peninsula and Upper Michigan, there are patches of sugar maple versus hemlock. The patches actually go back three or four thousand years – we’re able to prove that by drilling into the soil and getting the pollen at different layers that are thousands of years old,” Frelich told hikers. “We can see where sometimes maples will make advances, and there will be more maples, and then hemlock will push back in kind of little skirmishes. My observation with beech is they’re usually mixed in with the maples. Neither one is a particularly good competitor with hemlock. So I expect hemlock to win unless the woolly adelgid comes here.”

He noted beech bark disease on some of the beeches, “so a lot of the beech are going to gradually die,” which would impact some species of plants that are dependent on beech: beech drops and longspur violet.

“I hope those other plants won’t go extinct as the beech trees are killed off and maybe there will be some resistant beech trees that will repopulate the area,” he said.

2. Trees can develop resistance to invasive species. Invasive species and foreign tree diseases have found their way across oceans even before transoceanic travel and shipping, Frelich said. Over time, trees learn to resist the invasions and repopulate. Hemlock, for example, disappeared across its range about 5,000 years ago, and it took it a thousand years for it to fight its way back.

“American elm is reclaiming its territory now because the resistant ones have survived Dutch elm disease and they’re now spreading and there are 30 varieties of resistant elms available,” Frelich said.

He said he also has heard of some white ash surviving emerald ash borer and recovering.

“I hope people don’t run out and cut down every ash tree,” he said. “You need to leave some – well you need to leave a few million – just to be sure you have some resistant ones that repopulate the landscape.”

3. Perspective on earthworms. Frelich said trees and plants have had 150 years or so to adapt to European earthworms. Still, he said the surface of the forest floor within sight of the trail had probably sunk 2 to 3 inches in the past 150 years as earthworms bore holes and through erosion and over time, those bore holes collapse and soil compacts.

Frelich said there’s no way to know exactly how trees will respond to the new invasive jumping worms, since they’ve just arrived. 

4. Northern Wisconsin may be spared the woolly adelgid. The East Asian wooly adelgid slightly resembles an aphid and sucks at the base of hemlock needles. Frelich said the invasive species is killing trees in Pennsylvania and Ohio, but not in cold places where temperatures get to 15-below zero.  

“By Wisconsin standards, that’s not all that cold; by Minnesota standards, 15-below zero is your high temperature on a January day,” Frelich said. “Maybe it won’t take root here, or maybe the warming climate might let it take root here.”

5. Not every tiny tree is a young tree. Along a portion of trail below towering maples and beech, Frelich stopped and gripped the middle of a tree that was about 12 feet tall and had a crooked but strong trunk that was thinner than a broom handle. While hikers might just see a beech sapling, Frelich saw a wonder of nature.

“The crown of the tree is flat,” he said. “They don’t overlap each other and that’s what it does in order to survive in the understory when there’s not much light. Sugar maples will do that too. They’ll develop a flat crown. They might form three layers but the leaves are arranged so they don’t overlap, so they can harvest the most possible light.”

He said the tiny tree might be 80, 90 or 100 years old and was simply suppressed. When larger trees die or fall, the tiny old tree can then grow quickly. Along the way, he pointed to several other tiny deciduous trees and hemlocks which, if cut, might have rings that reveal surprisingly old ages.

6. Deer are the forest’s native enemy. Frelich said he’s witnessed how deer have cleared out Door County’s thick understory, unlike in the early 1960s when his parents built a lakeside cabin in what’s now Newport State Park.

“If you look through the forest here, you can see that it’s pretty clear up to about eye height,” he said. “That’s because of the deer.”

He said deer don’t like the taste of beech or ironwood.

7. Where have all the white birch gone? Frelich paused at a location where mature paper birch were growing and then remembered that some central portions of Peninsula State Park burned in the 1940s. He said the white birch pop up where woodlands have burned, and much of the peninsula forest was logged or damaged by fires, naturally and unnaturally, 100 and 150 years ago and prior to that. He said Door County has about one-fifth of the white birches that it had in the 1960s.