Iowa may have fewer big trees because of climate change

The bigger they are, the harder they fall, right?

When we lose big trees — the majestic and memorable — it hits us hard because we know it takes decades or even centuries for trees to reach those heights and breadths.

Iowa lost some of its largest trees, considered state champions among their species, in the Aug. 10, 2020, derecho that tore a swath through the middle of the state.

An increasing number of storms, warm days and dramatic temperature swings caused by climate change have the potential to disproportionately harm big trees. The growing conditions today also could affect the number of big trees we have in the future, said Ines Ibáñez, a professor of ecosystem science and management at the University of Michigan.

“We are looking at that stage — that germination, establishment and early survival — to see how it’s being affected by environmental change,” Ibáñez said. “Trees establishing now, those are the ones that will be the forest in 100 years. Just because we have these big trees now doesn’t mean biodiversity will be doing that well in the future.”

Iowa’s big tree inventory

For more than 40 years, Mark Rouw, of Des Moines, has been finding and measuring Iowa’s largest trees of each species. As a volunteer, he maintains the Big Trees of Iowa official registry of state champions and their runners up for the Iowa Department of Natural Resources.

During a 2021 update, Rouw noted many trees on the list were no longer standing after the derecho.

“I had so many big trees I’ve been monitoring so many years it’s almost like losing a friend,” Rouw said. “Especially some of those that were so big and impressive and unique that after they came down, you’re looking at the contenders and there’s nothing else that comes close.”

There’s the nearly 70-foot tall butternut in Lisbon that produced 32 10-gallon garbage bags full of nuts one summer. Gone.

There’s the 92-foot tall ponderosa pine fell in Ellis Park in Cedar Rapids. The state’s runner-up is a full two stories shorter.

A swamp white oak in Cambridge, in Story County, came down in the storm. The new state champion is in Iowa City’s Oakland Cemetery.

Climate change

It’s not just windstorms that can harm big trees. Much of Iowa has experienced dry to drought conditions in recent years. Lack of rain hurts crops, browns lawns and puts stress on trees.

Extremely large trees are more sensitive to drought, possibly because their height makes it harder to deliver sparse moisture throughout the tree or they absorb more sunlight with a higher, exposed tree crown, according to a 2015 article in the journal “Nature Plants.” The researchers analyzed data from 40 drought events in forests worldwide and found drought-related mortality increased with tree size in 65 percent of the events.

Ibáñez said redwoods — some of which are more than 200 feet tall — are more likely to experience these hydraulic issues than trees in the Midwest. Old trees with deep roots may actually be able to tap into more groundwater supplies than younger trees.

Floods can be just as bad on trees as droughts, with rushing rivers pulling the roots out from under massive trees or standing water cutting off the oxygen supply to a tree’s roots.

“Pests are one of the major threats,” Ibáñez added.

Bark beetles, for example, infest and reproduce in live trees. They usually exist in manageable levels, but drought, windfalls or other stressors on trees can cause an irruption of beetles, the U.S. Forest Service reported.

“The strong role of temperature in population growth, and the role that reduced precipitation can play in host tree stress, suggest that climate change-associated shifts in temperature and precipitation will influence bark beetle populations in future forests,” the report states.

It’s not all doom and gloom, Ibáñez insisted.

“It’s not depressing. It’s absolutely fascinating,” she said. “What may not be doing well now, but that will be doing well in the future?”

New champions

Rouw last week visited the grounds of the Brucemore estate, in Cedar Rapids, to measure a cluster of Atlantic white cedars recently spotted by Andy Dahl, University of Iowa arborist. Dahl and Rouw frequently meet up to measure Eastern Iowa trees, including the state champion black walnut on the UI Pentacrest.

Trees are scored on a point system. Each inch of a trunk's circumference at 4.5 feet above ground gets a point. Every foot of height gets a point and every foot of canopy spread gets a quarter point.

Rouw and Dahl measured several cedars, in each case starting with the trunk, and then moving out about 100 feet to use a range finder to measure the tree’s height through triangulation from the base of the tree. They then stretched the tape measure out on the ground to track the longest branches in each direction.

They paused to watch a crow pulling bits of bark for a nest.

“Final measurements show several cedars larger than the previous state champions,” Rouw said in an email after the visit.

The largest Atlantic white cedar at Brucemore had a trunk circumference of 4 feet 11 inches, height of 65 feet 4 inches and an average crown spread of 27 feet 5 inches for a total 131.205 points.

Near the cedars, they also found an English yew that tops the largest of its type measured elsewhere in Iowa.

“This is the biggest one I have ever seen!” Rouw said.

What’s next?

To increase the odds that future generations will have big trees to get excited about, we need to plant more trees and diversify the species planted, tree experts said.

“We’re looking at species of trees that can handle extremes in weather,” Dahl said about the plan for the UI campus, which has about 8,000 trees. “A couple of years ago, we had record cold, heat, drought and flooding in one year.”

Climate change is a wild card, but may help some tree varieties, especially if there are longer growing seasons, Ibáñez said.

“There are so many uncertainties, it’s hard to come up with a strategy that will hit it all,” she said. “Aim for high diversity. Have a system as diverse as possible. Maybe one of the species goes down, but you have other species that take over.

“The more trees you have, the higher chance you’ll have to get one that survives. So if you want to ensure your region has these big trees, plant as many as possible.”

Help with Iowa’s tree inventory

The Iowa Department of Natural Resources is seeking more volunteers to help with the state’s big tree inventory. Volunteers might contact owners of state champion trees to see if the trees still are standing, update contact information or visit trees to get measurements. To volunteer, contact Emma Hanigan, the Iowa DNR’s urban forest coordinator at (515) 249-1732 or Emma.Hanigan@dnr.iowa.gov.

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