University of Minnesota
College of Food, Agricultural and Natural Resource Sciences
Go to the U of M home page


Home > Solutions > Spring 2012 > The State of Minnesota's Trees

The State of Minnesota's Trees

After a year of drought, insects, windstorms and fire, how healthy are Minnesota's urban and recreational forests?

By Becky Beyers


Editor's note: We sat down with four CFANS tree experts at the end of 2011 to talk about how Minnesota’s trees survived a year of drought, fire and invasive species. Department of Forest Resources professors Gary Johnson, Marvin Bauer and research associate Lee Frelich joined Department of Entomology assistant professor Brian Aukema for a roundtable discussion.

Trees along a street with Minneapolis skyline.
Trees line a city street in south Minneapolis
Gypsy moth
Close-up of adult female Asian gypsy moth
Gypsy moth caterpillar
Gypsy moth caterpillar feeding on white oak
boreal forest
Climate change is affecting the growth of Minnesota's boreal forests
Tree bark showing effects of Emerald Ash Borer
Serpentine galleries created under tree bark by Emerald Ash Borer
Forest show new growth after fire.
New plants grow after fires spread through the BWCA in 2007
Emerald Ash Borer
Emerald Ash Borer
New trees sprouting
New trees sprout from the ashes of a burned forest
green ash tree
Scientists are collecting seed samples from green ash before the Emerald Ash Borer spreads statewide
Paper birch trees dying
Paper birch trees near Lake Superior are dying in large numbers due to drought periods

Q. Let’s start with Emerald Ash Borer. What happened in 2011, and where are we headed?

Brian Aukema: We found a couple of new infestations over the summer in the metro area that had both been there since 2007 and 2008. (It takes a little while to detect that trees have been infected with Emerald Ash Borer). So we know it is spreading, and that’s not unexpected. The ash trees in Minnesota, like most in North America, have never seen this insect before and are not equipped to deal with it, so we’ll continue to lose ash in the metro area and many other parts of the state.

It’s a matter of slowing the spread, not stopping it. Right now we’re going through our toolkit of management practices, quarantines, education—trying to keep people from moving firewood, which can slow the rate of introduction in new areas of the state. We’re also trying some new tools, like biological control agents. Along with the Minnesota Department of Agriculture, we’re introducing stingless wasps, which may be one more tool. But it’s not going to stop the spread.

Lee Frelich: We have 900 million ash trees in Minnesota. A lot of those are black ash in northern Minnesota swamps, which aren’t much for timber but provide great wildlife habitat. It would be a great loss for biodiversity in Minnesota if all those trees go.

Q. Let’s talk about some of the other hardships for Minnesota’s trees in 2011.

Frelich: The Pagami Creek fire was the biggest in the Boundary Waters since 1875, but if you look back in history it’s really not that unusual. From 1910 to the 1990s there were hardly any fires. But when you consider the last 400 years, we are returning to normal. The BWCA is like a sleeping giant that’s woken up, and we can expect more of these frequent large fires. There is no control over the big fires—all we can do is get people out of the way. The intensity is just too high.

While it’s normal for that ecosystem to have these big fires, there are hints that we’re going to see even more of these big fires than occurred historically because of climate change. It’s too early to tell for sure, though.

We are also seeing some invasive species in the burned areas. Earthworms are spreading very rapidly through the Boundary Waters area, which will have a huge ecological impact in terms of drying out the soil and reinforcing the impact of droughts, allowing nutrients to leach out of the terrestrial system and into the aquatic system. It makes the landscape more nutrient-poor where trees are growing, and adds to drought stress. We see that in maple trees throughout the state already. All of our maples in the southern two-thirds of the state are in an earthworm-invaded system, and they’re showing the exaggerated effects of drought stress.

Gary Johnson: Similar things are happening in the urban forest, but it’s not new. All these stresses have been going on for awhile, but a couple things have exacerbated it. One is reliance on too few tree species: We have way too many maples. The urban environment is so hostile to trees that the lifespan of a tree is only about 30-40 years in an urban area. The natural environmental stresses are different in urban areas, because we don’t have old trees and the younger trees are prematurely stressed.

We always have drought situations—boulevards and parking lots, and the concrete parking lots and impermeable surfaces. But this year will be interesting—I have never ever seen the soil as dry as it was this autumn. Because it’s so dry, the soil is going to be a lot colder this winter and it’s going to be below freezing temperatures much deeper in the soil. All you need is 15 degrees Fahrenheit, and the roots start dying.

Frelich: Even though the air temperatures have been warmer, the soil is colder, because there’s no snow cover. And that’s a bad combination, drought and deeply frozen soil.

Johnson: Brian was talking about the impending loss of trees in the southeastern part of the state—it’s nothing compared with western Minnesota. We’ve done surveys of most communities around the state and in the metro area it’s 20 to 30 percent ash trees, but in many western Minnesota communities, it’s not unusual for ash to represent 40 to 65 percent of a community’s tree population.

It becomes a huge social issue, especially in greater Minnesota. Who’s going to pay for the removal of these trees? Who’s going to pay for the planting and reforesting? These are communities that the state has turned its back on in terms of local aid to government. The towns are broke and the median incomes are way below the metro area. How are people going to pay for this? Ash trees can’t be left dead and standing. Within two years they become highly hazardous. They’re very, very brittle—they’re going to damage property, they’re going to damage utility lines. So it’s not a situation where you can say “we’ll get to it when we can.”

In western Minnesota especially, the trees are the only protection from the wind—there’s no topography that slows down that wind in the winter. In a community like Morris where you have the potential to lose half of your trees, that’s pretty significant. People will pay more for fuel and it will be a decline in their quality of life. And it’s a long-term issue—it will be 30 to 40 years before they can re-establish their community forests. I just hope they don’t plant all maples (laughs).

Marvin Bauer: My work is a little different—Brian, Gary and Lee are studying the trees themselves, whereas I’m mapping the land cover for Minneapolis, St. Paul and Woodbury. Trees are a very important part of urban communities with aesthetic, environmental and economic benefits, including air quality, water quality, stormwater management, energy conservation and property values. There are a lot of good reasons for cities to want to have a healthy urban forest. If you want to preserve and manage the current forest as well as plan for the future, that starts with knowing what you already have—that’s why we set out to map the tree cover using remote sensing—satellite imagery and data.

With that combination of tools, we mapped the urban land cover—trees, water, streets, parking lots, grasses and shrubs—down to about every 2 feet. You can see individual trees and map them. The cities can combine that data with other maps and use it for analysis and planning.

Cities do this for some of the reasons Gary suggests. Trees are critically important to cities. If we lose them, that’s going to be problematic. There’s a lot of value there—the cities are interested in preserving the current tree cover and enhancing it. Knowing where you could plant additional trees is an important question to be answered.

Q: What's next? It all sounds pretty grim. Should we be worried about trees in our state? 

Johnson: Trees become more vulnerable as their health declines. They can’t tolerate normal stresses like a healthy tree can. The trees are so stressed already that they can’t tolerate normal events. Then every once in awhile you have these catastrophic insects or diseases come along like the Emerald Ash Borer, where it doesn’t matter if a tree is healthy or not—they’re just going to eat it. These pests will kill their hosts. But it’s the normal things that are a big problem: Most insects and diseases that bother trees tend to be opportunistic—they go for the weaker trees. It’s a combination of poor selection of trees and poor siting that’s left them vulnerable, and that’s exacerbated by dramatic changes.

Aukema: Insects are so good at finding stressed trees. They’re super-sleuths. If a tree is stressed, they’ll find it. They’ll show us where the boundaries are—it’s true in urban and other environments. I’ve been looking at eastern larch beetle since I got here a year ago. I was amazed when I went up to Cotton (Minnesota) and looked at tamarack bogs. Eastern larch beetles like stressed trees and they’ll rattle around in a stand for 2-3 years and take out a few trees, and then disappear. But these beetles have been active here for 10 years and they’re working their way north, all the way up to Lake of the Woods. You have the same or less precipitation over a longer growing season, plus some drought stress, and you could have two generations of the eastern larch beetle per year.

It’s reflective of the changing environment and maybe the landscape. We’re seeing an increasing rate of insect invasions—we’ve got gypsy moth coming in from the east and it’s moving fast. Minnesota’s not immune. Who’s to say that this summer we won’t find Asian longhorn beetle in maples in Minneapolis? That would be a matter of “Whoa! OK, now we have to take out 4,000 maples on top of the 10,000 ash that have been infested and killed,” and who knows what’s next after that?

We are getting better at detecting them, and that’s important. But the rate of insect introductions is increasing. That reflects greater urbanization, increasing global trade. And travel barriers have been reduced.

We’ve inventoried trees in communities across the state, and I can tell you that in most places the three most common trees are maple, ash and spruce. You ask a pathologist which trees are most at risk for disease and he’ll say ‘maple, ash and spruce.’ Ask an entomologist and he’ll say “maple, ash and spruce.” And then you go to a community and tell people to stop planting so many maple, ash and spruce, but people don’t want to change. It’s a social battle.

I think with the changing climate we can look forward to higher drought frequency and more evaporation so more drought stress even with the same amount of rainfall. More big derechos, like the windstorm we had on July 1. It leveled 120,000 acres of forest when it got into northwestern Wisconsin. We’ll see more of those in a warmer climate, more drought and more bug problems. There are trees that are wind and drought resistant—burr oak, hackberry, elm, Kentucky coffee tree, American basswood. Those species of trees all grow in Kansas and Minnesota; maybe we should be looking at some of these forests we can already find on the landscape as ecological blueprints for future forests.

Johnson: I do see communities are improving in engaging citizens. More communities have water plans—the city of St. Paul gives you a device called the “water gator” to water trees on the boulevard. They give them to you, so the homeowner can take responsibility. Spend a dollar a week on water, and the new trees will be much healthier. St. Paul is not unique—a lot of communities are doing similar things. There’s more information, more civic engagement despite the economic problems we’re dealing with. So we’ve got that going for us.

Lee Frelich Brian Aukema Marvin Bauer
Frelich Aukema Bauer

Gary Johnson’s recommended Minnesota trees

Kentucky Coffeetree 'Discovery' Elm
Common Hackberry Ironwood
Bur Oak 'Sugar Tyme' Crabapple
Copper (River) Birch Eastern Larch
Three-Flowered Maple Thornless Cockspur Hawthorn
Ponderosa Pine Bitternut Hickory
Black Hill's Spruce Big-toothed Aspen
Norway Spruce 'Prairie Gem' Ussurian Pear
Concolor Fir 'Autumn Gold' Ginkgo
Ohio Buckeye 'Merrill' Magnolia
Northern Catalpa Common Witchhazel
Northern Pin Oak Canadian Hemlock
American Basswood Northern White Cedar
'Northern Acclaim' Honeylocust Minnesota Strain Red Bud
'Accolade' Elm Serviceberry


Discover CFANS

Alumni Updates

Events & Archives