MANKATO — Trees are under assault from a growing number of pests and now many more hardy, majestic specimens are suffering from wet weather and unpredictable winters.
"Environmentally, there are going to be some major changes," said Randy Schindle, a Mankato-based forester with the Department of Natural Resources.
On river bottomland, such as along Seven Mile Creek, the bases of trees have been under water all spring and in some cases since last fall.
Schindle said bottomland species such as silver maple, cottonwood and ash are designed to take lots of water.
"But the hard maple, oaks, walnuts, any of those types can't go forever under water," Schindle said.
Black Hills spruce, pines and other conifers that are common in the area are also dying if they are in soil prone to holding moisture.
"If they get wet feet, they don't like it."
Schindle visited one woman's property where she'd lost many Black Hills spruce. "They die quickly. They can't take water well."
Jason Lobitz, park foreman in North Mankato, said they've been seeing the effects of over-saturated soils, particularly in Bluff Park.
"It's been an ongoing battle there with standing water, but it's worse this year. I've never seen it pond in some of those spots, and I've been here 23 years."
Several mature hardwoods died last year. "Trees can take water for a number of years, but if it's too much, they die. Those roots need air, too," Lobitz said.
Water-killed trees have been a growing problem across the state. Nerstrand Big Woods State Park, northeast of Faribault, has lost 12 percent of forest due to lack of oxygen around flooded tree roots.
Schindle said these are trying times for foresters as assaults seem to come from all sides.
"Ash trees are taking a beating all over, but the water shouldn't affect them. The last 10 years ash trees have just been stressed. Some winters warmed up and the sap flowed and then it froze hard. Now we had a super-cold winter."
He said some of the ash grown here were from southern stock that aren't cold hardy for Minnesota.
Schindle and Lobitz this week inspected some ash trees that have died from an as-yet-unknown cause.
"They put out buds last fall but this year there were no leaves. I know of four but I'm sure there are more," Lobitz said. There were no signs of disease or bugs.
Schindle suspects the trees succumbed to some winter-related stress and is searching for answers from others in the DNR.
Tom Schmitz, New Ulm park and recreation director, said he hasn't seen a lot of damage to trees from excess moisture, but the city is preparing for something else he knows will be coming.
"We fully anticipate emerald ash borers coming in and decimating our trees. We're not planting any more ash, and we're selectively removing ash trees that aren't healthy," Schmitz said.
The city has an active Tree Advisory Commission made up of volunteers who make recommendations to the city. And the city has a Big Tree Contest each year.
"It's catalpa this year. Whoever finds the biggest one of that species, they get $200 for planting a tree anywhere in the city."
The event was initiated by Dr. Ann Vogel. Since it started 10 years ago, residents have scouted for the largest oak, linden, cottonwood and more.
So far the emerald ash borer hasn't been detected in this region but have been discovered mostly along the southeastern border of the state.
Beyond tree diseases and stresses, foresters are dealing with a growing list of invasive and exotic species.
"Garlic mustard is in Mankato now," Schindle said. The mustard spreads into high-quality woodlands upland and floodplain forests and crowds out native plans, affecting insect and bird life.
Oriental bittersweet is also spreading. The aggressive vine covers large trees with a thick canopy, killing them and shading the land under the tree canopy. Heavy with fruit and leaves, the vines can topple trees.
Oriental bittersweet is listed as a noxious plant, meaning landowners are required to remove it from above and below ground, usually by using herbicide.