Minnesota's oak trees are in danger. How Minnesota foresters are working to save them

CATHY WURZER: Perhaps you have seen this as you walk through the forest or maybe even your neighborhood. It's an oak tree. Green, normal-looking on the bottom, dead on top. Or perhaps a grand old black ash tree with a yellow ribbon tied around it signaling that this tree has been selected for removal to prevent the spread of emerald ash borer. There's a lot that our trees are facing because of the insects and extreme weather. And those who work with trees are a little worried.

The Minnesota DNR's Forestry Health Program, working with the State Department of Ag and other partners, are kind of like the CDC of tree health in Minnesota. Together, they survey state forests and work to problem-solve issues and pests, addressing tree health. Brian Schwingle is the Forest Health Program Coordinator with the DNR. He's on the line right now. Hey, Brian.

BRIAN SCHWINGLE: Hey, Cathy. Glad to be here.

CATHY WURZER: Thank you for taking the time. Well, if you had to make a list here, Brian, what are the five top issues affecting trees in Minnesota?

BRIAN SCHWINGLE: Ooh. Five top issues. Well, I'd have to start in Northeastern Minnesota in the Arrowhead where a native insect called the spruce budworm is-- it's kind of in peak outbreak mode up there, and it is a-- like I said, it's a native insect. It really likes to feed on needles of balsam fir and spruce.

So that's the biggest obvious forest health problem in the state. Just ahead of eastern larch beetle, which is a native bark beetle, it's in its 23rd straight year of outbreak, which is very unusual for a native bark beetle, that's partially being driven by a changing climate where our growing seasons are getting longer.

And so that has impacted about 70% of Minnesota's tamarac forests. Of course, most of your listeners hopefully heard of emerald ash borer. It's a non-native cambium feeder of ash. You can-- most people can think of cambium like the inner bark of a tree. It feeds on that.

And it's doing a lot of damage now from the border with Iowa all the way up to about Forest Lake, I would say. Oak wilt is-- I always like to say oak wilt is the invasive that everyone has forgotten. It's not new and shiny. It's been in Minnesota since the '40s, but it's just slowly pushing its way north. And so it's made a major impact to oak forests from about Little Falls over to our St. Croix State Park and all the way down to Iowa.

And then the last thing, like you mentioned in your intro, is the last couple decades have seen a lot of extreme precipitation events during the growing season, and that is taking its toll, and it's quite noticeable on burr oaks. And then the recent droughts have made an obvious impact on all of our oak species. So those are the big five.

CATHY WURZER: It's a lot. What are you most worried about? Sounds like the oak-- I'm thinking the oak trees down in Southeastern Minnesota, the savannas, those areas, I mean, I think that-- and with-- given climate change, are they most in danger?

BRIAN SCHWINGLE: You know, I don't like to be a doomsdayer with our forests. We're going to have our forests. As long as we don't turn into like a desert, we're going to have trees growing. And whether you think a forest is healthy or not totally depends on who you are and what you expect out of the forest.

So I don't want to mislead people into thinking that all of our forests are dying. I mean, if you drive around, most of our forests look fine. But to your question about where maybe burr oaks or oaks in general are suffering the most, actually, up in Central Minnesota, in particular, like Mora, Onamia, there is a-- even down to like Bram if you know where that is in Northern--

CATHY WURZER: Yeah.

BRIAN SCHWINGLE: --Isanti County, there is particularly wet spots and forests or forests-- or like oaks that are growing near wetlands, a lot have died in recent years. Everyone-- these droughts have just taken up the news, and understandably so in recent years, the last three years in particular.

But before, that in 2014, 2016, 2019, those years, for the entire State of Minnesota, if we just take a look at the growing season, they are in the top 15 wettest growing seasons on record going back to 1895. And trees, they like water, they need a lot of water, but they also need oxygen in the soil.

And if the soil profile is completely saturated for several weeks during the growing season, that suffocates roots, which translates into dieback at the crown. And then, as probably most people know just looking out in their yards, we've experienced a lot of drought recently the last three, four years. And it's just kind of like this one-two-three punch that's bringing down a lot of our oaks.

Another area where there's a lot of fast oak death is up around like Brainerd country, West Otter Tail, down to Little Falls. Those are red oaks that are dying pretty quickly. And they're dying-- really, it's being driven by the recent extreme drought, particularly of 2021.

But what happens is when trees are suffering from their roots being saturated and their roots dying from flooding, basically, and then being parched, they can't muster up chemical defenses anymore to protect themselves against pests that are usually harmless. But those pests, they can find those suffering trees and they can attack them, and there's one pest in particular, it's the main insect pest of oaks in the state, it's called two-lined chestnut borer. And it's having noticeable outbreaks in several areas of the state right now.

And they really go-- like you said, it's actually South Central Minnesota, down towards Albert Lea, there's quite a bit of death-- oak death going on down there. Twin Cities has seen a lot, up toward [INAUDIBLE] and then all the way north-- northwest of Bemidji, we're seeing widely scattered oak death.

CATHY WURZER: Wow, that's a lot.

BRIAN SCHWINGLE: Yeah.

CATHY WURZER: I'm wondering-- again, given climate change and the wild swings in weather, are there certain species of trees that you advise planting or not planting?

BRIAN SCHWINGLE: Well, I'm a burr oak fan, and even though we're seeing a lot of burr oak death, I think burr oak is a fantastic species for the future. And actually, other oaks, too, like northern red oak, white oak. They're really great species to plant white pine.

And all of those trees I just listed, they're pretty hardy. They can tolerate a lot of wet soils, they can tolerate a lot of dry soils. And I think one reason why we're noticing these big old burr oaks dying is because they can grow on tougher sites than a lot of other trees. But I would recommend people not shy away from burr oak.

Another key word though, is diversification. If you own a forest, even-- I mean, if you own a yard and want to plant trees, i think it's pretty important not to plant-- not to put all your eggs in one basket. You want to diversify the trees that you have on your land. And that will go a long ways in making your land more resilient to a changing climate.

CATHY WURZER: OK. A lot of good advice here, Brian. And really, before you go, though, too, for folks who do have trees in the yard, I've heard from many experts to get a soaker hose. Does that work really well, especially when it's dry? OK. Explain that.

BRIAN SCHWINGLE: Yes. I actually laid a soaker hose around my two burr oaks and turned it on because one of them was suffering from the droughts from the last two years.

CATHY WURZER: All right.

BRIAN SCHWINGLE: Yep. Run it once every week for several hours.

CATHY WURZER: All right. You got the advice from the horse's mouth here. Brian, I appreciate your time. Thank you so much.

BRIAN SCHWINGLE: Thanks, Cathy.

CATHY WURZER: Brian Schwingle is the Forest Health Program Coordinator with the Minnesota DNR.