Trees do more than fill the sky; dig deeper on their diversity

"Trees are very nice. They fill up the sky. Even if you have just one tree, it is nice too. A tree is nice because it has leaves. The leaves whisper in the breeze all summer long."

— From "A Tree Is Nice," by Janice May Udry, 1957 Caldecott Award winner

For the last year or so, we've all been out walking around more than usual. And we've started noticing trees. Of course they've always been there, and that's one reason we're noticing them — they endure. Pandemics, wars, storms, crises. They're a comforting symbol of permanence and beauty and stability, literally, rooted in our neighborhoods. Just seeing trees is calming.

Approaching on foot, instead of zipping by in the car, we can appreciate from afar the overall stature and shape, and drawing closer, the detail of leaves and bark, the scent, the sound of the wind through the leaves. And finally, toe to trunk, we realize the humbling scale of this oak. Or is it a cottonwood?

Like remarkable buildings, it's edifying to be able to identify the trees we meet and know a bit about them. To that end, the University of Minnesota Forestry Department and the Minnesota Department of Natural Resources have resources to help, whether you're in your backyard, on a neighborhood ramble, or taking one of the self-guided Tree Treks in the Twin Cities area. For example, there are opportunities in the Oakdale Nature Preserve and at Indian Mounds Regional Park in St. Paul.

The amount of Minnesota that's forested and the types of trees that make up those forests has changed over millennia due to climate and human intervention, at an ever-quickening pace since 1900. According to the DNR, before 1800, about 61% of Minnesota was forested, and 51% of those forests was old growth — ash, elm, maple, basswood, oak, red and white pines, white spruces, upland white cedar, and lowland conifers. Native Americans actively managed forests, setting small controlled fires to encourage new growth, enrich the soil, and make hunting easier. That explains why early European explorers described Minnesota's forests as "open" and "park-like," but those arriving in the late 1800s, when tribal populations and their management practices had decreased, described them as "dark" and "dense." Europeans saw those seemingly limitless stands of old growth trees as an open invitation, a resource for their use. Now, Minnesota is about one-third forested, with only small pockets of undisturbed old growth forest, protected by the DNR.

In addition to recognizing Native American forest management practices, the DNR and forest services have realized, somewhat belatedly, the importance of traditional ecological knowledge.

"Native tribes have used plants for medicine, food, housing, canoes, tools, spirituality. There's a concerted effort to tap into that knowledge," said Eric North,assistant professor of Urban and Community Forestry at the University of Minnesota.

The state lists 53 native tree species, though such a list is really only a snapshot of common, well-adapted trees during a particular time period. In the big picture of geologic time, the range of trees native to Minnesota has evolved, and continues to evolve.

"Trees that are considered native are those that were here when Europeans settled [mid-1800s], since they were the ones making these lists," North said. "Fossilized records show that at one point, gingkos had nearly worldwide range. That eventually shrunk to just China, but gingkos were reintroduced and grow very well here. So the idea of native versus nonnative is not static."

Climate change has affected the species of trees you are likely to encounter today as compared to those that proliferated here just 30 years ago, the blink of an eye in tree time.

"Even a temperature shift of a couple degrees means a tree survives or it doesn't," North said. "It's not just temperature alone — temperature affects rainfall, and what diseases or insects can survive. Or what competing plant species can grow here. Paper birches, for example, were native, but they are not doing well in the Twin Cities now. It's not cold enough. A hundred years ago, the bald cypress range didn't go much beyond Florida. Now there's a bald cypress on the St. Paul campus that's thriving. Even 20 years ago you wouldn't have seen many eastern redbuds here; now it's nearly native."

As Minnesota climate warms and the range of cold-loving trees shifts northward, arborists are encouraging newcomers to the Twin Cities via "assisted migration. "

"Climate is changing very quickly and obviously trees are stationary," North said. "We can plant them further north or south, at the edge of their current range. Of course, trying to figure out exactly where that is is hard — some seeds will have the trait that allows them to survive, and some won't — and planting a tree is a decades-long or even centuries-long investment."

Plant trials, experimenting with the limits of a species' range, are driven by the watch word in urban forestry today, North said — diversity. It was a lesson learned the hard way.

"In the 1960s and '70s, the Minneapolis urban forest was 90 percent elms," North said. "They grew well here, they were beautiful. When Dutch elm disease struck, all those trees went away. The city did a very thoughtful job of diversifying. Now there is no one species that accounts for more than 30 percent of the urban forest. There will always be the next pest or pathogen — emerald ash borer, for example — but ash trees are only 24 percent of the urban system. Diversity makes the system more stable."

North said green spaces — parks, forests, boulevards — are often seen in budgetary terms as niceties, not necessities. Of course, they are more than just window dressing. Trees clean the air and filter water, they keep flooding at bay, and are an essential part of the ecosystem. There is evidence that trees lower people's stress and improve outcomes of those recovering from illness.

Literally, pillars of the community, it might be worthwhile to know their names. So, take the University of Minnesota's Field Guide to Tree Identification with you on your walk. Start at the top with, "Is this tree coniferous or deciduous?" and go from there. Practice your tree identifying skills in a controlled environment by going on one of the Tree Treks. The trees on these self-guided tours are labeled.

Once you are pretty sure you know what a hackberry or a catalpa looks like, take your skills out into the wild wide world of Minnesota trees.

Sarah Barker is a freelance writer. She lives in St. Paul.