Mild winter foreshadows climate stress for forests

A snow-free landscape at Kinnickinnic State Park, near the Minnesota-Wisconsin border, on Dec. 18, 2011.

As I sit down to write this, the sun is shining, the sidewalks are sloppy, and it’s a balmy 46 degrees outside. It’s January in Minneapolis, but it feels like late March.

It feels fantastic, to take a deep breath outside without the burn of winter we expect here this time of year.

It’s also unsettling.

We’ve seen the news reports about how the winter-that-wasn’t has affected businesses and recreational activities. But as I walked the dogs Friday afternoon — gloves tucked in the pockets of my unzipped jacket — I wondered about the environmental impact of this weird and warm winter.

I called Lee Frelich, director of the University of Minnesota’s Center for Forest Ecology, to find out whether we’re likely to see consequences in our forests.

The short answer: it’s too soon to say. But a continued lack of cold or snow could cause stress for trees. And it foreshadows a climate shift that would bring huge changes to the region.

“One winter like this might not have a very big impact, but in the future, in a few decades, they mostly might be like this,” says Frelich.

Among the shorter-term risks: frozen roots, false starts, and a frenzy of pests.

Cold soil

A few winters ago, Frelich’s team buried temperature sensors near Seagull Lake, just south of the Canadian border in northern Minnesota. The devices recorded hourly temperatures four inches beneath the surface for two years. Not once did the temperature fall below 30 degrees. The reason? “Snow is a really good insulator,” says Frelich. Because there’s no snow on the ground this winter, the soil is actually colder than it was a year ago.

Ironically, Minnesota’s trees are less equipped to sustain cold ground temperatures than trees from Iowa or Missouri, where snow-less winter months are more common. A sharp cold snap without any snow to insulate the soil would mean widespread root damage, which would make trees more vulnerable to drought until their roots reestablished themselves. “If we ended up with an Arctic air outbreak without any snow on the ground, and the soil temperatures got down to, say, 15 to 20 degrees in the top couple of feet, we could have huge tree mortality.”

Leafing out

“Warm spells in the middle of winter can be quite stressful for trees because they could cause trees to come out of dormancy at the wrong time,” says Frelich. It would require several consecutive days in the 50s or 60s, like the record-setting warmth western Minnesota saw last week, in order for a tree to prematurely “leaf out.” If it turned cold again, Frelich says “the new buds that it had started to grow would be killed, and there would be major damage for the trees.”

Native Minnesota tree species aren’t used mid-winter warm-ups, but trees from further south aren’t fooled as easily. That’s why species like the catalpa tree, originally from the south, are the last to bloom in the spring. They sacrifice some of the growing season so they won’t be duped by a false spring in the middle of January. “They’re very conservative about leafing out,” says Frelich. “The problem is that when you move them north, they waste a significant part of the summer, so they’re not going to grow as well as trees here that take off right away.”

Bugs and pests

Trees have many tiny enemies that eat away at their leaves and bark and spread disease. Generally, prolonged deep freezes are good for Minnesota trees because they kill off many of the bugs and pests that torment them, says Frelich. Winters without extreme cold temperatures allow some of these pests to start the spring in greater numbers. Mild winters have allowed species like the mountain pine beetle to invade parts of the Rocky Mountains and Canada where it would have frozen to death in the past.

While this winter may seem like a gift to any creature trying to survive the elements, bugs that winter on the ground are likely having a tough time, says Frelich, because of the cold soil temperatures from the lack of snow cover. Bugs that spend the winter in crevices of the bark, meanwhile, are enjoying an easier winter than last year. “What’s good for one is bad for the other,” says Frelich.

So is Frelich enjoying the weather, or is it making him uneasy, too?

“Both,” he says. “It’s nice when I have to drive somewhere I haven’t had to worry about getting stuck in a blizzard, but on the other hand, it’s really scary to think that the climate could be changing this much, that winters like this might become common, if they did become common, what that would do to the natural resources of Minnesota would be huge. Even though one winter might not have a huge effect, if we had lots of winters in a row like this it would.”

Photo by Mike in Minnesota via Creative Commons

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