In northeastern Minnesota, paper birch trees along south-facing slopes are dying. Yet, many of the same trees on north-facing slopes are doing just fine.
The reason?
It's not certain, but two University of Minnesota scientists paint a picture of a forest increasingly under attack from a host of threats that will accelerate global warming-related changes there over the next 50 to 100 years. The south-facing birch, they suggest, appear to be an early victim.
"We're into climate change now, and we're starting to see the results in Minnesota,'' said Lee Frelich, director of the university's Center of Hardwood Ecology. "We can expect to see this dieback in many species of trees.''
Frelich and Peter Reich, regents professor in the U's department of forest resources, recently published a paper in the journal Frontiers in Ecology and the Environment that pulls together their research, as well as those of colleagues studying the effects of human-induced climate change along central North America's 1,700-mile prairie-forest border.
Here's their portrait: As temperatures increase, droughts, violent storms and fires will become more frequent. Insects, some new to the region, will become more numerous and damaging. Invasive earthworms will continue their march north, consuming the forest floor's protective duff and warming and drying the soil. White-tailed deer will become more numerous, eating away at the forest's ability to regenerate as it otherwise would.
"It's not just that it will be a few degrees warmer, but all these other threats are going to be more prevalent than they were in the past,'' Reich said.
Helpful factors, such as increases in nitrogen deposition and carbon dioxide levels that promote plant growth, won't be enough to offset them.
"Therefore, the prairie-forest border will move north faster than if there were a temperature effect alone,'' said Frelich, whose paper noted the shift could be as much as 300 miles and is occurring faster than earlier landscape changes.
It won't be one steady change, he said. Wet, dry, cold and warm cycles will continue, but warmer, drier ones will become more severe, making it harder for trees acclimated to existing conditions to thrive or even to survive.
"We might have the same amount of rainfall in given years, but if it all comes in two or three thunderstorms and there's a long gap in between, (the trees) respond to the long gaps,'' Frelich said.
Without dramatic intervention, the International Panel on Climate Change projects that summer temperatures for central North America could increase between 5 and 16 degrees Fahrenheit by the end of this century.
Frelich and Reich outlined similar projections before, but Frelich said many changes are occurring faster than even he expected.
Red maple trees, for example, are becoming more abundant and actually sprouting in burned-over areas near the Minnesota-Ontario border.
"I never thought red maple would reproduce there after a fire, but it is,'' Frelich said. "Obviously, it didn't read the textbook. That tells me the climate is not what it used to be.''
Frelich and Reich said many birch trees of all ages clearly are dying, and no explanation other than drought-related climate change appears to make sense.
Frelich said drier soil and warmer soil temperatures are to blame. But Reich cautioned that more study is needed to tie it to global warming.
"We really don't have much evidence one way or the other,'' Reich said.
Regardless, he said, people should ready themselves for changes.
"I view it as more of a wakeup call than anything else,'' Reich said, referring to the response he hopes the paper generates from policymakers and the public.
Dennis Lien can be reached at 651-228-5588.
