DULUTH -- Substantial areas of what is now Northland forest may someday change to savanna or grassland, according to University of Minnesota scientists detailing the impacts of climate change in a newly published research paper.

The paper predicts Duluth-area forest types -- red and white pine, birch and aspen -- will recede northeast to about Thunder Bay, while prairie-like habitats now common in southern and central Minnesota will move into the North-land.

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The scientific paper, published this month by the Ecological Society of America, also says many of the effects of a warming climate already can be seen and that more dramatic change will occur over the next 90 years.

The paper was written by Peter Reich and Lee Frelich of the university's Depart-ment of Forest Resources.

"We could become a mostly prairie state by the end of this century,'' Frelich said. "It's not going to retreat in uniform front. Islands of forest will remain in cooler, wetter areas and on north slopes. ... But the general line is already moving northeast.''

Reich said the paper is among the first to tie together multiple climate-related forest issues and discuss how they may work together to compound the impact of higher temperatures. For example, warmer weather spurs more insects, which cause more tree mortality, which spurs more fire, which changes the forest.

Higher temperatures also allow nonnative earthworms to move farther north. The worms destroy the bed of leaves on the forest floor. Sunlight then warms the soil, which dries quicker. Native trees can't regenerate and the forest eventually dies back.

"We need to look at all these things in combination to see what's going to happen in the future,'' Reich said. "There are many possible futures ... but they all don't look very favorable for our current species along the western line'' of the prairie-forest border.

Reich is overseeing an experiment on increased air temperatures on northern tree species in the university's Cloquet Forestry Center, where researchers are using electric heaters to produce temperatures expected in years to come.

The paper concludes the beneficial effects of increased carbon, which some scientists have suggested could mitigate the effects of higher temperatures on northern forests, won't be enough to overcome the multiple ill-effects of climate change.

Frelich scoffs at climate change doubters who cite Minnesota having one of its coldest Julys on record as evidence the earth really isn't warming. He notes the earth's oceans were the warmest this year in recorded history.

"Eighty percent of the reporting stations across the globe had warmer than average temperatures this summer. Only about 20 percent were colder. It should be about half and half,'' Frelich noted. "There will always be pockets of colder than normal air that float around that may disguise the change for a year or so, but it (warming) is still happening.''

Climate scientists note that while much of Minnesota is expected to see about the same amount of precipitation each year, rain and snow will come in less frequent but more intense storms. That means periods of drought followed by downpours that can create flooding.

That's what happened this summer and in several recent summers. After very dry conditions in June and July, Duluth saw heavy rainfalls in August. Total annual precipitation is near normal, but stress from that intense but brief summer drought can now be seen in some trees changing color and dropping leaves weeks earlier than usual.

"We might be the same (annual precipitation) at the end of the year. But if you keep having seven- or eight-week droughts every summer, the forests we have now won't survive,'' Frelich said. "What defines a forest climate is having rainfall once a week. If you don't get that, you have prairie. We've had these mini-droughts nearly every year for the past six or seven years.''