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Ah, fall in Minnesota — when leaves flutter gently down and mark the coming change of seasons. 

Or, as is the case in parts of the state this year, when oak leaves and branches tumble to the ground in a frenzy of destruction. 

This year is shaping up to be what’s known as a “mast year,” when oak trees, en masse, make an orchestrated bid for survival and grow oodles of acorns. It’s apparently an effort, at least in part, to simply out-reproduce their natural consumers, like squirrels, bears, weevils and other nut-eaters. 

 is a professor at the University of Delaware and author of “The Nature of Oaks.” He said the huge acorn crops come every two to five years, and may also be designed to thwart the long-term population growth of squirrels and other animals that feast on acorns.

“So, there’s going to be a lot of squirrels this year” amid the abundance of acorns, he said, “and next year there will be a lot of squirrels because they’ll have all this energy for the winter time, and they’ll have big broods. But then, next fall, there won’t be very many acorns, and the year after that, there will be even fewer — and those huge squirrel populations will crash. So it’s temporarily good for the squirrels, but in the long run, it crashes their population.”

Scientists aren’t exactly sure what triggers the mast years. They don’t seem to correspond to severe winters.

Experts don’t know, either, how all the oaks coordinate the effort — since an abundance of acorns on only a few trees wouldn’t outstrip the appetites of squirrels. It’s like a planned flood of acorns, intended to simply overwhelm consumers of the trees’ fruit. 

But it turns out squirrels can counterattack. 

“It’s weird, and people notice it,” said 

, a University of Minnesota extension service forestry expert in Cloquet. 

In some places, in some years, with some squirrels — the animals go on offense and climb into oak trees, spot clusters of huge tasty acorns at the tips of branches too small to bear even a squirrel’s weight, and just chew off twigs to get to the nuts. In some places, the otherwise green, healthy-looking leaves rain down in veritable carpets. 

“Once they’re down on the ground, they’re easier to collect,” Sagor said of the acorn crop.  

But it doesn’t happen everywhere. And it may be that some squirrels are just a little sharper than others. 

“It’s mostly gray squirrels,” said 

, a professor of wildlife ecology in Crookston, Minn., and a wildlife specialist for the U of M’s extension service. “I’ve heard of squirrels doing this in Pennsylvania and Ohio. I wouldn’t be surprised if it was just one of those things, that there’s a certain fraction of squirrels that figured out this is the best way to get the acorns down early.” 

And if they teach their offspring to do it, that may be a survival advantage as well. Although the mast year bounty happens outside of typical squirrel breeding season, it may have a significant impact on winter survival. 

Loegering says there’s a similar phenomenon in deer: landowners sometimes put paper caps on tree seedling buds to keep deer from browsing them and stunting the trees. For some deer herds, it works. Others seem to have learned that they just have to unwrap their snack first. 

“Some deer figure it out. Some don’t,” Loegering said. 

Sagor, the forest expert, said that the excessive litter of green oak leaves and twigs on the ground in parts of the state may appear alarming. But he says trees and squirrels have evolved together for a long time, through mast years and otherwise. 

“It’s just nature happening,” Sagor said. “Nature is crazy.”  

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Tim Nelson

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You may have noticed some leaves have already been falling around Minnesota — in some cases weeks before the annual autumn color change. That includes oak leaves, and even branches, that are raining down — and wouldn't you know it, squirrels are to blame.

Oaks vs. squirrels: A battle is underway in some Minnesota trees, and it’s nuts

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