Acorns galore thanks to ‘mast year’ for oaks prompting massive seed production around Chicago

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Last year’s mild winter has led to a bumper crop of acorns this fall.

The unusual abundance is the result of a “mast seeding event,” a phenomenon that happens once every few years, in which oak trees produce far more acorns than normal, experts say.

“The same species of tree coordinates and produces a ton of their seeds and drops them at once,” says Jessica Turner-Skoff, science communication leader at The Morton Arboretum. “It’s very location-specific, and it’s a really interesting phenomenon that has huge ecological effects.”

Jalene LaMontagne, population ecologist and quantitative biologist at DePaul University, says most years, wind-pollinated trees, like oaks, don’t produce many seeds. But if the weather is warm in the spring, the tree will grow more flowers than leaves, and each flower has more potential of being pollinated and eventually turning into an acorn. Jalene LaMontagne, population ecologist and quantitative biologist at DePaul University, says most years, wind-pollinated trees, like oaks, don’t produce many seeds. But if the weather is warm in the spring, the tree will grow more flowers than leaves, and each flower has more potential of being pollinated and eventually turning into an acorn. Anthony Vazquez / Chicago Sun-Times

The Chicago area is experiencing one of these mast years.

But why isn’t clear.

One possible explanation is “predator sedation,” Turner-Skoff says. This means oak trees might produce an overabundance of acorns so that animals won’t eat them all, allowing some of those seeds to grow into its own tree.

More acorns means more food for the animals that eat them — one reason you can expect to see more baby squirrels next spring.

Jalene LaMontagne, a DePaul University biological sciences professor, says the surplus in food can increase a species’ chance of survival, especially for animals, like squirrels, that stash food to eat over the winter, helping ensure their survival and future reproduction.

“Their population will increase over time, and then what eats them can also increase,” Turner-Skoff says. “It’s this really crazy cycle that, down the line, has an effect on the predator-prey cycle.”

Acorns from an oak tree near DePaul University. Acorns from an oak tree near DePaul University. Chicago Sun-Times

Another theory about what prompts a mast year could involve environmental clues scientists haven’t figured out. Producing acorns takes a lot of energy from trees. Pumping out mass quantities could result from environmental cues or phenology — the timing of life events — that prompt trees to boost up seeding.

“Those cues can signal to the tree that this will be a great year to reproduce because they have all the nutrients needed to grow them,” Turner-Skoff says.

Northern Illinois saw a relatively warm and less snowy winter than usual, according to Brett Borchardt, a National Weather Service senior meteorologist.

Most years, wind-pollinated trees such as oaks don’t produce many seeds. But, if the weather is warm in the spring, then those oaks will grow more flowers than leaves, and each flower has the potential of being pollinated and becoming an acorn, according to LaMontagne.

There are 55 species of oak trees native to the United States, 20 of them in Illinois. Other wind-pollinated species that also have mast years include walnut and hickory trees.

“Oaks can grow thousands and thousands of acorns, but next year there aren’t going to be very many,” LaMontagne says. “I encourage people when they’re out or in their neighborhoods to look up at the trees. We won’t see this again for a while.”

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