The Mystery of Mast Years

Last week I wrote about acorns clattering across my roof. As it turns out, nuts are raining down on many of your roofs, too! Commiserating over the loud, foot-rolling acorns makes me feel like part of an extended community. Are the oaks part of a similar community? And why are they suddenly attacking us with acorns!

Oaks are mast species, which means that all the trees in an area will produce a bumper crop of acorns at the same time, but only every two to five years. With hundreds of thousands of acorns available, the trees ensure that at least some of them will escape being eaten by chipmunks, squirrels, turkeys, blue jays, deer, and bears and survive until they can sprout and grow. This is known as predator satiation.

Red squirrels are seed predators on acorns.

In non-mast years, the acorn seed predators still survive, but at lower rates. When the oaks do mast, there aren’t enough critters to eat all of the acorns. The seed predators feed greedily and reproduce, but when there are few acorns the following years, their populations drop again. By being unpredictable with their mast years, oaks prevent seed predators from syncing up with the trees.

This same idea applies to parasites. Acorn weevils, knopper gall wasps, and acorn moths lay their eggs in developing acorns so that their larvae have an easy meal. A bacterial pathogen takes advantage of the holes they chew and causes “drippy acorn disease.” The result is the same as a chipmunk eating the seed, but the process takes longer. In addition, the parasites and pathogens are often closely tied to the acorn as a food source, and may not have other options. Chipmunks and other seed predators will eat from an extensive buffet of foods when acorns aren’t available.

While teaming up to satiate seed predators is clearly a good strategy for oaks, scientists are not so clear on how the trees coordinate, sometimes across hundreds of miles. It can’t be totally weather or resource driven, since variations in rainfall and temperature don’t fluctuate as much as the number of acorns produced. Certainly, an oak can’t produce tons of acorns if they are not healthy. But a year with plenty of rain doesn’t automatically result in acorns. During mast years a tree’s growth slows, so sometimes the trees need to put abundant resources toward making wood, not seeds.

One hypothesis about how oaks coordinate their mast years that seems to be gaining support in the scientific community is pollination efficiency. Oaks are wind pollinated. Their male flowers are dangly catkins that release pollen into the wind. The pollen needs to reach the pistils of the much smaller female flowers in order to fertilize the nascent seed. When oaks produce a ton of flowers at the same time and then have warm, dry weather, more female flowers will receive their dose of pollen. If the number of flowers oak trees produce fluctuates from year to year, this could translate into variable seed production, too. According to one study, this is true in “soft” climates, but not the Northwoods.

In harsh climates like ours, oak trees produce about the same number of flowers every spring. Having warm, dry weather that allows flowers to be pollinated AND to develop into acorns is essential, says Dr. Andrew Hacket-Pain who has used data from the Nature’s Calendar Phenology Project to study the correlation. A rainy spring, late freeze, or ice storm can easily ruin everything. Knowing how patchy storms can be in the Northwoods, I’m hesitant to believe that this could coordinate mast years across huge distances.

“In the old time, our elders say, the trees talked to each other,” wrote Robin Wall Kimmerer in her book Braiding Sweetgrass. Do they gossip about the weather like most Northwoods neighbors? Some mycologists theorize that the networks of mycorrhizal fungi who connect a forest by the roots may be the agent of coordination for mast years. “A kind of Robin Hood,” wrote Kimmerer, “they take from the rich and give to the poor, so that all the trees arrive at the same carbon surplus at the same time. Through unity, survival. All flourishing is mutual.”

Mice certainly flourish alongside acorns in mast years. The abundant food source means they have more babies. The same is true for the mice’s predators. Foxes, weasels, ticks, and even saw-whet owls may increase in number when mice are abundant. I’m excited for the potential uptick in owls, because the Museum has just started to recruit volunteers to help with a saw-whet owl study in Bayfield County. Check our calendar of events for details!

From mice to owls to chatting neighbors, oaks, and the mystery of their mast years, are at the center of our Northwoods community.

Author’s Note: Portions of this article are reprinted from 2015 – which was another mast year!

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too. Natural Connections 3 will be published in November 2025!

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Our Fall Calendar is open for registration! Visit our new exhibit, “Becoming the Northwoods: Akiing (A Special Place). Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.