As fall turns to winter, and green leaves and insects have substantially disappeared until next spring, fruits and nuts become increasingly important in the diets of animals that remain active in the cold. Among those, the hard nuts produced by a variety of trees—walnuts, hickories, and oaks, for example—are known as “mast.” Acorns are on the small side of mast crops, but they can be extremely numerous, and they play key roles in northern deciduous ecosystems.
Acorns, of course, are the nuts produced by oak trees. Acorns are rich in fat, carbohydrates, and protein (about 8, 12, and 2 grams, respectively, per 28 gram (1 oz) serving), and so they provide a nourishing meal for both wildlife and people. (Acorn-based recipes for human consumption usually start by processing them to reduce bitterness.) However, not all acorns are alike. Just like the oak trees themselves, acorns come in two major varieties.
Oaks in the “white oak” group (trees with rounded leaf lobes, like white oak and bur oak) produce acorns that mature in a single year. They contain relatively low concentrations of tannins, plant chemicals that confer a bitter taste and inhibit digestion. As such, they are preferred by wildlife. However, “white” acorns also germinate relatively quickly, and so they are most available in the weeks soon after they fall in autumn.
In contrast, acorns from “red” oaks (species with pointed leaf lobes, like red oak) require two years to mature. They tend to be slightly richer in nutrients than acorns from white oaks, but their higher tannin concentrations make them more bitter and less digestible. Nevertheless, “red” acorns germinate less readily than “white,” and so they persist longer and become increasingly important to wildlife as winter progresses.
A single oak tree can produce ten thousand or more acorns. However, not every tree produces in every year. In Ohio, something like 30 – 50% of oaks, on average, produce acorns each year. For reasons that are not well understood, some years—so-called “mast years”– are highly prolific for oaks across a geographic region, while others are much less so. 2023 has been a good year for acorns in our region; a walk through an oaky section of woods can feel like crossing a room full of spilled marbles.
Given the huge number of acorns produced by a single oak, the vast majority of acorns clearly do not end up as oak trees. Instead, they fuel the ecosystem in a variety of other ways. As noted above, one of those important roles is to provide food for winter wildlife, especially the warm-blooded (endothermic) animals—mammals and birds—that remain active through winter. Locally, acorns are favorite foods for white-tailed deer, mice, chipmunks and squirrels, raccoons, turkeys, woodpeckers, blue jays, and others. Acorns are particularly favored by deer. In a strong acorn mast year, deer are well fed, they stay in the area of acorn production, and more of them bear twin fawns. In contrast, in a year with low acorn abundance, deer wander in search of other options. Populations of mice, chipmunks, and squirrels also fluctuate with acorn abundance.
A number of animals collect and store acorns for later consumption and are important agents for dispersing the nuts away from their parent tree. Woodpeckers that hide and later retrieve stored nuts, like red-bellied woodpeckers, actually have larger brain areas devoted to memory (the hippocampus) than species that don’t. Squirrels and blue jays bury their stored acorns, thereby hiding them from potential competitors. Buried acorns that survive the winter are effectively planted and ready to germinate.
At the same time that birds and mammals are harvesting fallen acorns, another critter is enjoying that meal from the inside. Earlier in the year, acorn weevils—a long-nosed little beetle—lay their eggs inside immature acorns. The larva (grub) begins its development while the acorn grows on the tree. Then, in late fall or winter, after the acorns have fallen, the grub chews its way out and completes development in the soil. Squirrels and blue jays can distinguish acorns that host weevil grubs from those that don’t and avoid harvesting the buggy ones, presumably because the growing grub would consume the acorn meat during the weeks of underground storage.
Fallen acorns are part of another important set of interactions, too. White-footed mice eat acorns. They also serve as hosts for both the bacterium, Borrelia burgdorferi, that causes Lyme disease and for the black-legged tick, Ixodes scapularis, that transmits the bacteria to humans. (For some reason, white-footed mice have weak immune responses to infection and are not very fastidious at grooming, so they provide a welcome home to both the bacteria and the ticks.) In a strong acorn mast year, mice are well fed in the autumn and they produce a lot of offspring the following year. Immature black-legged ticks, meanwhile, wander the forest floor looking for a mammal to feed from; those that bite a mouse, quite likely picking up Borrelia in the process, are more likely to grow and mature into adults. That maturation takes a while, though, and so adult ticks that can bite humans appear in the year after the juvenile ticks feed on the mice. The result of all this is that the risk of Lyme disease for humans, transmitted by black-legged ticks, is strongly related to the population of white-footed mice one year previously and to acorn abundance the year before that. Lyme disease remains uncommon in Greene and Montgomery Counties (just one or two cases per year), but it is increasing significantly in Ohio, up from 67 cases in 2012 to over 750 ten years later. 2023 is a strong acorn year; watch out for Lyme disease in 2025.
The old English proverb notes that “mighty oaks from tiny acorns grow.” But while acorns do grow into oak trees, they also feed myriad wildlife, influence behavior and reproductive output of animals ranging from mice to deer, and underlie the risk for a serious disease of humans. As such, acorns are central players in Ohio’s forest dynamics, with connections in many directions. “No acorn is an island,” one might say.
Article and photo contributed by Dr. David L. Goldstein, Emeritus Professor, Department of Biological Sciences, Wright State University.