Bim … bim … bam. This late-summer sound announces that our oak tree produced acorns this year and is dropping them onto the metal roof of our shed. Along our road in Thetford, Vermont, and elsewhere, I see big patches of acorns beneath canopies of mature oaks.
Bounty like this is by no means a yearly occurrence. Oaks are among several species of tree that undergo heavy seed production sporadically – only once every two to five years. Furthermore, oak trees in a particular area synchronize their production, fruiting together to give a superabundance of acorns, which often gives the impression that acorns are plentiful everywhere.
The years of heavy production in a particular area are known as mast years – the word ‘mast’ being derived from the old English word, ‘maest,’ the noun for the forest’s fallen nuts that were used to fatten pigs. There is a good reason for mast years: Because of consumption by wildlife, as many as 500 acorns are needed to produce a single seedling. Flooding the forest floor with acorns is the oak trees’ way of ensuring reproductive success.
It’s no wonder certain animals like acorns. They are relatively large, highly caloric, rather thin-shelled and easy to digest. The nut can last for months without spoiling, making it perfect for caching. Dozens of animal species in New Hampshire and Vermont depend on acorns, including black bear; deer; red, gray and flying squirrel; mice; wild turkey; blue jay; red-headed woodpecker and ruffed grouse. Acorns are important enough that a mast year can send ripples through an ecosystem.
For example, after a mast year the population of mice, chipmunks and squirrels can explode. This can be good news for the mice and squirrels, and certain raptors, such as hawks and owls, which prey on them. But it can be bad news for some ground-nesting songbirds, such as thrushes, ovenbirds and juncos. That’s because by late spring the booming population of rodents soon finishes their remaining acorns and turns to eating the ground nesters’ eggs and young.
Mast years also orchestrate shifts in forest ecology that for humans can be both good news and bad. An example is the impact of acorn-consuming, white-footed deer mice on gypsy moths. The mice are the main predators of gypsy moth pupae, so one can argue that high mouse populations help protect our favorite shade trees from moth damage. (In experimental areas where white-footed deer mice were removed, greater numbers of pupae survived, resulting in 35 times more gypsy moth egg clusters laid the following year.)
But here’s the downside: White-footed deer mice are also a significant carriers of the bacterium that causes Lyme disease – a virus spread by black-legged ticks. (Newly hatched tick larvae feed on deer mice and become infected and can later, as nymphs or adults, pass the disease onto humans.)
Acorns are important for the breeding success of some of our largest mammals, such as black bear. Bears fatten up on acorns and other nuts before hibernation, when females give birth and nurse their cubs. Without acorns, females don’t always fatten up well and cubs die or are not born at all. Without acorns, bears must travel father in search of food, increasing the likelihood of being hit by automobiles or becoming nuisances around houses.
Fortunately, oaks are now relatively abundant, but research suggests they may be in decline. The number of young oak trees in the forest understory declined significantly between 1989 and 2000, according to a nation-wide census of U.S. timber resources by the U.S. Department of Agriculture.
They are declining, in part, because some of our forests have grown denser. While acorns can germinate in shade, young oaks need 50 to 75 percent sunlight to flourish. They need forest openings caused by fire, wind or other disturbances, yet the chances of fire have been greatly reduced thanks to modern forest-management practices.
Another big problem for oak trees in this region is the large population of deer, which kill the majority of seedling oaks by browsing on them.
Acorns, of course, are not the only form of mast consumed by wildlife. Mast is categorized into ‘soft mast’—berries and other fruits—and ‘hard mast,’ which is chiefly nuts. Historically, the ‘big three’ hard mast trees have been chestnut, beech and oak. But of these, the chestnut tree has been lost to chestnut blight and the beeches are succumbing to beech blight; so acorns have assumed new importance.
If we want our forests to sustain the diversity of wildlife we presently enjoy, then putting more oaks into the landscape should be a priority.
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