The acorn is an icon of autumn in the Northern Hemisphere. Each fall, the little capped nuts shower down by the thousands from the majestic branches of oaks, making them so commonplace underfoot that we rarely give much thought to them. Their tumbling to the ground, however, signifies more than seasons passing. As key components of northern forest ecosystems, the annual settling of acorns on leaf litter in cool woodlands promises new life, even as the senescence of fall blankets the earth.
Acorns, or oak nuts, are fruits, easily recognized by their scaly caps and by their hard, rounded outer shell, which protects the seedling leaves (or cotyledons) that develop from within the nut. Acorns start life as green, relatively soft fruits, later turning brown and hard when mature. The process of maturation can last anywhere from one to two growing seasons.
The scaly caps and smooth outer shells of acorns. (Photo by Jeremy D. Rogers)
Each of the some 450 known species of oaks (genus Quercus) produces acorns, giving rise to a great diversity of acorn traits. The shell of the nut may be egg-shaped, as seen in acorns of the chapman and scarlet oaks, or elliptical, such as in the mohr oak. Some species, including myrtle and pin oaks, have acorns that are nearly round. And acorn caps themselves range from shallow with hairy scales, as in the laurel oak, to deep and thick, such as in the swamp chestnut oak, to large and round, nearly covering the entire nut, as seen in the overcup oak.
Because acorns are rich in fat, they are a favorite food source of birds like blue jays and of mammals large and small, including deer, bears, squirrels, and mice. The survival of these animals is sometimes very intimately tied to the life cycles of oaks. In North America, for example, population sizes of gray squirrels and blue jays fluctuate annually in large part because of changes in the abundance of acorns produced by white oaks. Every 5 to 10 years, a stand of white oaks experiences an acorn boom, with each tree in the stand producing upwards of 7,000 acorns. In those years, populations of blue jays and gray squirrels grow rapidly, only to fall off again in years when acorns are less abundant.
A gray squirrel foraging for acorns. (Photo by Jeremy D. Rogers)
For squirrels in particular, however, not every acorn is created equally. Squirrels collect and sort acorns based on tannin content. The higher the tannins, the more bitter the nuts but the better their long-term storage underground. So, for example, acorns of red oaks have high tannin levels, making the nuts bitter and not easily digested. But these acorns keep well in underground stores, and hence squirrels bury the nuts in caches, eating them in the winter, usually only after stores of tastier nuts have been depleted.
Acorns of white oaks, in contrast, have relatively low levels of tannins, rendering them tasty and easily digested. Squirrels tend to immediately eat or store these acorns. For storage to be effective, the squirrels must first remove the nut embryo, otherwise the acorn will germinate before the onset of winter, using up many of the nutrients that make it a valuable food source. White oak acorns attempt to bury themselves for germination by sending out a thick taproot soon after they fall to the ground. This allows the seedling to take root before the ground freezes. The acorns of red oaks, on the other hand, have adapted to predation by generating large amounts of tannins, slightly reducing their culinary appeal to animals but taking advantage of animal caching behavior.
The propagation of oak seedlings and the survival of acorn predators is a delicate balance. Oaks are a climax species, they appear in the later stages of succession of forest growth. But they also expend large amounts of energy to produce few seeds in hard fruits capable of surviving animal predation and cold winters. And at the center of it all is the humble acorn, an unsuspecting nut whose autumn glory is made all the more amazing by the fundamental place it fills in the lives of oaks and forest animals.