When Nature Goes Nuts
An astonishing array of animals is linked in some surprising ways to the mighty oak and its bounty
More than 200 years ago, seven-year-old Ephraim Farrar walked to the front of a schoolroom in Ipswich, New Hampshire, and recited a verse written for him by a townsman, David Everett. History doesn´t record whe-ther the lad earned an A for his elocution skills, but both he and Everett gained a measure of immortality because of two lines that can be found in any book of quotations: "Large streams from little fountains flow. Tall oaks from little acorns grow."
True on both counts! But if you spend an autumn afternoon sitting under an oak that sprouted around Ephraim´s time and watch a plump gray squirrel bury acorns at the rate of one a minute, while a tom turkey scratches away the coppery leaf litter to cram its crop with dozens of the nuts, a new rhyme might pop to mind: "From tall oaks billions of acorns fall. And creatures wild will eat them all."
Indeed, the acorn crop in an oak forest can reach 700 pounds per acre in a good mast year, when one ancient tree with an immense trunk and a spreading crown could yield 15,000 nuts. Yet by the end of November, most of them will be gone. Packets of energy that are easy to open and digest, acorns are a significant food item for some 150 species of birds and mammals and typically make up at least 25 percent of the diets of black bears, raccoons, gray and fox squirrels, wild turkeys and white-footed mice, to name a few. White-tailed and black-tailed deer, meanwhile, eat oak foliage along with bushels of acorns.
In some cases--notably for tree squirrels, woodland mice, blue jays and acorn woodpeckers in the West--having an ample store of the burnished brown nuts, which range in size from a half-inch to 2-1/2 inches in length, can be critical to overwinter survival. For example, the clown-faced acorn woodpeckers live in family groups as large as a dozen or more birds that stockpile acorns in communal granary trees--one nut to a hole--for use when in-
sects are not available. Each woodpecker generation drills a few hundred new holes in the bark, leaving the tree unharmed; one huge granary in California with 50,000 holes represented 100 years of excavation. But when such stores are depleted, acorn woodpeckers wander away from their territory in search of food. And if the mast shortage is widespread, most of the birds will starve.
In the Pacific Coast states, oaks are second in importance behind pines as a natural source of wildlife food. East of the Rockies, they rank number one. Biologists have linked acorn crop failures--caused by late frost that kills the buds or spring rains that interrupt flower pollination--to poor black bear reproduction and meager antler growth on whitetail bucks.
However, scientists have only begun to understand the complex relationships between acorns, animals and plants in the forest ecosystem. Recent research has shown, for example, that a bumper acorn supply leads to a high risk of getting Lyme disease for people in suburban and rural areas. (The worst areas for Lyme disease are the northeastern, north-central and far western states.) Yet at the same time, an abundance of acorns lessens the likelihood of severe gypsy moth defoliation, which can stress and kill oaks.
Researchers have also learned why tree squirrels discriminate between types of acorns--and how a decision to eat a nut on the spot rather than cache it influences the genetic makeup of an oak forest and the rate at which a particular oak species colonizes new areas. And foresters have made an ominous prediction: Trees of the oak genus have dominated much of the country´s deciduous forests for thousands of years and comprise nearly half of our eastern timberlands. But if present trends continue, within a century oaks will be only a minor component of the ecosystem.
The Latin name for the oak genus, Quercus, means "beautiful tree," and the United States is blessed with 58 native oaks that reach at least small-tree size, although in the arid West some species are more commonly found as shrubs. Botanists tell us that our oaks originated in Mexico, evolving from evergreen trees to deciduous trees as they migrated northward into colder climates after the last glaciation. Oak diversity also changes from south to north--from 28 species in Alabama to 8 in Minnesota, for instance. Only 9 kinds of oaks managed toeholds in southern Canada.
Taxonomists divide the North American oaks into two groups: white oaks, which have leaves with rounded lobes, and red oaks with pointed, spine-tipped lobes. But there´s a more significant difference: The acorns of white oaks mature in one growing season and are sweet to the taste. (Native Americans taught European settlers how to grind them into flour.) Red oak acorns take two years to mature and are bitter with tannin but contain three times more fat. White oak seeds sprout soon after they fall to the ground. Red oak acorns lie dormant during the winter months and sprout in the spring. All of these factors influence the behavior of squirrels and other small animals at harvesttime, which in turn has had a major impact on the disperal and regeneration of oaks.
Fox and gray squirrels neither hibernate nor store a lot of body fat; the extra weight would be a handicap when climbing trees. So they scatter-hoard thousands of acorns, burying them one at a time in a pattern that discourages pilferage. The big arboreal rodents rely on memory and the nuts´ odor to recover their bounty in time of need--even from under a foot of snow. And when the crop is modest, nearly all of the cached acorns will be eaten by spring. But after peak acorn yields, when as much as 75 percent of the reserves are left, oak seedlings will sprout up all over the place. More than a few will sprout from caches outside the forest edge. While squirrels generally hide acorns within 150 feet of the source, they also use adjacent prairies, pastures and fields for storage sites. Blue jays, meanwhile, often carry acorns a mile or two before hammering them into the ground.
Moreover, nearly all of those widely scattered seedlings will be red oaks, and biologists Michael Steele of Wilkes University in Pennsylvania and Peter Smallwood of the University of Richmond in Virginia looked for an explanation. An acorn, they report, has three parts: the cup, an outer shell and the kernel, which consists of two lipid-rich seed leaves (called cotyledons) enclosing a tiny embryo at the point of the nut. Those fatty layers enable oak seedlings to shoot up faster and higher than competitive species with much smaller seeds. The scientists knew that the higher energy content of red oak acorns would enhance their value to wildlife, but Steele says, "We suspected that high tannin levels would make them less palatable and digestible."
Early in the study, however, the researchers watched gray squirrels prying off the caps of red oak acorns, gnawing the top of the shell and discarding the rest. Chemical tests confirmed their theory that the highest levels of noxious tannins are located near the bottom of the red oak acorn. They noted that squirrels buried a lot of the half-eaten acorns, and because the embryos were undamaged, those seeds germinated almost as often as intact ones.
Other hoarding animals, including flying squirrels, white-footed mice and blue jays, share the tree squirrels´ preference for red oak acorns. On the other hand, the rodents and bird eat the quick-sprouting white oak acorns as soon as they hit the ground, before the taproot drains the stored energy.
In a nutshell, red oaks have evolved a successful strategy for broad dispersal of their seeds, which counts on the hunger of a few nonhibernating woodland rodents plus the blue jay and their mutual distaste for tannin. Meanwhile, the fast germination of white oak acorns ensures that some of a tree´s seeds will escape predation because they quickly become unpalatable. The flip side of that coin is that most of the white oak seedlings, which are shade tolerant, will be clustered around the parent trees, guarding the old homestead against competitive tree species instead of colonizing distant areas.
The spacing of bumper acorn crops about three or four years apart is no accident either. "The evolutionary response of oaks to seed predation is to produce more acorns than the sundry forest animals could possibly eat," says animal ecologist Rick Ostfeld at the Institute of Ecosystem Studies (IES) in New York´s Hudson Valley. But he notes that if there were large crops every fall, populations of squirrels, mice and deer would increase until no amount of acorns could satiate them. So the oaks intermingle good mast years with poor ones, during which many of the seed predators starve. "Trees aren´t as stupid as they look," he says.
For several years, Ostfeld and IES chemical ecologist Clive Jones have been documenting a two-act drama whose lead character is the white-footed mouse, a woodland creature with deer-colored fur, big eyes and ears, a long tail and a fondness for both acorns and gypsy moth pupae. The mice also carry in their bloodstream the bacteria, or spirochete, that causes tick-borne Lyme disease in humans. The main supporting player is the white-tailed deer, the favorite host animal for adult black-legged ticks.
Because well-fed white-footed mice can raise several litters of young a year, nesting even in midwinter, the population will peak the summer after a big acorn crop. Gypsy moth caterillars feed on oak leaves during the spring and early summer, then make cocoons and pupate for two weeks. The inch-long pupae are "tender, tasty morsels," Ostfeld says, and the mouse hordes will eat nearly all of them. But when mouse numbers crash, as is inevitable after a year or two of poor mast production, gypsy moth populations can climb to outbreak levels where the hairy caterpillars denude entire hillsides, damaging or killing thousands of trees.
Meanwhile, white-tailed deer eat little else but acorns when mast is abundant and many of them are infested with adult ticks that were picked up in brushy hangouts. The ticks mate, the females drop off to lay their eggs in the leaf litter and the pinpoint-size larvae hatch the next summer at a time when the oak woods are overrun with mice, which are the favorite host for young ticks. Many of the mice carry the Lyme disease bacteria, and tick larvae that take a meal of blood from one of them will molt the coming spring into infected nymphs that transmit the bacteria back to other mice--or to humans. The nymphs then molt into adult ticks, hitch a ride back to the woods and the cycle begins anew.
The risk of getting Lyme disease is highest the second year after a big acorn crop. Ostfeld also notes that white-footed mouse population explosions affect other species. "Mice also eat the eggs of ground-nesting birds like juncos and ovenbirds."
As to the uncertain future of eastern oak forests and their animal communities, William Healy of the Northeastern Forest Experiment Station in Massachusetts warns that "a decline in the abundance of oaks will alter the organization and function of wildlife communities. Seeds of species that are replacing oaks, primarily red maple, sugar maple, sweet birch and yellow poplar, are of considerably less value to wildlife than acorns."
Healy and his U.S. Forest Service colleagues point to logging, deer browsing and a century of wildfire suppression, gypsy moth defoliation and drought as "the agents responsible for the decline and mortality of mature oak, and for regeneration failures." Hardwood forests in some regions are being ground up by gigantic chip mills that supply wood chips to papermakers as far away as the Pacific Rim. Biologists worry that these timberlands will be clear-cut again before any replacement trees grow large enough to produce good acorn crops--40 to 45 years in the case of the northern red oak. Deer populations that sometimes exceed 30 animals per square mile eliminate oak seedlings. Stress from drought and defoliation makes oaks vulnerable to attacks by fungi and boring insects. And fires that would kill competing trees but not oaks, which have thick bark and deep roots, are quickly extinguished.
In particular, red maples--once considered mainly swampland trees--are spreading like aggressive weeds into uplands throughout the East where they were rare a century ago. Foresters call the tree a "super generalist" because it grows under almost any condition--wet or dry areas, rich or poor soil, open sunlight or deep shade. In fact, red maples accounted for 90 percent of the saplings after gypsy moths ravaged oak forests in Pennsylvania´s Allegheny Mountains in the 1980s. The maple´s whirlybird seeds fall in spring, when they´re not needed by wildlife, and are too small to offer much sustenance to squirrels and larger animals. Red maple foliage, moreover, is unappetizing to deer.
Healy says, "The prognosis for reversing the current trend is not good. Factors that cause mortality of mature oaks are unlikely to change, nor are the cultural practices that inhibit oak regeneration. Economic and social forces discourage management to sustain oaks." In a nutshell, a century from now the nation´s eastern forests will be very different from today´s oak-dominated woods, and for acorn lovers that is sobering news.
Field editor Les Line once watched a gray squirrel dig a rectangular hole about 3 inches wide, 10 inches long and 12 inches deep in the snow to recover a single acorn.