The Nature of Oaks

October

When Our Property was no longer mowed for hay, the invasive plants from Asia—multiflora rose, autumn olive, Japanese honeysuckle—that had been kept at bay by the mowing exploded everywhere. These species are incapable of supporting the wildlife we hoped to attract to our new home, so a typical weekend at the Tallamys’ involved whacking out the root balls of huge multiflora rose bushes with my mattock. It was satisfying work, especially when we got to stack the whacked bodies up in giant piles, but it did leave open ground wherever we had removed a bush. The following spring we were thrilled but somewhat befuddled to find seedling white oaks and beeches popping up in many of these disturbed areas. Befuddled because I couldn’t for the life of me figure out how they had gotten there. We had no white oaks or beeches on our property and no mature trees nearby from which squirrels could have moved seeds even if they had carried them a good distance. I knew that acorns and beech nuts, unlike the seeds of many other plants, do not remain viable in the soil for more than a year, so there was no chance they had sprouted from a seed bank many years old. I was stumped!

An ancient mutualism

I love such ecological puzzles and blame them for all the times I walk down the hall but have no idea why I am walking down the hall. My mind is usually elsewhere. Fortunately, I hadn’t puzzled over the oak seedling conundrum too long before I saw a picture in a photography magazine of a blue jay flying with an acorn in its beak. A quick literature search revealed that I had indeed stumbled on the answer: it was blue jays that were bringing acorns and beech nuts to our property and planting them for us everywhere the soil was disturbed enough to make tapping the seeds below ground easy. Although blue jays are the only jays in much of the eastern United States, there are eight species of jays in North America and 40-some worldwide. They all share a common ancestor that evolved about 60 million years ago in what is now southeast Asia, the same time and place that oaks evolved. Oaks and jays are thought to have hit it off right from the start; oaks made large nutritious seeds that are the perfect size and shape for jays to eat, and, in their attempt to store these seeds for long periods, jays became the quintessential acorn dispersers. Over the eons, jays became so dependent on oak acorns that they adapted both physically and behaviorally to acorn traits. The small hook at the pointed end of a jay’s beak is designed to rip open an acorn’s husk, and a jay’s expanded esophagus (its gular pouch) enables it to carry up to five acorns at once while in flight.

Just because a jay can carry more than one acorn at a time does not mean it takes them all to the same place (Bossema 1979). Whereas many birds cache groups of seeds for use during periods of drought or cold, jays bury them as singletons, just beneath the surface of the ground, at sites dispersed throughout their winter territory. These can be over a mile from the acorn-bearing tree, which makes jays the undisputed champions among acorn dispersers and explains where blue jays had found acorns to bring to our property; there are a number of large white oaks and beeches within a mile of our house. The idea, of course, is that each jay will remember all the places it has buried an acorn and then know exactly where to go to retrieve the seed for food as needed during the winter. But apparently this is more of a mental challenge than most jays are up to. And who can blame them; a single jay can gather and bury up to 4,500 acorns each fall, but it typically remembers where only a quarter of them are buried before springtime. And if a Cooper’s hawk manages to eat a jay in December, that jay retrieves none of its acorns. The end result is that each jay plants somewhere in the neighborhood of 3,360 oak trees every year of its seven- to 17-year life span! It’s no wonder that jays have enabled oaks to move about the earth faster than any other tree species.

Blue jays routinely carry acorns for winter storage over a mile from the parent oak.

Moving acorns away from the parent tree—where they would surely lose in the competitive battle for light, nutrients, and water—provides oaks with an enormous ecological service, but oaks may derive another important benefit from their relationship with jays beyond the obvious advantages of seed dispersal. Like all organisms, oaks have always had to contend with diseases that attack them over their long life span. In recent decades, however, oaks have been besieged by new diseases introduced from other continents. Sudden oak death and oak wilt are two that have hit oaks hard in several areas of the country. In both cases, however, a small percentage of the oaks in natural and planned landscapes have shown some degree of resistance to these diseases. When disease hits an area, it is the resistant oaks that produce the most and best acorns, and it is jays that preferentially disperse those seeds bearing resistance genes. Although many oaks die quickly from introduced diseases, the seeds of those that remain are spread throughout the countryside by jays, assuring that future generations of oaks will be better able to survive infection. This is natural selection at its best but only works when the partnership between oaks and jays is thriving. The white oak I planted as an acorn in our front yard produced its first acorns last year, so now the ancient coevolved relationship between jays and oaks can play out right in our yard.

Jays, of course, are not the only birds that love acorns. Acorns are a critical component of winter diets for turkeys, Lewis’s woodpecker, and many ducks (especially the beautiful wood duck), and they are taken opportunistically by tufted titmice, bobwhite quails, red-bellied woodpeckers, yellow-shafted flickers, eastern towhees, American crows, and white-breasted nuthatches. For the most part, these birds eat acorns as they find them, but a notable exception is the acorn woodpecker of western oak woodlands. Acorn woodpeckers are specialists that store hundreds of acorns for winter use in individual holes they have drilled into snags. They are a colonial species that uses the same acorn storage tree year after year. A single acorn snag can accumulate 50,000 holes ready to receive acorns each fall. If you live in the West and are lucky enough to have an acorn snag within view of your house, you can cancel your subscription to Netflix; watching your acorn woodpeckers work your tree will be entertainment enough!

The list of mammals that rely on acorns for winter forage is also a long one. We all know how much gray squirrels love acorns, but so do red and flying squirrels, chipmunks, rabbits, black bears, white-tailed deer (acorns make up as much as 75% of a white-tailed deer’s diet in late fall), opossums, raccoons, white-footed mice, and voles. And no wonder! Acorns contain large amounts of protein, carbohydrates, and fats, as well as calcium, phosphorus, potassium, and niacin. If it hadn’t been for a healthy supply of acorns from various oak species, many of the animals just listed would have been devastated when American chestnuts disappeared from eastern forests after the introduction of chestnut blight from Asia.

Unlike blue jays in the East, the strikingly beautiful acorn woodpecker stores acorns in pockets it chips out of oak bark throughout its range in western states.

Masting

Speaking of acorns, have you ever noticed that every once in a while oaks produce an outsized crop of acorns? And it’s usually not just one oak here and there, but often nearly all the oaks in an entire region that produce an extraordinary number of acorns in the same year. An exceptional example was seen in the fall of 2019 when members of the red oak group from Georgia through Massachusetts synchronously produced vast numbers of acorns. This phenomenon is called masting, and it has begged explanation for centuries. When I was in graduate school I was taught that masting is an adaptation against acorn predation. Acorns are such a valuable source of food for so many types of animals that if oaks predictably produced a moderate number of acorns each year, the squirrels and the deer, the mice and the jays, the ducks and the towhees, and all the other creatures that rely on acorns to get them through the winter would increase their population sizes to meet the available food supply. This would not be good news for oaks, because every year, large populations of acorn-eaters would end up destroying nearly every available acorn, and oak reproductive success would plummet. But if oaks unpredictably and synchronously produced many more acorns than there were acorn predators to eat them—that is, if they produced a mast—some acorns would escape the predatory scramble for acorns and germinate.

There is a second advantage for oaks that mast, whether it is by adaptive design or simply a fortunate consequence of masting that evolved for other reasons (Koenig and Knops 2005). During mast years, there is unlimited food for acorn predators. This removes one of the biggest factors that limits population growth, and thus birds, squirrels, mice, deer, and so forth typically make more babies successfully during mast years than at other times. Unfortunately for all these hungry new mouths, the year following a mast year is typically (but not always) a bust year for acorn production; the number of acorns drops below that of most non-mast years, and many acorn predators perish. This boom-or-bust approach to acorn production helps keep acorn predator numbers well below what would make oak reproduction iffy during most years.

A second explanation for oak masting has nothing to do with outpacing acorn predators. There is evidence that masting evolved to improve pollination. Oaks within particular taxonomic lineages may synchronize their reproductive effort to maximize pollination efficiency (Pearse et al. 2016). For the most part, oaks are wind-pollinated, and wind-pollinated plants, not surprisingly, are at the mercy of the vagaries of the wind. Simple probability statistics tell us that pollination success will increase if there is more pollen blowing around when the female flowers on oaks are mature and open. Synchronizing the release of pollen in some years, therefore, results in lots of successful pollination and a mast crop of acorns. Why don’t oaks synchronize pollen release all the time? Well, don’t forget those predators, which would respond by increasing their reproductive rate if masting were predictable. But even if oaks “tried” to synchronize pollen release every year, the time window during which pollen production could be synchronized with other oaks and with the maturation of female flowers is very short. If, by chance, it rains or is unseasonably cold just when female flowers are ready to receive male pollen grains, many flowers would go unfertilized and acorn production would be low (Kelly and Sork 2002).

Whether to allocate limited resources, to outpace acorn predators, or to increase pollination efficiency, oaks like the white oak produce a bumper crop of acorns every so often in a process called masting.

There is yet a third hypothesis that offers an explanation for masting in oaks, and it has to do with energy allocation (Ostfeld et al. 1996). In most years there are not enough resources (water, nutrients, and sunlight) to produce the energy required for oaks to both grow and make a lot of acorns all at the same time. Making acorns requires a great deal of energy, so during mast years, oaks grow very little. This is evident if you closely examine oak growth rings after the fact. Accordingly, the energy allocation hypothesis suggests that oaks partition available energy; some years they allocate it to growth, other years they direct energy toward reproduction.

It may be obvious by now that, like so many ecological explanations, these three hypotheses are not mutually exclusive. Oaks could simultaneously enjoy selective advantages from masting behavior to allocate limited resources, to outpace acorn predators, and to increase pollination efficiency. You can decide for yourself which explanation makes the most sense as you watch all the wildlife activity around your oak the next time it masts.

November

Each Fall The Acorns dropping from your tree create a “get ’em while you can” feeding frenzy among your local wildlife. If for some reason you ever feel the urge to compete with the squirrels, jays, chipmunks, and other creatures for a few handfuls of acorns, put them in a plastic bag. After a day or two, you will likely notice small, cream-colored, legless insect larvae accumulating in the bottom of the bag. Closer inspection will reveal a small hole in a large percentage of your acorns. The Sherlock Holmes side of your personality will correctly deduce that the larvae have tunneled out of your acorns through those tiny holes. If you then place a larva on some loose dirt, you can watch it wiggle itself beneath the surface and out of sight in less than a minute.

Protein supplements

What you have witnessed is one of the most abundant creatures taking advantage of your oak tree yet one that is most easily missed: the acorn weevil, any one of some 22 species in the genus Curculio. Weevils are fascinating insects and belong to the largest family of animals, the Curculionidae, containing at least 83,000 species worldwide. With so many species, you would think we would be bumping into weevils every day. We don’t though because most adult weevils are nocturnal, and most weevil larvae are subsurface feeders: they spend their entire larval development inside a seed like an acorn, chestnut, or hickory nut, or tunneling into a root underground. If you do encounter an adult weevil, there is no mistaking it for something else: weevils appear to have huge noses! So huge, in fact, that it can be longer than the weevil’s entire body. In reality, weevils have no nose at all; what appears to be its nose is actually an elongated extension of the head capsule itself. The weevil’s mouth, bearing tiny mandibles, is at the very tip of that extension.

In some years, acorn weevils will lay eggs in 30% of the acorns produced on a tree.

And this may be the adaptation that has allowed weevils to be the most speciose family in the world. The evolutionary innovation of the oddly extended head capsule has provided weevils with a tool no other beetle possesses—a drill, in effect, that gives immature weevils access to food that is beyond reach of predators and parasitoids. Here’s how it works in acorn weevils. When a female is ready to lay eggs, she finds a developing acorn, typically in mid-July, and uses her “nose” to chew a tiny tunnel right into the heart of the seed. When the tunnel is complete, she turns around and deposits an egg at the surface of the hole and then plugs the hole with her feces. When the egg hatches, the larva wriggles down to the end of the tunnel, where it eats and grows on the acorn’s innards for the next two months. Once the acorn falls from the tree, it’s a race against time for the weevil larva; it must get out of the acorn and into the relative safety of an underground chamber before something eats the acorn and ends the weevil’s short life. If an acorn bearing a weevil larva is eaten by an acorn predator, it’s too bad for the weevil, but good news for the consumer, for acorn weevils boost the protein content of acorns considerably.

An acorn weevil larva chews a hole in the side of its acorn and then wriggles through it so it can pupate within the soil.

Even after it chews an exit hole out of the acorn and drops to the ground, the weevil is in danger from shrews, mice, and dozens of arthropods that would consider a chance encounter with this soft ball of protein and fat a lucky event indeed. If the larva succeeds in getting a few inches underground, it stretches up and down, twisting and turning in every direction until it has formed a comfy underground chamber where it will molt into a pupa and remain for the next two years. Somehow the weevil pupa knows when two years have passed; it then molts one final time into its adult form. If someone or something has not stepped on the soil above the chamber, compacting it so tightly that the adult weevil is trapped forever, the weevil will dig its way to the surface, find a mate and a developing acorn, and repeat the process once again.

Acorns, weevils, and ants

Nothing is wasted in nature, including acorns that have already been used by acorn weevils. Acorns are the perfect shape to provide housing for colonies of tiny Temnothorax ants and are large enough to hold 100 or so ants comfortably. Gaining access to the cavity within an acorn, however, is a formidable challenge for ants half the size of a grain of rice. Fortunately for these ants, when an acorn weevil exits its acorn, it leaves a hole just big enough for the ants to come and go but not so big that ant predators can easily get into the acorn as well.

It might seem as if Temnothorax colonies that have found a cozy acorn with a ready-made predator-proof front door have few ant worries, but not so, for several Temnothorax species are what is known as slave-making ants. That is, they raid and enslave other nearby Temnothorax colonies. Once through that convenient door too small for predators but not too small for themselves, the slave-makers throttle the queen, kill the workers, and then kidnap the pupae of the victim’s colony. When those pupae become adults, they serve at the pleasure of their masters, rearing brood and foraging for food the rest of their lives. If you have the patience and the eyesight, you can watch such trauma unfold beneath your oak tree from late fall through winter and on into spring.

When acorn weevil larvae tunnel out of their acorn, they leave a hole just the right size for Temnothorax ants to turn the acorn into a house.

Temnothorax longispinosus adults move their larvae into a newly abandoned acorn.

Blastobasis

Following a very similar sequence of events, the acorn moth (Blastobasis glanulella) is another insect that develops within acorns, although it is not nearly as numerous as acorn weevils. I speak of the