Have you ever made a pact with another person to look after and protect that person as long as you are alive and able? If you are married you certainly have. How about with a pet? Have you ever vowed, silently or not, that as long as you are alive and able you will make sure that animal is treated properly? Well, I have made such a pact with a plant—a tree. An oak tree, to be exact, that lives in my backyard. Perhaps eliciting human affection is a survival mechanism for oaks. The oak must be a tree not uncommonly spared the ax, considering that in America there are more oaks than any other tree species considered “famous and historic.”
The Druids also protected oak trees. They believed that the trees held spirits that could whisper prophecies to them. Druid sanctuaries were connected to the very oldest oak in the forest. These “oracle oaks” were considered so sacred that anyone who cut one of them was executed. The Druids would likely see a connection between E. E. Cummings cutting a venerable old oak and his death from a stroke hours later.
Your last summer at your farm
like a young man again you cut down
an aging, great New England oak.
Oh you are big and you would not start to stoop
even on that absolute day.1
The most famous tree in Maryland, the state tree, in fact, was the Wye Oak. That white oak (Quercus alba) was the first tree in the history of the United States to receive formal government protection. Imagine! A tree that elicited affection from a legislative body! At 460 years of age it was the largest and oldest specimen of its kind in the nation. I passed within a few miles of the Wye Oak many times, but never made the detour to see it, reassuring myself that I would do it someday. It had been there since 1543; what was the hurry?
It is a lesson difficult for us to learn, but where there is life there is death. And two years ago the mighty oak blew over in a storm. I never got to see it, but a botanist friend pressed a branch for me that now hangs on my office wall. Its presence there is a lesson to me.
THIS MORNING, early, I saw a gray squirrel (Sciurus carolinensis) crawling through the branches of the oak in my backyard. When the squirrel got as far out as it could go on the smaller branches, it made a big leap to the adjacent magnolia tree (Magnolia soulangiana). I have witnessed that squirrel take this route many mornings. After crawling through the magnolia’s outer branches the squirrel climbed down the trunk to the ground. Both the squirrel and I were looking to see where the cats were at that point, but the only animals that gave the squirrel any trouble were the birds who swooped in screeching and scolding. Cats have been the death of many young squirrels, I thought, but squirrels have been the death of many young birds, too.
The squirrels were not always in my yard. They are there now because of the oak tree. It is a cherrybark oak (Quercus pagoda); at least I think it is. Oaks are difficult to tell apart. There are forty-three kinds of oak in the eastern United States alone,2 and they often hybridize, which makes identification even more complicated. I have frequently seen botanists, too time limited for a thorough identification, just give up and put a tree into one of the big categories of “white oaks” or “red oaks” without determining the species. If you want to sound like one of these “experts,” all you need to know is that white oaks have rounded leaf lobes and red oaks have pointed leaf lobes.
One of the problems with identifying oaks to species is that their leaves are highly variable. If you pick twenty leaves from an oak tree, chances are that each one will look slightly different from the others. And the leaves from one tree may look very much like the leaves from another tree of a different species. Besides looking at the leaves it is necessary to look at the buds, the bark, and especially the acorns. The problem with examining acorns is that they are usually out of reach on the tree, and once they fall to the ground they are rapidly eaten— or hidden—by the many species of wildlife that depend on these nutritious morsels, like the gray squirrels in my yard.
For eleven years I watched my oak tree produce acorns and wondered why there were no squirrels to eat them. What had happened here before I came that caused the squirrels to be absent? The only thing I could imagine was a combination of young men with shotguns and territorial dogs; both had lived here before me. Finally, in year twelve, the squirrels returned to the tree, and there they have been living and breeding ever since. I am happy to see them— the landscape is much richer with them in it—and I will happily sacrifice a few flower bulbs to keep them here.
ALTHOUGH IT MAY BE DIFFICULT for humans to distinguish oak species, the squirrels are able to tell. Acorns from different species vary in the amount of nutritious fat and bitter tannins they contain, and also in the time of year they germinate. These factors determine whether a squirrel will eat an acorn immediately or bury it to eat later in the winter or the spring.3 If an acorn is of the type that sprouts in the fall, the squirrel will eat it right away— a germinated acorn is not as tasty as an ungerminated one. If they cannot eat the acorn right away they nip out the tiny embryo from the nut, thus killing the seed and preventing germination. Acorns high in tannins store best through the winter. The squirrels know that and hide those acorns. Some are forgotten, and they may eventually sprout. So, even though the squirrels are responsible for killing many of the seeds, they are also important seed dispersers. From the tree’s point of view, it is OK to have ninety-nine of every hundred seeds eaten if the remaining one is taken to an area where it will germinate and grow.
Insect infestation is another factor that determines whether a squirrel will eat or store an acorn. A small beetle (Curculio) that looks like a miniature anteater feeds on acorns. The weevils crawl out of the ground during the summer months and begin climbing the nearest oak tree. They climb high into the branches hoping to find another weevil of the opposite sex. After mating, the female makes her way to a maturing acorn. At the end of her long, curved snout are jaws that can gnaw a circular tunnel in the acorn. The material she chews from the tunnel is digested as food, but that is not the reason she makes the tunnel. When the tunnel is completed the female turns around and extends her ovipositor—a telescoping egg-laying appendage—into the hole, lays an egg, and seals the hole with a bit of her poop. A small white dot, from this fecal pellet, is the only way to tell from the outside that the acorn has a weevil egg in it. In a week or two the egg hatches into a small, wormlike larva that feeds on the flesh of the acorn. Eventually the mature acorn falls to the ground. Rather than avoiding these acorns, the squirrels relish the protein-rich condiment. Some of the larva-containing acorns escape discovery and produce the next generation of weevils. The full-grown larva chews an exit hole— the original hole made for the egg being much too small to allow passage of the fat larva—and crawls out of the acorn and down into the ground. There it writhes around in the soil creating a small space where it will spend the next year. At the end of the year the larva pupates and emerges as an adult weevil, which crawls up the nearest oak tree, and the cycle begins again.
Almost as wonderful as this life cycle is the fact that someone managed to discover it. Who could possibly have had the patience to follow and record the behavior of these insects? How long did he or she wait at the base of an oak tree for the adult weevils to emerge? Describing such life cycles and relationships is more natural history than what we today consider science. How many other ecological relationships are still undiscovered because natural history is no longer given the respect it once had and our biology students are now studying genomes instead?
THE OAK TREE and the weevils also provide a food source for another organism—the acorn moth larva (Cydia splendana). The fertile adult female, a tiny black-and-white moth, lays her eggs on oak leaves. When the larvae hatch they crawl toward the nearest acorns, their preferred food. But neither the adult moths nor the little larvae have a way to get through the acorn’s shell. They depend on the holes made by the oak weevils and larvae. No oak tree: no squirrels, no weevils, no acorn moths. And how many other organisms would we lose as well? Without this oak my yard would be a less ecologically rich and interesting place.
IN SOME YEARS oak trees produce a bumper crop of acorns— far more than in other years. Studies have shown that in these “masting” years the mice that feed on acorns are also more abundant— and since mice are a preferred food of owls and other raptors, masting oak trees mean more owls.4 Want owls? Plant oaks. The mice that aren’t eaten by hawks and owls can repay the oak for providing the food that kept them alive over the winter by feeding on the pupae of gypsy moth caterpillars during the summer. The gypsy moth caterpillars (Lymantria dispar) relish oak leaves and can defoliate whole forests, weakening the trees as a result. But eventually a caterpillar will stop feeding and form a pupal case in which it will metamorphose into a moth. The trees in a gypsy moth–infested forest have dark, brittle pupal cases hanging on their trunks. When mice eat these pupae, they help to prevent another generation of moths from maturing, mating, and laying eggs. Fewer acorns: fewer mice, more gypsy moth caterpillars.
Acorn production can also affect the abundance of deer. White-tailed deer are more likely to have twins the spring following a mast year.5 Besides acorns, deer also like to eat tree seedlings. When acorns are scarce, more of the seedlings will be “browsed,” resulting in less cover, and potentially greater mortality for songbirds.
BIOCHEMISTS HAVE WORKED OUT the intricate pathways of photosynthesis and aerobic respiration, and have even mapped the human genetic code; but the field we call ecology, the connections between organisms and their environment, is a huge canvas of the unknown. We put a few strokes on the canvas when we learn about connections— such as the one between the mice and the gypsy moths— but there are still far more gaps than there is knowledge to fill them.
For instance, I was the first to experimentally identify the pollinator of a particular rare wildflower.6 I was also the first to describe the plant’s mating system, the time it takes the seeds to mature, and some of the animals that feed on the seeds. Many nonscientists assume that we already know these facts about most plants, but the truth is that these details are unknown for the majority of non–crop plant species.
We are beginning to understand the connections between oak trees and other species, but we still have much to learn. When it comes to other plant species we have almost everything to learn. We have been to the moon but we still don’t understand what is in our own backyards.