The Forest Unseen: A Year's Watch in Nature

by DavidGeorge Haskell

  Other plants in the mandala take a slower road. The toadshade trillium pokes up a trio of dappled leaves between the Hepatica and spring beauty plants, but it is not seeking a quick spurt of growth. Toadshade trillium’s leaves have few enzymes with which to harness sunlight, so they cannot match the ephemerals’ growth rate. Their thrift is rewarded when the tree canopy closes; low levels of enzymes are cheap to maintain, so the trillium can make a sugary profit in the deep shade of summer. We are at the starting line for the annual botanical race for the mandala’s limited space. Evolution has produced a wonderful diversity of running styles: Carolina spring beauty is a muscled-up sprinter, trillium is a lean distance runner.

  The bright burning lives of the ephemerals ignite the rest of the forest. Their growing roots reinvigorate the dark life of the soil, absorbing and holding the nutrients that would otherwise be flushed out of the forest by the spring rains. Each root secretes a nutritious gel, creating a sheath of life around its hairy tip. Bacteria, fungi, and protists are a hundred times more abundant in this narrow halo, and these single-celled creatures provide food for nematodes, mites, and microscopic insects. The grazers are preyed upon by even larger soil-dwellers such as the bright orange centipede that shimmers back and forth over the mandala as I sit watching. The centipede is longer than my hand is wide, so big that I can see each segment on its legs as the body undulates between the sources of its life, the flowers.

  A few days ago my contemplation of the flowers was broken by a fiercer predator than this centipede. A palm-sized ball of gray fur shot out of the ground, then dove back into another hole, accelerating like a dustball pulled into a vacuum cleaner. A few minutes later I heard rustling and high-pitched squeaks from the other side of the mandala. I saw just enough of the sooty fur and stubby tail to know that the terror of the leaf litter was prowling the mandala: a short-tailed shrew.

  Shrews live short, violent lives. Only one in ten survives longer than a year; the rest get burned out by their furious metabolism. Shrews breathe so frantically that they cannot survive long aboveground. Their outrageously rapid breathing would desiccate and kill them in the dry air.

  Shrews feed by snapping at prey then chewing poisonous saliva into their victims, sometimes killing the animal they have caught, sometimes paralyzing it for storage in a dungeon of horrors, a larder of living but incapacitated prey. So ferocious are shrews that they eat whatever is before them. Mammalogists despair of them. If a shrew gets caught with mice in a live trap, the scientists return to find a pile of bones overseen by an angry gray warden.

  The squealing that I heard was just the lower range of the shrews’ repertoire. Most of their chattering is too high for my ears to detect. These highest calls are shrew sonar. Shrews send out ultrasonic clicks and listen for the reflected sound waves, using echolocation to find their way around burrows and to locate prey. These “terrestrial submarines” therefore navigate mostly by sound. Their eyes are tiny, and mammalogists disagree about whether shrews can see images or whether they merely perceive patches of light and dark. As with the snails, shrew vision is a mystery.

  The soil’s food web reaches its zenith in the shrew. Only owls will eat shrews; everything else gives them a wide berth, fearing their vicious teeth or the acrid taste of their scent glands.

  There is kinship with humans here. The first mammals were shrewlike creatures terrorizing the snails and centipedes of the Mesozoic. Our ancestors were shrill and vicious, leading a caffeinated existence in dark corridors. An analogy with our current state of being is tempting. Thankfully we’ve lost the poison fangs and pungent glands.

  The spring ephemerals also touch off life’s fire aboveground. Small black bees fly from flower to flower, rejecting all but Carolina spring beauty flowers. Here the bees bury their heads, slaking their thirst for the strong sugar water that we call nectar, then whir their legs through the flower’s pink pollen-bearing anthers. The bees emerge looking like chocolate drops dusted with rosy icing sugar. They fly off with fat saddlebags of pink pollen hanging from each hind leg.

  The flying candies are all female bees, recently emerged from their winter burrows. Each female flies around seeking a new nest site in either a patch of soft soil or an old log. The bees dig tunnels into their chosen home and paint shiny secretions on the walls of a nest chamber. These secretions hold the walls together and keep water away from the delicate offspring. The mother mixes pollen and nectar into a ball, then lays an egg on the ball and seals it into a small mud-walled cell. The larval bee that hatches from the egg will munch its way through the pollen paste and emerge several weeks later, its body built wholly out of flowers. This dependence on pollen and nectar will continue for the rest of the bee’s life. Bees eat nothing else; they are the original “flower power” creatures.

  Once emerged, the offspring of some species of woodland bee fly off to breed on their own. Many other species stay at home, forgoing the opportunity to lay their own eggs. These helper bees take over the foraging duties, allowing the foundress, their mother, to specialize in egg laying. This communality is favored by two forces, one external to the bees and one built into their genes.

  The crowdedness of the bees’ environment encourages stay-at-home offspring. Most of the forest floor is too rocky, too wet, or too deeply covered in leaf litter to make an adequate nest. Competition for nest sites is intense, and female bees that try to strike off on their own face a serious risk of failure. Staying at home is a safer bet; if you’ve been born there, then by definition your mother has a successful nest hole.

  The bees’ genetics tip the scale further in favor of helping mother. Female bees are born from eggs fertilized by sperm stored from the mother’s autumnal love flight, so they have, as do humans, two copies of all their chromosomes, one from mom and one from dad. In contrast, males develop from unfertilized eggs, so they carry only one set of chromosomes, inherited only from their mother. Therefore all bee sperm cells are identical. This curious genetic system produces even more curious kinships. Sisters within a bee colony are very closely bonded, a supersorority of chromosomes. Whereas human siblings share, on average, half their genetic individuality, these bee sisters share much more. The half of their DNA that they inherit from their father is identical; the half they inherit from their mother is split evenly among sisters. The average of their parents’ contributions therefore comes to three-quarters of their genes via common descent. If the bee mother mated with more than one male, this relatedness drops a little but still remains high enough to affect the evolutionary process.

  Evolution’s accountant rewards those animals that assist close kin and ignore more distant relatives. Normally this means that raising one’s own offspring is the best strategy. But female bees’ genes have primed them to be as open to helping mom as they would be to leaving home and breeding themselves. So, when the mother bee fills her springtime nest with fertilized eggs, she is hatching a cohort of daughters for whom leaving home is risky and staying at home is very attractive. Male bees feel a different set of forces. No strange relatedness rewards them for staying at home. Sons therefore behave like aristocratic cads, hanging about the nest, ambling about in search of nectar, and focusing their energies on the pursuit of virgin queens. Their sisters have little patience with them and will sometimes forcibly expel them from the nest.

  Brother-sister tensions are not the only source of conflict in the bee nest. Worker bees sometimes try to sneak their own eggs into the nursery. The queen responds by eating the eggs and releasing odors that suppress egg laying in her uppity daughters, reinforcing the already strong pull of genetic relatedness. Sometimes several overwintering females will found a colony together, causing a tug-of-war over who lays the most eggs. The winner usually becomes queen, but her co-foundresses continue to try to lay eggs themselves.

  Fraught family lives are not the only source of woe in bee nests. Defenseless larvae and concentrated stores of pollen and honey make alluring targets for raiders. Many of these rai
ders are out in force today over the mandala’s flowers. Bombyliid flies or “bee flies” are the most specialized and successful of these pirates. Adult bombyliid flies are innocuous, comical even. They dart from flower to flower, supping on nectar with a rigid proboscis that points the way for the orange feather-duster-fluffy body. But this cuddly flower-tippling buffoonery ends when the female drops her egg in front of a bee nest. The egg hatches, and a wormy larva crawls into the nest to dine on the bees’ pollen and honey stores. The worm then molts into a predaceous larva that consumes the bee larva whose cell has been robbed. Sated, the fly larva encases itself and waits underground. When the ephemerals kick-start the mandala’s life next spring, the bombyliid flies crawl out of their pupal dens, metamorphosed from pirates into clowns.

  A pattern emerges as I watch the bees and flies in the mandala. Adult bombyliid flies show no discernment in their choice of flowers, stopping at every flower to sip nectar or eat pollen. The bees are fussier, preferring spring beauty and rejecting the nectarless flowers of rue anemone and Hepatica. These preferences are the hem of a huge and complex cloak of relationships. Hundreds of species of insects and flowers interact in this forest every spring, each trying to buy success for its offspring through sugary bribery or subsidized hauling of pollen. Some, like the bombyliid flies, are numerous but only partly successful at transferring pollen. Others, like the fussy bees, are rarer but more effective pollinators.

  This intricate web of dependency dates back one hundred and twenty-five million years to when the first flowers evolved. The oldest fossil flower, called Archaefructus, had no petals, but its pollen-bearing anthers had flags on their tips. The botanists who described the fossil believe that these extensions may have been used to attract pollinators. Other ancient flowers also appear to have been insect-pollinated, further supporting the idea that insects and flowers have been partners since the first flowers evolved. How this marriage came about is unknown, but it seems likely that flowering plants evolved from fernlike plants. These ancestors produced spores that attracted insects looking for an easy meal. The ancestors of the flowers turned the plague of insect predators into a blessing by producing conspicuous displays to attract these spore munchers, then producing so many spores that the insects’ bodies would be coated. The predators inadvertently carried some of this sporey dust onto the next flower, increasing the fecundity of the spore producer. Eventually the spores got wrapped in a package, the pollen grain, and the true flower was born. The bees and spring beauties in the mandala reenact the main theme of the original relationship. The bees, or their larvae, eat most of the pollen they gather, transferring only a small number of pollen grains from flower to flower.

  The core of the relationship between flowers and insects is unchanged, but the details and frills have been elaborated enormously. An insect flying across the mandala is bombarded with scents, billboards, and lures, all trying to coax her to the flowers’ storefronts. The bombyliid flies answer all these calls, stopping in at every flower. Most bees are more selective. Sometimes this selectivity produces specialization: a flower designed for one insect, an insect’s brain tuned to one flower. The orchids carry this to exquisite extreme, mimicking the aroma and appearance of a female bee, inducing mating by the male, whose ardor is then converted into an orchid postal system.

  The mandala has a small number of specialized flowers. Toothwort’s tubular flowers exclude small bees, allowing only long-tongued bees and flies into the narrow nectar tube. Some bee species feed exclusively on spring beauty flowers, choosing to be faithful to one flower for the sake of efficiency. But these examples of specialization are conspicuous exceptions to the promiscuity of the plants and their pollinators in the mandala. Spring’s brevity promotes this unusual preponderance of generalists. The ephemerals are caught between the cold weather of early spring, which stops pollinators from flying, and the tree canopy’s emergence, which steals the light that the spring ephemerals need to grow and produce seeds. This is no time for pickiness. Plants need any help they can get from the insects, regardless of whether the insect is a faithful bee or a haphazard fly. All the flowers in the mandala, except for the toothwort, produce cup-shaped flowers that are available to any insect. The starburst is open wide, welcoming all the woodland pollinators to its blazing show.

  April 2nd—Chainsaw

  A mechanical whine starts abruptly and cuts through the forest, jangling my nerves as I sit at the mandala. A chainsaw is ripping through wood somewhere to the east. This patch of old-growth forest is protected, supposedly free of chainsaws, so I leave the mandala to investigate. After scrambling across a rock scree and climbing up a stream bank, I find the source of the sound: a golf course maintenance crew felling a dead tree at the edge of one of the cliffs above the forest. The golf course runs to the edge of the cliff, and dead trees evidently do not fit with the golfing aesthetic. The maintenance crew bulldozes the felled tree over the edge of the cliff, then moves on to other tasks.

  The sight of a cliff being used as a disposal chute is irritating, but the dumped tree will provide some extra salamander habitat. I am relieved that the cutting was not below the cliff line in the old-growth forest itself. The mandala’s intense starburst of flowers is special, and nearly unique, because the chainsaw has never stripped this hillside of its cover. Salamanders, fungi, and solitary bees also revel in the tangles of huge fallen logs and deep leaf litter. Logging, particularly clear-cutting, kills many of these inhabitants of the woods, and their populations take decades, sometimes hundreds of years, to recover.

  Stripping the mountainside of trees turns the forest soil from a moist duff to an oven-baked brick. Ground-nesting bees, wet-backed salamanders, and the creeping stems of ephemerals dry up and die in the parched soil. Only when the forest has regained its leaf litter, canopy, and dead wood do the creatures start to return, but this revival is slow, constrained by the lack of old dead logs to act as nurseries and by the sluggish dispersal abilities of flowers and salamanders.

  So what? Why should we leash our accelerating desire for wood and paper for the sake of saving a springtime explosion of forest biodiversity? Can’t the flowers look after themselves? After all, disturbance is natural. The old “balance of nature” cliché went out of fashion decades ago. Now the forest is a “dynamic system,” constantly assaulted by wind, fire, and humans, always in motion. Indeed, we can turn the question on its head and ask whether we need to go out and do some clear-cutting, to replace the fires that used to clear large areas of forest but have been suppressed by land managers for nearly a hundred years now.

  These questions are the roots of a thorny growth of arguments that choke academic conferences, government reports, and newspaper editorials. Does the forest need the buzz of chainsaws, or does it need time to renew itself unperturbed by log hunters? We are tempted to use nature as a model, but nature provides a Baskin-Robbins of justifications. Which flavor of forest life cycle would you like: the annihilating force of an ice age; or an ancient, undisturbed mountainside; or the dancing mischief of a summer tornado?

  Nature, as usual, is not providing the answer.

  Rather, we are thrown back a moral question: what part of nature do we wish to emulate? Do we aspire to the uncompromising, all-controlling weight of an ice sheet, imposing our glacial beauty on the land, retreating every hundredth millennium to free the forests’ slow regeneration? Or do we seek to live like fire and wind, cutting swaths with our machines, then moving on for a while, hitting at random intervals, at random locations? How much wood do we need? How much do we desire? These are questions of time and of magnitude. We can cut every two decades, or we can cut every two centuries; we can focus our extractive desires, or we can let them run across the whole; we can strip the forest bare, or we can remove just a few trees.

  Our collective answer to this question grows out of the values of millions of landowners, pruned and guided by society’s two clumsy hands, the economy and government policy. The forest is shattered li
ke a broken windshield by surveyors’ lines, so the diversity of these values plays out in a mosaic across the continent. Despite this chaos, patterns emerge from the aggregate. We are neither an ice age nor a windstorm but something altogether new. We have changed the forest on the scale of an ice age, but at a pace accelerated a thousandfold.

  In the nineteenth century we stripped more trees from the land than the ice age accomplished in one hundred thousand years. We hacked the forest down with axes and handsaws, hauling it away on mules and railcars. The forest that grew back from this stripping was diminished, robbed of some of its diversity by the scale of the disturbance. This was a windstorm on the scale of an ice age but similar to a tornado in its crude physical messiness.

  Cheap oil and expensive technology have now brought us to the second phase of our relationship with the forest. No longer do we cut by hand and haul with animals or steam; gasoline engines do it all, accelerating our extraction and increasing our control. Oil’s power and our minds’ cleverness gave us another tool: herbicides. In the past, the forest’s regenerative power limited our ability to direct the land’s future. The forest would burst back, prepared for the ax by millions of years of wind and fire. Now “chemical suppression” is the tool of choice to knock back trees whose genes tell them to resprout. Machines clear the forest, cutting then bulldozing the remaining “debris.” Then the helicopters move in and rain herbicides on the remnants, forestalling a resurgence of green. I have stood at the center of these clear-cuts and seen no green to the horizon in nearly every direction: an arresting experience in Tennessee’s normally lush summer.

  All this effort is directed at preparing the land to receive a new forest, a monoculture of fast-growing trees. Depending on the tree and the soil type, the rows of trees are then sprayed with fertilizers to replace some of the nutrients that were removed from the old, outmoded forest. If you squint, the resulting tree plantations look something like forests. But the diversity of birds, wildflowers, and trees is gone. Suburban backyards have more biological diversity than these shadows of real forests.