The Forest Unseen: A Year's Watch in Nature (25 page)

Tuning in to the network has clear advantages for the forest’s animals. Awareness of potential danger gives listeners a head start in deciding how to react. But the advantages of actively contributing to the waves of information are not so obvious. Why call when you see a predator? Why not eavesdrop on others but keep mum? Calling attention to yourself by making a loud sound when a predator approaches seems nonsensical.

For animals with kin nearby, the costs of alarm calling may be outweighed by the need to protect family. Although it is late in the season, some chipmunks and squirrels around the mandala have youngsters with them, so their squeaks and trills give advanced warning to their offspring. But many animals use alarm calls when family are not present, so other benefits must also be in play. Some alarm signals are designed to actively communicate with the predator, drawing attention to the animal in the moment of danger. In doing so they may reap the paradoxical benefit of telling the predator who and where they are. From the predator’s perspective, prey that have seen your approach and are poised to flee are likely to be hard to catch. The predator’s time would be better spent looking for unwary prey. Alarm calling can therefore provide a direct benefit to the caller by advertising the unprofitability of an attack and thus buying safety. “I’ve seen you—you can’t catch me. Move on.”

White-tailed deer take this advertisement one step further. As they run from predators they pump their tails up and down, flashing a white rump and undertail at the pursuer. Their run is interspersed with long upward leaps, losing time that could be spent hoofing forward. Those flashing, prancing displays must have a function beyond telling the predator that it has been seen: running away is already a clear signal that the deer has detected the predator. It is possible that the deer is communicating its vigor and hence ability to escape. Only healthy deer can afford to punctuate their run with wasteful flourishes; weak or sick deer cannot risk their lives with time-wasting bounding displays. This idea has not been thoroughly tested in white-tailed deer, but similar puzzlingly exaggerated displays in gazelles do seem to be honest signals of condition.

The animals’ acoustic network has an invisible analog among the forest’s plants. When insects chew on leaves, they trigger a physiological reaction from the host plant that not only deters further damage to the host but also alerts neighboring plants. Damaged leaves turn on genes that produce a flush of chemicals. Some of the defensive chemicals
evaporate and perfume the air around wounded plants. The wet interior of the neighboring plants’ leaves is soaked in these molecules and, like aroma in a human nose, the molecules dissolve and move into the surrounding cells. Here the chemicals turn on some of the same genes that produced defensive chemicals in the original plant. Unwounded plants around a damaged colleague therefore become less palatable to insects. The trees are listening.

When I sit or walk in the forest I am not a “subject” observing “objects.” I enter the mandala and am caught up in webs of communication, networks of relationships. Whether or not I am aware of it, I change these webs by alarming a deer, startling a chipmunk, or stepping on a living leaf. Dissociated observation is not possible in the mandala.

The webs change me also. Every inhalation carries hundreds of airborne molecules into my body. These molecules are the aroma of the woods, the combined fragrance of thousands of creatures. Some aromas are so pleasing to humans that we have domesticated them, extracting “perfumes.” At least one such perfume, jasmonate, is an alarm chemical, communicating danger among plants. Perhaps our olfactory aesthetics reflect a desire to be wedded to nature’s struggle?

But perfumes are the exception. Most of the forest’s molecules bypass my sense of smell and dissolve directly into my blood, entering my body and mind below the level of consciousness. The effects of our chemical interpenetration with plant aromas are largely unstudied. Western science hasn’t stooped to take seriously the possibility that the forest, or the lack of it, might be part of our being. Yet forest lovers know very well that trees affect our minds. The Japanese have named this knowledge and turned it into a practice,
shinrin-yoku
, or bathing in forest air. It seems that participation in the mandala’s community of information may bring us a measure of well-being at the wet chemical core of ourselves.

T
he forest’s colors are gradually turning. The spicebush in the mandala is mostly green, but a few leaves are flecked with yellow. Color in the ash next to the spicebush is faded, and the outer leaves are drying and bleached. Above me, the maple and hickory still show their summer colors, but the leaves of a large hickory upslope have all turned tan and gold. A few scattered leaves have fallen, refreshing the top surface of the leaf litter and giving a quiet crunch to moving animals.

A winged maple seed flashes past my face. It whirs in a blur of light, like a flying knife at the circus. The seed helicopters down, then strikes the leaf of a toothwort herb, falls between two dead leaves on the forest floor, misses a sandstone pebble, and lodges vane up, seed down in a crevice in the humus. A fine place to germinate—this was a lucky fall.

April’s maple flowers have finally ripened and, after months of slow growth, helicopters are scattered all across the forest floor. A few nestle in dark openings in the litter, but most are exposed on the dry surfaces of leaves or rocks. For all the whirling drama of their flight from the canopy, the ultimate fate of the maple seeds is determined by the particularities of where they land. Rough surfaces excel at catching windblown seeds, so mossy rocks snare more seeds than do bare rocks. The leeward sides of trees accumulate more seeds than the windward. Animal
predators destroy seeds by eating them, or inadvertently spread and sow them by storing them for a meal that never comes because of forgetfulness or death.

There is little that wind-dispersed seeds can do to select a premier germination spot. They don’t get carried to a fertile ant nest like
Hepatica
seeds, or deposited like cherry seeds in a pile of manure, or wiped onto a convenient branch by the beak of a mistletoe-carrying bird. But the maple seed’s helplessness about the choice of its final destination does not mean that the seed has no power. The seed’s skill comes before the final landing.

This morning, no seeds were falling on the mandala. Now, in the late afternoon, they rain so densely that their crackling impact on the ground sounds like a forest fire. This is no coincidence. The thin strip of tissue that holds the seed to its parent is weakest on dry afternoons. These afternoons are also when the wind is strongest—trees time the release of seeds to catch the best wind. Of course, the tree has no central air traffic controller to tell seeds when to depart. Instead, the materials used to fasten the seed to the mother tree, as well as the shape and strength of the attachment, determine when and how seeds will be released. Millions of years of natural selection have tuned the design of these release mechanisms.

There is more to the trees’ strategy than simply dumping seeds into dry air. Flying seeds have two roads ahead of them. The “low road” carries them down from the canopy to the forest floor around the parent. These seeds travel, at most, a hundred meters or so from home. The “high road” takes seeds above the canopy, into the open sky where they may travel miles.

Few seeds take the gravity-defying high road, but it is of great importance to the fate of tree species. Rare long-distance dispersers have a strong effect on the genetic structure of species, on the ability of species to persist in fragmented landscapes, and on how fast species move in response to retreating ice ages or advancing global warming. Like
human history, the narratives of ecology and evolution hinge on the actions of a few individuals that travel across continents and settle far from home.

Maples try to buy a ticket on the
Mayflower
by contriving to launch seeds into gusty updrafts. They do so by preferentially releasing seeds in the upward-blowing air of eddies and gusts but holding tight in downdrafts. Many wind-dispersed trees concentrate their seeds at the top of the canopy, increasing the chances of launching a seed that can catch an updraft. Maples in the mandala have an extra advantage. The prevailing winds blow unobstructed across the valley below and are then deflected upward by the steep slope on which the mandala sits. The wind therefore gives the mandala’s seeds an extra skyward puff in their fight against gravity.

Each tree casts a “seed shadow” that is darkest, most densely packed with seeds, in the tree’s immediate surroundings but theoretically extends across the entire continent. A glance upward confirms that the maple seeds that flutter onto the mandala are almost all the stay-at-home variety, having fallen from trees within easy gliding distance. Mixed among them are a very small number of competitors from other parts of this forest, or perhaps a rare seed that traveled here on a thermal updraft, like a vulture, from tens or hundreds of miles away.

The long reach of seed shadows makes the study of seed dispersal difficult. It is easy to gather information about the vast majority of seeds that stay close to their parents. But the offspring that get flung into the open sky are nearly impossible to track—yet they are the key players in the grand story of each species’ history.

Lacking a drone spy plane with which to track soaring seeds, I turn my attention to the maple seeds on the mandala’s surface. The diversity of forms is remarkable, particularly in the shapes of the wings. Some have three times the surface area of others. A few are ruler-straight; others curve downward like a boomerang, and others arch upward. On most seeds, a notch indents the wing where it meets the seed, but some are notchless. The angle and depth of the notch varies, as does the fatness
of the wing. The mandala is a botanical air show, with every airplane wing shape on display, and a few shapes that no human engineer would dare use.

The diversity of shapes causes the maple seeds to fall with very different styles. The most obvious seeds are those that don’t fly but plummet. One in five seeds lands joined to its sibling. These pairs don’t spin at all but plunge and smack into the ground under the tree. Singletons with small or hunched wings also drop without spinning. These are exceptions, however. Most seeds fall freely for a second or two, and then start to spin. The wing rotates so that its rib, the fatter edge, slices through the air with the wing’s thin vein following. This spinning airfoil generates lift, slowing the fall. A drifting seed can obviously glide farther from the parent than can a seed that falls like a stone. But the extra time in the air also increases the likelihood that a turbulent updraft will cause it to float upward. Whether through shallow descent or lucky ascent, the wind smears the trees’ seed shadows outward, reducing competition among siblings and dropping hopeful progeny across a wide area.

Botanists call seeds that produce their own lift samaras. Technically, a samara is not a seed but a special fruit, formed by the mother’s tissues, holding the seed inside. Ashes and tuliptrees also produce samaras, although neither generates the same amount of lift as the maple’s spinning blade. The maple’s asymmetry gives it an advantage. Its samara is designed to flow through the air like a bird or an airplane wing, with a slicing leading edge. Ash and tuliptree samaras are symmetrical and cannot achieve the maple’s elegant spin. Instead, they twist rapidly about their long axes as they fall, preventing the wing from biting into the air. These species rely less on their own airfoil and more on the strength of the wind to carry their seeds. Accordingly, ashes and tuliptrees cling tightly to their samaras, letting them go only when the wind rages hard.

Maple samaras live in a little-known border country between the aerodynamics of fast, large objects, like cars and airplanes, and the
aerodynamics of slow, minuscule objects, like motes of dust. Airplanes experience their surroundings as relatively free of friction, but dust flecks are so small that friction is about all that they experience. In other words, as an object gets smaller, its world comes to resemble a jar of cold molasses: hard to swim through but easy to float in. The size and speed of samaras puts them in low-grade maple syrup, perhaps appropriately. Engineers have shown that this syrupy air forms a swirl over the leading edge of a rotating blade. This miniature vortex sucks on the upper surface of the spinning samara, slowing its descent.

The aerodynamic consequences of the variety of maple samara shapes are hard to assess. But students of maple samaras have tossed seeds from balconies and drawn two general conclusions. First, wide wingtips produce turbulence and likely slow the wing’s spin, reducing lift. Curved wings are likewise less able to generate lift than straight wings. So, fat-tipped, curvy samaras are poor fliers in the ordered air of a laboratory building. But most of the seeds in the mandala have fat tips and curves. Are the samaras defective versions of an elusive perfect form? Or do maples know something that we do not about the advantages of fat-tipped, curved, or otherwise “deficient” wings?

Wind in a forest is a confusion of swirls and puffs. The forms of the samaras seem to me to be botanical incarnations of the wind’s complexity: a wing for every eddy, a curve for every gust. This diversity of biological form is not restricted to samaras but is a general theme in the forest. Close examination of almost any structure here—leaves, animal limbs, twigs, or insect wings—reveals variability everywhere. Some of this irregularity comes from the different environmental contexts that individuals find themselves in, but much of it has deeper roots in genetics, made possible by the reshuffling of DNA in sexual reproduction.

The fact that individuals differ subtly from one another seems a minor detail of natural history, but such variability is the basis of all evolutionary change. Without diversity, there can be no natural selection
and no adaptation, as Darwin knew when he devoted the first two chapters of
On the Origin of Species
to variation. The samaras’ diversity therefore points indirectly to the invisible workings of evolution. From these varied forms will be chosen the next generation of maples, specially adapted to the winds blowing through this mandala.