The maple has one more physiological trick to help it cope with embolisms. Sugary sap rises forcibly up maple trunks in the early spring, flushing out air and restoring the old xylem’s integrity after winter’s hard freezes. Maples can therefore use old xylem for extra water-carrying capacity, whereas hickories are restricted to the current year’s growth. The maples’ springtime flow of sap is powered by cycles of nighttime freezes and daytime thaws in their twigs. This explains why sap flows heavily in some years and hardly at all in others. When temperatures see-saw between sharp nighttime frosts and sun-warmed days, sap flows prodigiously; when the weather is uniformly tepid, the flow is stanched.
The contrast between the leafy maple and the somber hickory comes down to a matter of plumbing. The trees seemed at first to be prisoners of the unyielding laws of physics. The constraints imposed by water’s evaporation, flow, and freezing circumscribe their lives. But trees are also masterful exploiters of these same laws. Evaporation is the cost that trees pay for opening their leaves, but evaporation is the force that silently and effortlessly moves hundreds of gallons of water up tree trunks. Likewise, ice is the enemy of springtime xylem, but ice powers the maples’ early flow of sap, again without cost to the tree. In two different ways, both maple and hickory have turned the tables on their constraints and turned adversity into triumph.
April 14th—Moth
A moth shuffles its tawny feet over my skin, tasting me with thousands of chemical detectors. Six tongues! Every step is a burst of sensation. Walking across a hand or a leaf must be like swimming in wine, mouth open. My vintage meets the moth’s approval, so his proboscis unfurls, rolling down from between the bright green eyes. Unrolled, the proboscis juts straight down from the moth’s head, like an arrow pointing at my skin. At the point of contact, the proboscis’s rigidity softens and the tip flops backward, pointing between the moth’s legs. I feel cool wetness as the moth slaps the tip around, seeming to search something out. I lean toward my finger, squinting through a hand lens in time to see the tip worm itself into the groove between two ridges of my fingerprint. The proboscis stays lodged in this furrow, and fluid flashes back and forth in the pale tube. The sensation of moistness continues.
I watch the moth feed for half an hour and discover that I cannot dislodge my guest. At first I hold my finger still, cautiously moving only my head. After several minutes my body protests at such stiffness, so I move the finger. No reaction. I wave the finger, then blow on the moth. Again, the moth continues its work. Pokes with my pencil end fail to stir the animal. A large fly also visits and dabs my hand with wet kisses from its toilet-plunger mouth. This bristly fly shows more normal insect reactions and takes flight as I lean close. The moth, however, sticks like a tick.
The moth’s antennae hint at the cause of the vigorous attachment to my finger. The antennae arch out of the head, reaching forward by nearly the length of the moth’s body. Closely spaced ribs jut out from each antenna’s spine. The moth is thus crowned with two threadbare feathers. These plumes are covered with velvety hairs. Each hair is peppered with holes that lead into a watery core in which sits a nerve ending, waiting for the right molecule to bind to its surface and trigger a response. Only males have such exaggerated antennae. They comb the air for scent released by females and fly upwind, guided to a mate by their enormous feathery noses. But finding a mate is not enough. The male must provide a nuptial offering to his mate. My finger provides him with an essential ingredient for this gift.
Diamonds may be the crystal of choice for wooing humans, but moths seek a different, altogether more practical mineral, salt. When the moth mates he will pass to his partner a package containing a ball of sperm and a packet of food. This food is generously seasoned with sodium, a precious gift that looks forward to the needs of the next generation. The female moth passes the salt to the eggs and thus to the caterpillars. Foliage is deficient in sodium, so the leaf-munching caterpillars need their parents’ salty bequest. The moth’s arduous attachment to my finger prepares him for mating and will help his offspring to survive. The salt in my sweat will make up for deficiencies in caterpillar diets.
The morning is sunny and comfortably warm. Summer’s heat has yet to arrive, so I am barely sweating. This makes the moth’s task harder and provides a poor chemical mix for his gift. Copious sweat would be much better. Human sweat is made from blood with all the large molecules removed, like soup passed through a sieve. The blood fluid passes out of our vessels, seeping around the spaces between our cells and into the coiled tubes at the bottom of sweat ducts. As the fluid passes up the sweat ducts the body pumps sodium back into its cells, reclaiming the valuable mineral. The faster the sweat moves, the less time the body has to recapture the sodium, so when we pour with sweat there is little difference between the mineral mixture in our sweat and that in our blood. We are literally sweating blood, minus the lumps. When the sweat moves out of us sluggishly, we produce fluid that has less sodium and proportionally more potassium, a mineral that the body expends little effort on reabsorbing. Plant leaves have plenty of potassium, so male moths are not interested in it and void any that they suck up with the sodium. Some of what the moth is taking from my skin will therefore pass into his feces and thence back to the soil.
Despite providing the moth with barely a trickle of the wrong flavor of sweat, I am a mammal worth clinging to. Humans are one of the few animals to use sweat as a cooling mechanism, so salty skin is seldom found in the mandala. Naked salty skin is rarer yet. Bears sweat and so do horses, but their bounty is hidden under a layer of hair. Horses never visit the mandala. Bears are very rare, although remains in local caves show that they were common before the arrival of gunpowder. Most other mammals sweat only on their paw pads or lip margins. Rodents don’t sweat at all, perhaps because their small bodies make them particularly vulnerable to dehydration.
Blood fluid oozing from pores is therefore an unusual treat at the mandala. My skin’s meager sweat is a feast compared to the scarcity of sodium in the forest. Rain puddles sometimes are worth sucking at, but they are seldom rich in sodium. Feces and urine are saltier, but they dry quickly. I am the best bet today. Not wanting to carry the moth out of the forest when my sit at the mandala is over, I must pry his grasping feet from my skin, then run away.
April 16th—Sunrise Birds
A peach stain soaks into the darkness on the eastern horizon, then the whole dome of the sky lightens, bleeding from darkness to pale luminosity. Two repeated notes ring through the air; the first is clear and high, the second is lower and emphatic. These tufted titmice keep up their rapid two-part rhythm as a Carolina chickadee starts a whistled melody, four notes that fall and rise like a nodding head. The peach spreads up from the horizon, and a phoebe calls with a whiskey-and-cigarette-roughened voice, rasping out its name, phwe-beer, like a broken bluesman.
As the sky’s pallor brightens, a worm-eating warbler rattles a caffeinated castanet. The dry buzz unleashes a confusion of songs from all directions, a jumble of tempos and timbres. The black-and-white warbler wheezes lazily, whee-ta whee-ta, from an upside-down perch under a tree limb. The hooded warbler rings out from a sapling, twirling the notes twice around to gather speed, then flinging them to the sky, wee-a wee-a WHEE-TEE-O. From the west comes a yet louder song. Three rich tones wash over the forest like repeated waves, then break down into rippling eddies. The tin-whistle song of the Louisiana waterthrush seems inspired by the flow of the streams along which it lives, yet the song’s cadence and volume carry the sound above the water’s roar.
Peach turns to pink, and color spreads wider over the horizon. The sky’s vault brightens enough to reveal the partly closed chickweed flowers in the mandala and to give form to the boulders and stones that define the mandala’s edge. As the world fades into view, Carolina wrens sing, vying with the waterthrush to produce the loudest song in the woods. The wrens sing year-round, but today I hear them with fresh ears, the familiarity of their song stripped away by the spri
ngtime flush of sound. No other bird, save the now-departed winter wren, can match the vigor of their acoustic attack or the exuberance of the uncoiling energy in their song.
The wren’s music is answered by a Kentucky warbler from farther down the slope. The warbler echoes the wren’s theme and tone but holds back, like a diver bouncing endlessly on a board, never daring to take the plunge. Then another song bursts from the canopy, lisping like a black-and-white warbler, but the song breaks out of the pattern, accelerates, then twitters. I cannot identify the bird and, more frustratingly, cannot find it with my binoculars. Perhaps this is the dawn “flight song” of a warbler? These flight songs are out-of-character virtuoso solos given during arching flights high above the forest. They are seldom recorded and, in my limited experience, are highly variable. What role they play in the birds’ lives is unknown but, if nothing else, they must provide a rush of creative release for birds that spend the rest of their day repeating just a few syllables.
Woodpeckers add their boisterous voices to the performance. First the red-bellied woodpecker lobs its quivering cry across the mandala, then an answer flies back, the maniacal laugh of the pileated woodpecker. Blue jays punctuate the woodpeckers’ volley with alternating rasps and whistles. As the glow in the sky intensifies, half a dozen goldfinches fly eastward, bouncing in the air just above the forest canopy, like flung stones skimming over water. Each bounce is accompanied by a twitter, ti-ti-ti, ti-ti-ti.
The whole sky flashes pink for a moment, then yellow surges up from the east, brightening the mandala. Color sinks back to the horizon again, leaving milky light across the rest of the sky. A red-eyed vireo greets the glow with his regularly spaced bursts of whistling. Some bursts end on a rising note, “where am I?”; others conclude on a low note, “there you are…” The vireo questions the forest, then answers over and over, lecturing into the midday heat when other birds have retired from the podium. As befits his professorial temperament, the vireo seldom descends from the heights of the canopy and is usually detected only through his bright, repetitious song. The vireo is joined by a brown-headed cowbird. Cowbirds are brood parasites, laying their eggs in the nests of other birds. This emancipation from parental duties leaves the cowbirds free to pursue the pleasures of courtship. The male’s song has taken him two or three years to perfect and sounds like molten gold falling, solidifying, then ringing out as it strikes stone. A burst of precious liquidity combined with the ringing of metal.
The heavens shine blue now, and the sunrise’s colors have faded to a pastel belt of cloud in the east. A northern cardinal chips loudly on the slope below the mandala, each note like struck flint. These crisp calls are the counterpoint to turkey gobbles rising up from the valley below. The forest has muffled the turkey’s distant sounds, adding what Thoreau called the “voice of the wood nymph” as the sound is bounced and squeezed through the vegetation. We are in turkey-hunting season, so the gobbles are as likely to be gobble-mimicking humans on a gastronomic quest as they are real turkeys searching for love.
The fading dawn colors revive momentarily, and the sky shines with lilac and daffodil, layering colors in clouds like quilts stacked on a bed. More birds chime into the morning air: a nuthatch’s nasal onk joins the crow’s croak and a black-throated green warbler’s murmur from the branches above the mandala. As the colors finally fade under the fierce gaze of their mother, the sun, a wood thrush caps the dawn chorus with his astounding song. The song seems to pierce through from another world, carrying with it clarity and ease, purifying me for a few moments with its grace. Then the song is gone, the veil closes, and I am left with embers of memory.
· · ·
The thrush’s song flows from the syrinx buried deep in his chest. Here membranes vibrate and squeeze the air that rushes out of the lungs. These membranes circle the confluence of the bronchi, turning a toneless exhalation into sweet music that ascends the trachea and flows out of the mouth. Only birds make sound this way, using a biological hybrid between the flute’s swirling tube of air and the oboe’s vibrating membranes. Birds change the texture and tone of their songs by adjusting tension in the muscles that wrap the syrinx; the thrush’s song is sculpted by at least ten muscles in the syrinx, each one shorter than a grain of rice.
Unlike our voice box, the syrinx offers little resistance to the flow of air. This gives small birds the ability to ring out louder songs than the huskiest human. But despite the efficiency of the syrinx, birdsong seldom carries farther than a stone’s throw. Even the turkey’s explosive gobble is quickly swallowed up by the forest. The energy that propels the sound is easily absorbed and dissipated by trees, leaves, and the sponginess of air molecules. High-pitched sounds are more easily absorbed than bass notes, whose long wavelengths let them flow around obstacles rather than bounce away. The beauty of birdsong, especially descant birdsong, is therefore a blessing available only at close range.
Not so the sun’s gift. The photons that created this dawn have traveled one hundred and fifty million kilometers from the surface of the sun. But even light can be slowed and filtered. This slowing is most dramatic inside the sun’s belly, where photons are born from the fiery union of pressurized atoms. The sun’s core is so dense that it takes ten million years for a photon to struggle to the surface. Along the way, the photon is continually blocked by protons, which absorb the photon’s energy, hold it for a moment, then release the energy as another photon. Once the photon finally bursts free from millions of years trapped in the sun’s molasses, it zips to earth in eight minutes.
As soon as photons reach our atmosphere their paths are again strewn with molecules, albeit molecules that are millions of times less densely packed than those in the pressed mass of the sun. Photons come in many colors, and some colors are more vulnerable to being impeded by the atmosphere. Red photons have wavelengths that are much longer than the size of most air molecules, so, like a turkey gobble in a forest, they flow easily through the air and are seldom absorbed. Blue photons have wavelengths that more closely match the size of air molecules, so this short wavelength is absorbed by the air. An air molecule that absorbs a photon jiggles with the excitement of the ingested energy, then pops out a new photon. The ejected photon is shot out in a new direction, so the tidy stream of blue photons is scattered into a ricochet of light. Red light is not absorbed and scattered, so it flows straight on through. This is why the sky is blue; we are seeing the redirected energy of blue photons, the glow of billions of excited air molecules.
When the sun is overhead, photons of all colors reach our eyes, even though some blue ones are redirected along the way. When the sun is low on the horizon, photons have a sloping path to cut through the air, so more blue light is stripped out. The red dawn light bathing this Tennessee mandala was therefore born in the blue morning skies over the Carolina mountains to the east.
The light and sound energies washing over the mandala find a point of convergence in my consciousness, where their beauty quickens a flame of appreciation. There is convergence also at the start of the energy’s journey, in the unimaginably hot, pressurized core of the sun. The sun is origin of both the dawn’s light and birds’ morning songs. The glow on the horizon is light filtered through our atmosphere; the music in the air is the sun’s energy filtered through the plants and animals that powered the singing birds. The enchantment of an April sunrise is a web of flowing energy. The web is anchored at one end by matter turned to energy in the sun and at the other end by energy turned to beauty in our consciousness.
April 22nd—Walking Seeds
The springtime flush of flowers is over. A few chickweed flowers and a geranium are all that is left of the month’s glory. Spent flowers rain down from above, bringing to earth evidence of the maple and hickory trees’ prodigious reproductive efforts. Hundreds of maple and hickory flowers are lying within the mandala. Unlike the gaudy blooms of the spring ephemerals, these tree flowers are bland and unassuming, with no obvious petals or colored adornments. This extrem
e Puritanism of dress suggests that sex among the mandala’s trees is a business very different from the ephemerals’ effusive festival of nectar and color. These trees have no one to impress. Wind carries their pollen, so insect eyes and palates need not be bribed; flowers can be stripped down to their utilitarian essentials.
Pollination by wind is a particularly useful strategy for early-blooming trees. The spring ephemerals live in a relatively warm, sheltered microclimate, yet they struggle to find pollinators. The tree canopy’s microclimate is more exposed and is even less friendly to the insects of early spring. Wind is not in short supply, however. Maples and hickories have therefore broken the ancient contract with insects, using physical rather than biological methods to transport their pollen. Increased reliability comes with an unfortunate decrease in precision. Bees deliver pollen directly to the stigma of the next flower. Wind does not deliver anything. Rather, it disperses whatever is caught in its motion, much to the distress of both flowers and human sinuses. Wind-pollinated plants must therefore release vast drifts of pollen. They are like castaways stranded on an island, throwing millions of bottles into the water for want of a dependable postal service.
Unlike the hermaphroditic wildflowers, maple and hickory produce two kinds of flower, male and female. Male flowers dangle from twigs so that the slightest movement of air will stir them. Maple trees hang clusters of these flowers from wiry filaments. Each filament is a centimeter or two long and ends in a tuft of anthers, pollen-producing structures that look like tiny yellow balls about the size of a comma on this page. Hickory’s anthers are strung on fuzzy garlands called catkins, each one about as long as a finger. In both species, the anthers nestle in groups under small umbrellas, presumably to stop the rain from washing pollen away. The female flowers are more stubby, having no need to cast large quantities of pollen into the wind. Their stigmas intercept wind-borne pollen to start the fertilization process. Little is known about the aerodynamics of the stigmas, but they seem to be placed in the windiest parts of the plant and to be designed to encourage air to curl around them, forming eddies that slow the air and thus deposit pollen grains.
The Forest Unseen: A Year's Watch in Nature Page 9
