The Forest Unseen: A Year's Watch in Nature (11 page)

Last week, the forest’s green lay across the ground, a carpet of photosynthesis that ended at ankle height. Now the maples are unfurling leaves and dangling green flowers from branches. Like a tide rising, the green glow is reclaiming the forest from the ground up. The upward surge floods the mountainside with a sense of renewal.

Sugar maple branches hang over the mandala, and their new leaves block the sun’s rays, shading the understory. Of the hundreds of spring wildflowers, only a dozen remain; the maple has snuffed their spark. But not all the trees around the mandala are in leaf. The maple’s exuberance contrasts with the dour, lifeless pignut hickory that stands on the other side of the mandala. The hickory’s massive gray trunk rises straight to the canopy where it holds out dark, bare branches.

The contrast between the maple and the hickory expresses an inner struggle. Growing trees must throw open the breathing pores on their leaves, allowing air to wash the wet surfaces of their cells. Carbon dioxide dissolves into the dampness before it is turned to sugar inside the
plants’ cells. This transformation of gas into food is the trees’ source of life, but it comes at a cost. Water vapor streams out of the leaves’ open breathing pores. Every minute, several pints of water are exhaled into the air by the maple above the mandala. On a hot day, the seven or eight trees whose roots penetrate the mandala send several hundred gallons of water out of their leaves as vapor. This reverse waterfall quickly dries the soil. When the supply of water is exhausted, the plant must close its breathing pores and cease growing.

All plants face this trade-off between growth and water use. But trees have another devilish layer of difficulty. By thrusting their leaves skyward they have become slaves to the physics of their plumbing systems. Inside each trunk lies the vital connection between earth and sky, soil’s water and sun’s fire. The rules that govern this connection are stringent.

Inside the trees’ leaves, sunlight causes water to evaporate from cell surfaces and drift out of breathing pores. As vapor wafts away from wet cell walls, the surface tension of the remaining water tightens, particularly in the narrow gaps between the cells. This tension yanks more water from deep in the leaf. The pull moves to the leaves’ veins, then down the water-conducting cells in the tree’s trunk, finally all the way to the roots. The pull from each evaporating water molecule is minuscule, like a breath of wind tugging at a silk thread. But the combined force of millions of evaporating molecules is strong enough to haul a thick rope of water up from the ground.

The trees’ system for moving water is remarkably efficient. They exert no energy, instead letting the sun’s power draw water through their trunks. If humans were to design mechanical devices to lift hundreds of gallons of water from roots to canopy, the forest would be a cacophony of pumps, choked with diesel fumes or run through with electrical wires. Evolution’s economy is too tight and thrifty to allow such profligacy, and so water moves through trees with silence and ease.

Yet this efficient water-lifting system has an Achilles’ heel. Sometimes
the rising columns of water are broken by air bubbles. These embolisms plug the flow of water. Winter weather makes these blockages more likely because air bubbles form when water freezes inside water-conducting cells. These are the same bubbles that haze ice cubes in kitchen freezers. Thus icy weather peppers the trunk with air gaps that wreck the trees’ plumbing. Maple and hickory have found two different solutions to this challenge.

With its bare branches, the hickory looks wintry and inactive, but this is an illusion. Inside, the tree is building a whole new plumbing system, readying itself for the flowers and leaves that will emerge in a couple of weeks. Last year’s plumbing system is useless, blocked by embolisms. So, hickory trees spend the first part of April growing new pipes. Just below the bark, a thin sheet of living cells wraps the trunk. These cells divide and create the season’s new vessels. The outer layer of cells, those that lie between the bark and the sheet of dividing cells, will become phloem, a living tissue that transports sugars and other food molecules up and down the tree. The new cells formed on the inner side will die and leave their cell walls to become the xylem, or wood, that conducts water up the trunk.

Hickory xylem tubes are long and wide. These pipelines offer little resistance, so the flow of water is prodigious when the tree finally leafs out. But the width of these tubes makes them particularly vulnerable to blockages by embolisms. Once blocked, they become useless and, because the tree has relatively few of these wide conduits, the flow of water drops significantly with just a few embolisms. This design means that hickories must delay the growth of their leaves until the danger of frost is past. The trees miss out on the warm sunny days of spring, but they recoup these losses when their pipelines are thrown open later in the season. The hickory is therefore like a sports car—kept off the road by ice until late in the spring, but outstripping all rivals on warm summer days.

Hickory trunks have one more problem. Their wide, long xylem tubes are weak, like thin-walled straws. These tubes cannot hold up
heavy branches or cope with the force of wind pulling on leaves. Therefore, later in the year, after the springtime xylem has grown, the tree grows thick-walled, smaller-bored xylem vessels. This summer-grown xylem provides the structural support that the water-carrying vessels lack. The yearly alternation is visible in cut hickory wood as a “ring porous” pattern of wide porous cells separated by denser wood.

If hickories are sports cars, then maples are all-wheel-drive passenger cars. Their xylem is frost-resistant and lets them leaf out weeks before the hickories. But, come summertime, maples will lag behind hickories in their ability to carry water and thereby feed on sunlight. The maples’ xylem cells are more numerous, shorter and narrower than the hickories’, and they are separated by comblike plates. Unlike the broad, open tubes of hickory, the maple’s design confines embolisms to the small cells in which they form. Because maples have so many small tubes, each embolism blocks just a tiny fraction of the trunk. Unlike the ringed patterns in hickory wood, maple wood is more uniform, showing a “diffuse porous” pattern. These differences are visible in furniture and other wood products—maple is smooth-grained, whereas hickory has regular rows of pinholes.

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.

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.

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.