The Forest Unseen: A Year's Watch in Nature (29 page)

The quality of light also changes under a break in the canopy. Leaves absorb some wavelengths of light better than others. In particular, they absorb red light, but “far red” light passes through them. Far red, or infrared, is invisible to humans because its wavelength is too long for the receptors in our eyes. But plants can “see” both red and far red light. Growing twigs use the relative proportions of the two wavelengths of light to tell where they are in relation to other plants.
Under the canopy and in crowded conditions, far red light dominates because leaves of competing plants absorb most of the red light. But under open skies the proportion of red light surges. Twigs respond by changing their architecture, spreading branches wide and stretching their tips into the light.

Trees accomplish “color vision” through a chemical in their leaves. This molecule, called phytochrome, can exist in two different forms. The switch between forms is activated by light: red causes the molecule to flip into the “on” position; far red causes it to flip “off.” Plants use the two forms to assess the ratio of red and far red light in their environment. In the reddish light of the canopy gap, phytochromes in the “on” position dominate, causing trees to grow bushy branches toward the gap. In the forest shade, far red dominates, and trees grow upward with spindly trunks and few side branches. The whole plant is permeated with phytochromes, so trees act like large eyes, sensing color over their entire bodies. Ralph Waldo Emerson, who claimed to be a transparent eyeball, open to the woods, would perhaps appreciate the trees’ superior abilities in this regard.

The vegetation directly under the gap is unmistakably transformed by the infusion of light, but the canopy’s rupture also bleeds sunshine into the surrounding forest, even to the mandala, which is firmly nestled under an umbrella of maple and hickory. Saplings are growing faster on the eastern side, and branches that face this direction are more vigorous than their westerly peers. The mountain slope here has a northeastern tilt, so the light gap reinforces a preexisting bias.

The ground-hugging layer of herbaceous plants is also affected by the gap. Leafcups are absent from the western half of the mandala and grade upward from stunted plants at the mandala’s center to vigorous ankle-high individuals at the eastern edge. These plants are adapted to growing in forest gaps and reach knee height in the center of the opening. The tallest plants will reach my shoulders next year when they flower after completing their second and final year of growth. The other herbaceous plants,
Hepatica
and cicely, show little sign of leaping
at the light and seemingly grow just as well in the shady western half of the mandala as in the east. This surface uniformity may hide more subtle effects, because these plants respond to higher levels of light not by growing taller but by setting more seed or sending out more rhizomes.

Within five years, the gap will be choked with saplings racing for the canopy. Mature trees on the edges will reach into the gap, grabbing light from above the saplings. In ten years, one or two of the saplings will have won and the dozens of losers will be dying. This struggle is short in comparison to the centuries that mature trees live once they reach the canopy, but intense competition among young trees has a large effect on the forest’s composition. In the diverse forests of Tennessee, no one species consistently wins the sprint for the canopy, a reflection of the variegated soil and temperate climate.

The fallen hickory tree and the broken twig are two points on a broad continuum of disturbance in the canopy. On one end of this continuum are massive disruptions such as hurricanes, which visit rarely, every hundred years at most in this part of Tennessee. At the other end of the spectrum are the tiny holes left in the canopy’s scaffolding by the squirrel’s damaging feet. These holes are short-lived and small-scale, creating the sunflecks that boost the growth of ephemerals and low-growing saplings. Wood rot and winter ice storms also create small holes in the canopy. I hear a large branch crashing down every few hours, particularly in winter. Disturbance on an intermediate scale is also common, with windstorms being by far the most regular source.

Storms in the forest have a more primal character than storms playing over tamed urban land. A vigorous cloudburst is exhilarating, a burst of sensory delight with its leafy smells, gray light, and chill. But a full tree-ripping storm pushes the senses beyond exhilaration or thrill, into fear. As rain patter turns to squall, the canopy heaves under the pressure of the wind. Tree trunks saw to and fro, flexing beyond what looks possible, then slashing back. All my senses wake, my eyes
dart. Then the floor bucks. As trees oscillate, they yank on roots and lift the ground. As if I’m walking on the deck of a yawing boat, my feet stumble. The storm confuses—my eyes are blurred by streaming rain, ears jammed with the roar of wind in leaves; the ground lurches underfoot. This confusion focuses into an urge to run, but unless rocks or other shelter is nearby, running goes nowhere safe. Periodically a severed tree limb crashes through the branches. The imagination takes off, and every snap becomes the sound of falling timber. In these storms I’ll either scamper to shelter, if it is available, or huddle against a sturdy-looking trunk, feeling its weight heave against my back. What I fear most is the fall of a full-sized tree, but the fear has no outlet, so I sit wide-eyed until the storm eases. At the storm’s peak, there is strange comfort in my powerlessness. Nothing I do can influence the lashing world that I’m caught in, so surrender follows, and with it comes a curious state: mental clarity wrapped in an electrified body.

Violent storms hit the mountainside dozens of times each year. But they are short-lived, and their physical damage tends to be focused on small areas—a patch of older maples here, a loose-rooted giant buckeye there. The forest is pockmarked with the gaps formed by these falls. For some species, such as the sugar maples, a canopy opening offers an accelerated route to the top. But maples are shade tolerant, so their growth does not require canopy openings. For other species, however, gaps are the only hope. Tuliptrees and, to a lesser extent, oaks, hickories, and walnuts need bright light to grow, so the persistence of these species depends on the irregular patchwork of disturbance across the forest. The tuliptree seeds that landed in the shady part of the mandala have little hope of germinating and surviving their first year. Those that landed twenty feet east of here will slake their greedy thirst for sunlight and vie to be the one-in-a-million seed that fulfills its potential and reaches the canopy.

The canopy’s renewal depends paradoxically on its being split open to let light reach the ground. Any change in the dynamics of gaps will therefore affect the viability of the forest. This makes me particularly
concerned about the spindly tree growing at the back of the gap next to the mandala. This tree has grown several feet since the spring, thrusting its two-foot-wide, heart-shaped leaves into the opening. Fast-growing alien species such as this
Paulownia tomentosa
, or Princess Tree, are spreading through the eastern forests, taking over the forest by invading light gaps and outgrowing native species.
Paulownia
, and its invading partner,
Ailanthus altissima
, the Tree of Heaven, produce thousands of wind-dispersed seeds and thereby spread rapidly. They especially favor roadside edges and logged forests, but like most pioneers will readily invade openings after smaller-scale disturbances.

Fast-growing invaders are particularly harmful to the regeneration of native trees that require full sunlight to grow: oaks, hickories, walnuts, tuliptrees. When
Paulownia
and
Ailanthus
sprout in a gap, they smother the slower-growing natives. In forests that are heavily disturbed by fire, logging, or housing development, nonnative trees can quickly erode native tree diversity.

The study of twigs seems esoteric. But this impression is dangerously wrong. Counting back through bud scars, tallying yearly growth, I not only see the struggle among native and foreign trees, I read the ledger of the world’s atmosphere. Each twig yearly adds a few inches, and these inches, combined across the forest, create one of the world’s biggest stores of carbon.

When we count all new growth—twigs, leaves, thickening trunks, extended roots—the mandala likely took ten or twenty kilograms of carbon from the air this year, a pile of sticks about as big as a small car. Summed over the world’s surface, forests contain about twice as much carbon as the atmosphere, over a thousand million million tons. This vast store is our buffer against calamity. Without forests, much of the carbon would be in the air as carbon dioxide gas, baking us in an awful greenhouse.

As we’ve burned oil and coal, we’ve returned long-buried carbon
stores to the atmosphere. But forests have saved us from the full brunt of the resulting climate change. Half our burned carbon has been absorbed by forests and the oceans. Lately, this buffering effect of forests has diminished—there is a limit to the rate at which trees can soak extra carbon from the atmosphere, especially as we accelerate our burning of fossil fuel. Nonetheless, forests continue to shield us from the more dire consequences of our profligacy. The study of twigs and bud scars is therefore the study of our future well-being.

I
lie facedown at the edge of the mandala, readying myself for a dive under the surface of the leaf litter. The red oak leaf below my nose is crisp, protected from fungi and bacteria by the drying sun and wind. Like the other leaves on the litter’s surface, this oak leaf will remain intact for nearly a year, finally crumbling in next summer’s rains. These surface leaves form a crust that both hides and makes possible the drama below. Protected under the shield of superficial leaves, the rest of autumn’s castoffs are pulverized in the wet, dark world of the litter. Yearly, the ground heaves like a breathing belly, swelling in a rapid inhalation in October, then sinking as the life force is suffused into the forest’s body.

Below the red oak leaf, other leaves are moist and matted. I tease away a wet sandwich of three maple and hickory leaves. Waves of odor roll out of the opening: first, the sharp, musty smell of decomposition, and then the rounded, pleasant odor of fresh mushrooms. The smells are edged with a richer, earthy background, the mark of healthy soil. These sensations are the closest I can come to “seeing” the microbial community in the soil. The light receptors and lenses in my eyes are too large to resolve the photons bouncing off bacteria, protozoa, and many fungi, but my nose can detect molecules that waft out of the microscopic world, giving me a peek through my blindness.

A peek is about all that anyone is given. Of the billion microbes that live in the half handful of soil that I have exposed, only one percent can
be cultivated and studied in the lab. The interdependencies among the other ninety-nine percent are so tight, and our ignorance about how to mimic or replicate these bonds is so deep, that the microbes die if isolated from the whole. The soil’s microbial community is therefore a grand mystery, with most of its inhabitants living unnamed and unknown to humanity.

As we chisel away at the edges of this mystery, jewels fall out of the eroding block of ignorance. The earthy smell that embraces my nose comes from one of the brightest jewels, the actinomycetes, strange semicolonial bacteria from which soil biologists have extracted many of our most successful antibiotics. Like the healing chemicals in foxglove, willow, and spirea, the actinomycetes use these molecules in their struggle with other species, secreting antibiotics to subdue or kill their competitors or enemies. We turn this struggle to our advantage through medicinal mycology.

Antibiotic production is a small part of the huge and varied role of actinomycetes in the soil’s ecology. There is as much diversity within the feeding habits of this group of bacteria as exists within all the animal kingdom. Some actinomycetes live as parasites in animals; others cling to plant roots, nibbling on them while fighting off more damaging bacteria and fungi. Some of these root dwellers may turn against their hosts and kill the plant by belowground assassination. Actinomycetes also coat the dead bodies of larger creatures, breaking them down into humus, the dark miracle ingredient of productive soils. Actinomycetes are everywhere but seldom enter our consciousness. Yet we seem to have an intuitive understanding of their importance. Our brains are wired to appreciate their distinctive “earthy” smell and to recognize the aroma as the sign of good health. Soil that has been sterilized, or that is too wet or dry for most actinomycetes, smells bitter and unfriendly. Perhaps our long evolutionary history as hunter-gatherers and agrarians has taught our nasal passages to recognize productive land, giving us a subconscious tie to the soil microbes that define the human ecological niche.

The other members of the microbial community are harder to pin down in the complex smell that rises from the earth’s abdomen. Fungal spores contributed to the acrid mustiness; bacterial decomposers released sweet aromas from the remains of dead leaves. Tiny wafts of methane rise up from sodden patches where anaerobic microbes hide. Many other microbes live beyond the reach of my nose. Bacteria grab nitrogen from the air and pass it into the biological economy. Others take nitrogen from dead creatures and send it back into the air. Protists graze on fungi and bacteria that encrust decaying leaves. This secret microbial world has existed for a billion or more years. The bacteria, in particular, perform biochemical tricks that have fed them since the earliest years of life, three billion years ago. The smell in my nose therefore comes from a hidden world that is broad and deep, complex and ancient.

Microbes may be invisible, but my window into the soil offers plenty else to see. Lightning-white fungal strands crackle over black leaves. Pink hemipteran bugs dance around orange spiders. A ghostly white springtail moves over the dark crumbs of last year’s decayed leaves. Everything lives in miniature. A buried maple seed towers over the animals like a mansion dwarfing its owner. The largest living creature is a rootlet, one tiny part of a plant, perhaps a sapling or tree. It is barely thicker than a pin, but it dominates the small hole that I have bored into the litter.