The Forest Unseen_A Year's Watch in Nature

by David George Haskell

  A forest without large herbivores is an orchestra without violins. We have grown accustomed to incomplete symphonies, and we balk when the violins’ incessant tones return and push against the more familiar instruments. This backlash against the herbivores’ return has no good historical foundation. We may need to take the longer view, listen to the whole symphony, and celebrate the partnership between animal and microbe that has been tearing at saplings for millions of years. Good-bye shrubbery; hello ticks. Welcome back to the Pleistocene.

  February 16th—Moss

  The mandala’s surface is a tumult of water, crackling as the clouds fire volleys, pause, then loose more artillery. Battalions of rain blown in from the Gulf of Mexico have assaulted the forest all week. The world seems made of flowing, exploding water.

  Mosses exult in the wetness. They arch into the rain, swollen green. Their transformation is remarkable. Last week they hung parched and bleached on the mandala’s rock faces, beaten down by winter. No longer. Their bodies have tapped the clouds’ energy.

  My own wintertime desiccation has created a thirst for wet, green renewal that moves me to a closer look. I lie at the mandala’s edge and lean my face to the mosses. They smell of earth and life, and their beauty rises exponentially with nearness. I am greedy for more and pull out a hand lens, pressing my eye against it as I creep closer.

  Two types of moss intermingle on the rock face. Without removing them to the laboratory to examine the shape of their cells under a microscope, I cannot definitively identify them, and so I observe them without naming. One species lies in fat ropes, each rope wrapped in closely spaced leaflets. From a distance the stems look like living dreadlocks; a closer view shows the leaflets are arranged in repeating graceful spirals, like green petals repeated over and over. The other species stands erect, its stems branching like miniature spruce trees. The growing tips of both species are green as baby lettuce. Color darkens behind the tips, shading into the olive green of mature oak leaves. Luminosity dominates this world; each leaf is one cell layer thick, so light dances and flows through the moss, giving it an internal glow. Water, light, and life have united their powers and broken winter’s lock.

  Despite their verdant vigor, mosses get little respect. Textbooks write them off as primitive holdouts from an earlier time, prototypes that have been superseded by more advanced plants such as ferns and flowering plants. This notion of mosses as evolutionary leftovers fails on several counts. If mosses were backward hicks dying out in the face of superior modernity, we would expect to see fossil evidence of an early period of glory, followed by a slow descent into obscurity. But the scant fossil evidence shows the reverse. Further, fossils of the first primitive land plants bear scant resemblance to the carefully arranged leaflets and elaborate fruiting stalks of modern mosses.

  Genetic comparisons corroborate the fossils’ story, showing that the plants’ family tree split into four main branches, each of which has been separated from the others for nearly five hundred million years. The order in which these branches split is still controversial, but the liverworts, creeping alligator-skinned lovers of stream margins and wet rock faces, may have been the first to diverge. The ancestors of the mosses broke away next, followed by the hornworts that are the closest relatives to ferns, flowers, and their kin. Mosses have evolved their own way of being, a way that is not now, nor ever was, just a halfway house to a “higher” form.

  I gaze through my hand lens and see water caught everywhere in the moss. In the angles between leaves and stems, water is caught in curved silver pools, trapped by surface tension. Droplets don’t flow, they clasp and climb. Moss seems to have erased gravity and conjured rising snakes of liquid. This is the world of the meniscus, the lip of water that pulls itself up the wall of a glass cup. And moss is all glass edge, an architecture that invites then traps water in its labyrinthine core.

  The relationship of moss to water is hard for us to grasp. Our plumbing is internal, all buried pipes and pumps. Trees likewise keep their conduits below the skin. Even our houses are plumbed from within. Mammals, trees, houses: these belong in the world of the very large. The microworld of moss operates under different rules. The electrical attraction between water and plant cell surfaces is a powerful force over short distances, and moss bodies are sculpted to master this attraction, moving and storing water on their complex facades.

  Grooves on the surface of stems wick water from the mosses’ wet interiors to their dry tips, like tissue paper dipped in a spill. The miniature stems are felted with water-hugging curls, and their leaves are studded with bumps that create a large surface area for clinging water. The leaves clasp the stem at just the right angle to hold a crescent of water. These trapped drops are interconnected by water trapped in woolly hairs and surface wrinkles. Moss bodies are swampy river deltas miniaturized and turned vertical. Water creeps from slough to lagoon to rivulet, wrapping its home in moisture. And when the rains stop, the moss has captured five to ten times as much water on its body as it contains within its cells. Moss carries a botanical camel’s hump as it trudges through long stretches of aridity.

  Mosses work out of a different architectural textbook from that of trees, but the end result is arguably as complex and certainly as successful at long-term evolutionary survival. But the sophistication of moss design does not end with the transport and storage of water. When the rains began a week ago, they triggered a cascade of physiological changes that made possible today’s lush growth. Water first wrapped the desiccated moss, then seeped into the thin wooden walls of each cell and slicked the surface of the dry raisins within. These shriveled balls were dormant living cells, and each one’s skin was primed to sponge the rain’s gift. The cells swelled, the skin pushed against the wooden wall, and life returned.

  The push of thousands of cells plumped the plant and raised the moss out of its winter slackness. At the corners of each leaf, large curved cells ballooned with water and levered the leaves away from the stem’s axis, opening spaces to hold water and angling the leaves’ faces skyward. The inner concave leaf surfaces hold water. The outer convex surfaces harvest sun and air to make the mosses’ food. The rain-induced swelling turned each leaf into both water harvester and sun catcher, root and branch.

  Inside the cells, havoc reigned. Inrushing water jumbled the cells’ innards. Wetted membranes loosened so fast that some of the cells’ contents leaked away. These sugars and minerals were forever lost to the plant, the cost of flexibility. But disorder did not last. Before drying, the moss prudently stocked its cells with repair chemicals. Now that the cells have swollen, these chemicals restore and stabilize the cells’ flooded machinery. As soon as the moistened cell regains its balance, it will replenish the supply of repair chemicals. The cell will also infuse itself with sugars and proteins that help pack away machinery when conditions dry out.

  Mosses are thus equipped at all times to cope with either drought or flood. Most other plants take a more relaxed approach to emergency preparedness, building their rescue kit from scratch when times get hard. This kit building takes time, so rapid drying or wetting will kill the laggards but not the mosses.

  Careful preparations are not the only way that mosses overcome drought. They can endure extremes of aridity that would crisp and destroy the cells of other plants. By loading their cells with sugar, dry mosses crystallize into rock candy, vitrifying and preserving the cells’ innards. Desiccated moss would be tasty were it not for the fibrous coating and bitter seasoning of the candied cells.

  Half a billion years of life on land have turned mosses into expert choreographers of water and chemistry. The lush thickets of moss over the mandala’s rocks illustrate the advantages of a limber body and nimble physiology. The surrounding trees, shrubs, and herbs still wear winter’s chains, but mosses are unshackled and free to grow. Trees cannot make use of the early thaw. Later, the tables will turn, and trees will use their roots and internal plumbing to dominate the mandala’s summer, shading the rootless
mosses below. But for now, the trees are paralyzed by their hulking size.

  The mosses’ late winter eagerness produces benefits that extend beyond their own growth. Life downstream from the mandala profits from the mosses’ hold on water. The rainstorm’s kinetic energy rakes the hillside, yet the water streaming off the mandala is clear. There is no hint of the mud and silt that bleeds from the fields and towns around. Mosses and the thick forest leaf litter sponge moisture and slow scouring raindrops, turning the artillery assault on the soil into a caress. As the water flows down the mountain, the soil is held in place by a weave of herb, shrub, and tree roots. Hundreds of species work the loom, interpenetrating their warp and weft, turning out a tough, fiber-filled denim that rain cannot tear. By contrast, fields of young wheat and suburban lawns have sparse, loose-woven roots that cannot hold the soil.

  The mosses’ contributions go beyond acting as the first line of defense against the eroding power of water. Because they have no roots, mosses harvest water and nutrients from the air. Their rough surfaces trap dust and can snatch a healthy dose of minerals from a breath of wind. When the wind carries acidity from tailpipes or toxic metals from power plants, mosses welcome the junk with wet, open arms and draw the pollution into themselves. The mandala’s mosses thus cleanse the rain of industrial detritus, clasping and holding heavy metals from car exhaust and the smoke of coal-fired power stations.

  When the rain departs, the mosses’ sponginess retains water, then slowly releases it. Forests therefore nurture life downstream from themselves, shielding rivers from sudden muddy surges and sustaining flow during dry spells. Evaporation from the wet forest creates clouds of humidity and, if the forest is large enough, generates its own rain. We usually take these gifts without consciousness of our dependence, but economic necessity sometimes jolts us out of our sleep. New York City decided to protect the Catskill Mountains rather than pay for a man-made water purification plant. The millions of mossy mandalas in the Catskills were cheaper than a technological “solution.” In some watersheds in Costa Rica, downstream water users pay upstream forest owners for the service provided by the forested land. Thus the human economy becomes modeled on the reality of the natural economy, and the incentive to tear up the forest is reduced.

  In the mandala, the rain continues its pounding. From where I sit, I hear two streams roaring, one on either side of the mandala, both at least a hundred meters away. The rain’s volume has turned the sound of their usual quiet trickle to a thundering roil. After an hour or more of huddling in my waterproof clothes, I feel oppressed by the incessant violence. But the mosses seem more at home than ever. Five hundred million years of evolution has given them mastery of wet days.

  February 28th—Salamander

  A leg flashes across a crevice in the leaf litter. The stub of a tail follows and then disappears into layers of wet leaves. I resist the urge to peel away the leaves; instead I wait, hoping that the salamander will surface again. Several minutes later, a shining head thrusts out, and the salamander sprints into the open. It pushes down another hole, reappears, bursts into a run, trips over a leaf stem, and somersaults ungracefully into a hollow. Shaken, the salamander rights itself and climbs out of the depression, finally ducking its head to slide under a dead leaf. Cold mist thickens the air, and I can see only a few feet ahead of me, but the salamander shines as if it were illuminated by a clear ray of sunlight. The dark, smooth skin is freckled with silver. Small red streaks flow down the animal’s back. The skin is impossibly wet, a cloud condensed into animate matter.

  Like mosses, salamanders thrive on moisture, but salamanders cannot use the mosses’ strategy of drying up and waiting out the days between rains. Instead, they follow cool, humid air like nomads, moving in and out of the soil as the humidity changes. In winter they creep down between rocks and boulders, escaping the freeze and living as troglodytes in the subterranean darkness, up to seven meters belowground. In the spring and autumn they climb back up and ply the leaf litter, pursuing ants, termites, and small flies. Summer’s drying heat pushes them back underground, although on wet summer nights salamanders burrow back to the surface to feast without danger of dehydration.

  The salamander is twice as long as my thumbnail. Its neck and legs are slender, marking it as a member of the genus Plethodon, perhaps a zigzag salamander or a southern redback. The fact that all Plethodon species are variably colored and poorly studied reinforces the imprecision of my identification. Then again, no one is quite sure what a salamander “species” really is, which suggests that nature doesn’t conform to our desire to draw firm lines.

  The salamander is small, so it is likely a juvenile, hatched late last summer. Its parents courted last spring, with delicate footwork and tender cheek rubbing. Salamander skin is a patchwork of scent glands, so the cheek rubs convey chemical whispers and pheromone love poems. When the couple has become acquainted, the female lifts her head and the male slides under her chest. He walks forward and she follows, straddling his tail in a conga dance for two. After a few steps, he deposits a small cone of jelly topped with a packet of sperm. He moves forward again, waggling his tail, and the female follows. She stops and uses her muscular vent to pick up the sperm. The dance breaks up and the salamanders wander on their separate ways, never to interact again.

  The female seeks out a rock crevice or hollow log in which to lay her eggs. She then wraps herself around them, remaining in the nest hole for six weeks, longer than most songbirds sit on their eggs. She rotates the eggs to stop the developing embryos from sticking to the sides. She also eats any egg that dies, preventing mold from growing and killing the whole clutch. Other salamanders may visit the nest hole, looking for an egg snack, and the brooding mother chases them off. Motherless broods invariably get infected by fungi or eaten by predators, so this vigil is crucial. Once the eggs hatch, her parental duties are finished, and the mother will renew her depleted energy reserves by feeding in the leaf litter. The young salamanders are miniature versions of the parent and strut across the forest floor, feeding themselves without assistance. The Plethodon scuttling across the mandala therefore lives its whole life without dipping a toe into a stream, puddle, or pond.

  This breeding process demolishes two myths. The first is that amphibians are dependent on water for breeding—Plethodon is a nonamphibious amphibian, as slippery to classify as it is to hold. The second myth is that amphibians are “primitive” and therefore don’t care for their young. This latter fallacy is embedded in theories about the evolution of the brain claiming that “higher” functions such as parental care are confined to “higher” animals such as mammals and birds. The mother’s careful vigil shows that parental solicitude is more widely spread in the animal kingdom than hierarchical brain scientists suppose. Indeed, many amphibians care for their eggs or their young, as do fish, reptiles, bees, beetles, and a menagerie of doting “primitive” parents.

  The juvenile salamander in the mandala will spend another year or two feeding in the leaf litter before it is large enough to become sexually mature. Plethodon sets to this task of feeding with carnivorous gusto. Salamanders are the sharks of the leaf litter, cruising the waters and devouring smaller invertebrate animals. Evolution has discarded Plethodon’s lungs to make its mouth a more effective snare. By eliminating the windpipe and breathing through its skin, the salamander frees its maw to wrestle prey without pause for breath. Plethodon has struck a deal with evolution’s Shylock: better tongues bought with a few grams of lung. The salamanders are living it up on their three-thousand-ducat loan, conquering the wet leaf litter across the eastern forest. The gamble is paying off at present, but the usurer may yet call in the debt. If pollution or global warming changes conditions in the leaf litter, Plethodon species will be ill suited to cope. Indeed, projections of habitat change caused by global warming suggest that mountain salamanders will suffer major declines as their cool, wet habitats disappear.

  No one knows how Plethodon salamanders arrived at their lu
ngless condition. Their relatives all have lungs, although those that live in mountain streams have rather small lungs. Cold streams have plentiful oxygen, so stream-dwelling salamanders can use their skin as a breathing organ. Perhaps the terrestrial lungless salamanders evolved from these stream-dwelling kin? This was biologists’ favorite explanation until researchers looked more closely into the geological record. The rocks told an inconvenient story: the eastern mountains were small undulations when Plethodon salamanders evolved. Such gentle inclines could not have produced the cold, rapid streams inhabited by the small-lunged salamanders. So, we are left without a historical narrative for the Plethodon’s lungless condition.

  The mandala is almost large enough to contain the whole world of this animal. Adults are territorial and rarely stray more than a few meters; some individuals move farther downward into the soil than they do across the surface of the litter. This rootedness accounts for the diversity of woodland salamanders. Because they seldom move far, the salamanders on different sides of a mountain or valley are unlikely to interbreed. Local populations therefore adapt to the particularities of their habitat. If this divergence keeps up for long enough, separate populations may come to look different and have different genetic characteristics. Some may even get called different “species,” depending on the current taxonomic fashion. The Appalachian Mountains are ancient rocks, and their southern end, where the mandala sits, has never been covered by a killing sheet of ice-age glaciers. The salamanders here have therefore had time to explode in a burst of diversity that is unmatched anywhere on the planet. This diversity partly accounts for why salamanders are so difficult to classify into species.