The Forest Unseen: A Year's Watch in Nature (6 page)

Fossil evidence of these large herbivores abounds in caves and swamps across the eastern United States. These fossils provided fuel for the nineteenth-century debate about evolution. Darwin thought these animals were further evidence for the idea that the natural world is always in flux. He commented, “It is impossible to reflect on the state of the American continent without astonishment. Formerly it must have swarmed with great monsters; now we find mere pygmies compared
to the antecedent, allied races.” Thomas Jefferson disagreed, believing that giant sloths and other creatures must still be alive. After all, why would God create them, then kill them off? Creation reflected God’s perfect handiwork, therefore nature would unravel if pieces were allowed to fall away. Jefferson instructed the explorers Lewis and Clark to bring back reports of these creatures from their trek to the Pacific coast. The expedition found no evidence of living mastodons, sloths, or any other extinct creatures. Darwin was right; pieces of creation can be destroyed.

Like the footprints left by the deer’s visit to the mandala, the passing herbivores have left signs in the architecture of some of our native plants. Honey locust and holly trees have thorny stems and leaves. These thorns are deployed only to three meters’ height, twice as high as any living herbivore can reach but exactly the right height to deter the extinct mega-browsers. The honey locust is doubly lost because its seedpods, which are two feet long, are too large for any living native species to consume whole and thus disperse the seeds, although they are perfectly sized for large extinct herbivores such as mastodons and ground sloths. Osage orange’s milky softball is another fruit whose seed-dispersing partner has died. Similar fruits on other continents are eaten by elephants, tapirs, and other large herbivores of the kind that exist only as fossils in North America. These widowed plants wear history on their sleeves, giving us a glimpse into the bereavement of the whole forest.

The structure of ancient forests is forever hidden from us, but the bones of extinct browsers and the stories of the first Americans suggest that this was not an easy place for shrubs and saplings to thrive. North American forests have experienced fifty million years of browse, followed by ten thousand years of drastically reduced mammalian herbivory, then one hundred strange years of no browse at all. Might the ancient forests have been patchy and sparse, kept trimmed by herds of wandering herbivores? Certainly these herbivores had enemies of their own, which are now gone, or almost so. The sabertooth cat and the dire
wolf are extinct; the gray wolf, mountain lion, and bobcat are rare. In the western United States, the giant American lion and the cheetah both preyed on the plant browsers. The existence of these many species of large carnivore is further evidence of the abundance of herbivores. Giant cats and wolves need giant herds of food. The only places in the world that sustain large populations of carnivores are well endowed with browsers. After all, carnivore flesh is just plant matter passed up the food web. So, abundant fossil evidence of large predators is strong evidence for heavy browse on plants.

Humans have eliminated some predators but have lately added three new deer-slaying creatures: domestic dogs, immigrant coyotes invading from the west, and automobile fenders. The first two are effective predators of fawns; the latter is the main suburban killer of adults. We face an impossible equation. On one hand, we have the loss of tens of species of herbivores; on the other we have the replacement of one type of predator by another. What level of browse is normal, acceptable, or natural in our forests? These are challenging questions, but it is certain that the lush forest vegetation that grew in the twentieth century was unusually underbrowsed.

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.

T
he 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.