Just as the forest itself can only be described in superlatives, the same is true of the geographical setting. The dimensions of the Amazon basin are unlike those of any other river valley in the world. The conventional conception of a river valley is of a meandering depression that drops steadily toward some sea or lake, some destination where water can find its own level and come to rest. But the Amazon basin is a great flat plain, shaped more like a crepe pan than a bowl, that covers an area nearly the size of the contiguous United States. The preponderance of the river system lies within Brazil, but the Amazon’s tributaries extend into eight other countries that ring the basin from northeast to southwest: French Guiana, Suriname, Guyana, Venezuela, Colombia, Ecuador, Peru, and landlocked Bolivia. To the north and south, the basin is contained by the Guyana Shield and the Brazilian Shield. These weathered uplands were formed in the Archean era, more than two billion years back in Earth’s five-billion-year history. The rock, scoured by eons of rain and wind, is some of the oldest material on the surface of the planet. The western rim of the basin is the towering wall of the Andes mountain range, which curls like a spine along the Pacific coast of South America, never more than 100 miles from the ocean. On the time scale on which geological changes are measured, the Andes are newborn, having pushed upward some twenty to forty million years ago when the plates of the earth’s dynamic crust beneath the Pacific collided with the continent.
There is geological evidence that before the Andes rose, much of the basin drained west into the Pacific. Once the mountain chain blocked that route, the basin filled with water and became a brackish, landlocked lake. Over time, hundreds of feet of sediment accumulated on the bottom. Then, somewhere around the site of the present town of Obidos, this inland sea broke through and forged a route east to the Atlantic, carving a channel that now is more than 200 feet deep in places, making the Amazon the deepest river in the world. Today, the Amazon has its beginnings in the snows of the Andes. Some 18,000 feet above sea level, at a place in southern Peru just over the continental divide from a river that cuts westward down to the Pacific coast, a lacy network of ice-fed streams joins in a downhill rush to form the most distant headwaters of the river. Although it starts its life just 100 miles from the Pacific, the river seeks its lowest level in the opposite direction and must travel 4,007 miles before it reaches an ocean. The river is perpetually replenished by the water vapor that sweeps west from the Atlantic, falls as rain, evaporates from the forest’s own humid mass, blows farther west, and then finally collides with the mountains and falls as precipitation once again.
After its initial plunge—some 16,000 feet in the first 600 miles —the river steadily gathers strength from a widening system of tributaries and takes on the name Solimões. The Solimões, which at its source rips away at the young, crumbling rock of the still-rising Andes, is so heavily laden with silt that it is opaque. It settles down as it reaches the flat basin, and the rest of the way to the Atlantic is barely a downhill ride. In fact, over its final 2,000-mile run to the ocean, the total vertical drop of the Amazon is just a little over 100 feet.
Near the geographical center of the basin, the Solimões is joined by a major river running south from the ancient Guyana Shield, the Rio Negro. The Rio Negro is clear but as dark as strong tea, containing little sediment but many dissolved organic compounds leached from rotting vegetation. (The similarity to tea is no accident; among the dissolved compounds are tannins, the same organic acids that give tea its hue.) Once these two great streams are joined, the river takes on the name Amazon and acquires a pale tan hue.
For the next 600 miles, the distance between banks is often 5 miles or more, occasionally 18 miles. In the widest stretches, it is difficult to see the riverbanks from the deck of a small boat in the central channel. Parallel to the river, the land appears to have been scratched deeply by the claws of a gargantuan cat. The furrows, many of them flooded, are created during the wet season, when the water overflows the banks and scrapes away at the landscape.
The mouth of the Amazon is incompletely plugged by a grassy island the size of Switzerland. There is no way to visualize the volume of water that flows past this island; suffice it to say that it is greater than the combined flows of the next eight rivers on the ten-biggest list. It is a fifth of all the fresh water flowing from all the world’s rivers into all the world’s oceans. Where the river reaches the coast, a tan stain blossoms over the blue ocean, billowing outward as if someone has just spilled a great pot of tea with milk. More than 100 miles from the coast of South America, the silty exhalation of the river still flows fresh and unmixed atop the denser salt water. It was this freshwater outflow that in February of the year 1500 caught the attention of the first European to find the river—a Spanish sea captain who had previously sailed with Christopher Columbus. Vincente Yanez Piñon followed the freshwater current to the mouth of the river, which he named the Sweet Sea, failing to conclude that it might be a river because it so dwarfed any he had previously seen.
Although the forests of the Amazon cover little more than 2 percent of the earth’s land surface, they account for 15 percent of the total terrestrial plant biomass—trunks and stems and leaves and roots. The ecosystem is a high-revving photosynthetic engine, fueled by the steady, seasonless tropical sun. The plants are in metabolic overdrive; everything is either in a state of birth or death, growth or decay, attack or defense. Where one tree has fallen to crumble and rot, dozens of green seedlings and saplings suddenly spring up to vie for the blast of sunlight entering the hole in the canopy.
Despite the seeming extravagance, it is an efficient, miserly system that picks up after itself and wastes nothing. Anything that dies is quickly dismantled and reabsorbed. The Amazon forests must be self-sufficient, because the ancient soils beneath most of the basin have long ago been leached clean of minerals and nutrients. Only 4 percent of the Amazon has what agronomists consider fertile soils.
Water that evaporates from one leaf soon condenses again and falls as rain. The dense sieve of roots and fungi that carpet the forest floor allows few of the nutrients in the rainwater to escape into the streams, rivers, and eventually the sea. Indeed, stray compounds have been so completely filtered from some Amazon tributaries that the water is almost as pure as if it were distilled.
The vastness of the Amazon rain forest contributes to the impression that it is a continuous, unchanging mass of jungle. But the region actually includes many forest types. When the rains of the wet season combine with the maximum snowmelt, the rivers of the Amazon basin rise 40 feet or more, inundating enormous stretches of forest. This seasonally flooded forest—called várzea or igapó, depending on which part of the basin you are in—makes up only 2 percent of the total forest area. Even so, a majority of the Amazon’s human population lives in the flooded portions, probably because that is where the soils are regularly enriched by silt and because these areas are close to the rivers. Until roads were cut into the forest, almost all significant human settlement occurred within a few miles of the rivers.
The flooded forest is an ecosystem all its own, where plants must survive for months underwater and fishes swim for part of the year through the treetops, eating fruit and playing an important role in distributing seeds. The terra firme, firm ground, that makes up the rest of Amazonia tends to have aged soils that have been washed clean of nutrients over millennia. In some areas, there is natural savanna, in others, low forests that are little more than a tangle of lianas. But huge swathes of terra firme support rich rain forest. This forest has evolved in such a way that it is nearly independent of the substrate. The system feeds itself and waters itself, recycling nutrients and holding water in its biomass like an enormous sponge. It is somewhat like hydroponic agriculture, in which plants can thrive on sterile sand—or even suspended in racks—as long as they receive moisture, nutrients, carbon dioxide, and sunlight.
Within any single patch of rain forest, there is also an initial impression of monotony: all you see are
columnar trunks, thickets of vines and creepers, mats of decaying leaves. It is only after you have walked for a while in a mature stand of rain forest that individual elements begin to stand out: trees with flying buttresses, hanging plants, climbing plants, plants with fruit clustered high in the air, or plants—like cocoa—in which the fruit grows directly out of the tree trunk. You notice that your feet are intermittently shuffling through foot-long canoe-shape leaves, then plate-size hand-shape leaves, then a dusting of purple flower petals dropping from unseen blossoms 100 feet above. A Morpho butterfly flits past, like an animated origami masterpiece folded from a sheet of electric-blue foil (one naturalist described these seven-inch forest dwellers as “the bluest things in the world”). As you walk on, a tree above you clacks quietly in the wind and a woody pod falls at your feet. It is the seed pod of Cariniana micrantha, a relative of the Brazil nut. Out of its end pops a perfectly fitted cap. In the exposed cavity you see tightly packed regiments of seeds, each with a feathery tail that would have allowed it to soar away from the parent tree—if the monkey that dropped the seed pod had left it on the tree to ripen a little longer.
The dizzying complexity of the forest exists at all levels. Every time you turn to focus on a particular object, perhaps a rotting log, that object then splits into individual elements—fungi, beetles, ants, and a pile of aromatic wood dust where a nest of termites has been hard at work. Look even more closely, and those fragments would split apart yet again. The leaf-cutter ants crossing the log are carrying chunks of plant material into their subterranean fungus garden. Spores of the fungus are planted on this food source, and later the ants harvest the fruiting bodies that the fungi produce. The ants cannot digest the leaves themselves, and the fungi have evolved to an extent that they can no longer live anywhere but in nests watched over by ants. Meanwhile, the termites’ guts harbor bacteria without which they cannot digest the tough cellulose skeleton of the tree. The beetles are attacking a pile of monkey droppings. And, at the point where the log appears to be slowly sinking into the earth, a thick mat of fungal threads called mycorrhizae are reducing the dead wood to its chemical constituents. These constituents include traces of minerals such as phosphorus, a coveted commodity in Amazon forests.
The fungi have a symbiotic relationship with the surrounding trees, and if you were able to trace the strings from which that fungal mat is woven, you would see that they emanate from nearby shallow tree roots. Although the fungi are a separate organism from the tree, neither can thrive without the other. Where the fungus interlaces with the tissue of the tree root, there is an ongoing exchange of goods. The tree provides the fungus with carbon-based compounds that help it grow, and the fungus provides the tree with recycled phosphorus and other minerals. Slowly, as you absorb more and more of the details around you, the spectacular complexity and interrelatedness of the rain forest become overwhelmingly apparent.
Even so, that which can be seen at eye level or on the forest floor is just a taste of the richness of the forest. It is in the canopy—the topmost layers of branches and foliage 100 feet or more above the floor—that the incredible biological bounty is most apparent. Many researchers have noted the similarity between tropical rain forests and coral reefs. In both ecosystems, life occurs in strata, with the richest array of life forms in the layer closest to the sun. In the case of a coral reef, that is the shallowest part, where innumerable fishes and invertebrates rely for their food on phytoplankton and corals sustained by photosynthesis. In the rain forest, it is the canopy. This stratum has been called the last great unexplored frontier of the natural world. While the coral reef has been made accessible by scuba gear, there is still no simple way to wander through the treetops. In the shadows beneath the canopy, less and less grows and thrives until finally—on the equivalent of the deep sea floor, where little light penetrates—there is the thin brown layer of rotting mulch and the tangle of runners and buttressed tree roots, much of which is simply there to support the rich community far above.
The forest’s layered look has been determined by the location of the crucial fuels that it needs to thrive. The architecture of the forest has resulted from a sort of tug-of-war between the need to absorb water and nutrients from the earth below while competing with neighboring plants for the light coming from the sun above. The elevated canopy came into being as competing plants evolved different ways of reaching above each other. Even though the tropics are bathed in almost twice as much sun as regions at the latitude of Paris, there never seems to be enough to go around.
Evolution has solved this dilemma in several ways. The most straightforward is the tree trunk. The forest giants, such as the Brazil nut tree, rise twenty stories or more, hoisting their foliage above the main mass of the canopy. In these enormous trees, water and nutrients are pulled up through the trunks by the vacuum created as water evaporates from the surface of leaves far above; the pull of the vacuum can exceed 2 to 3 tons per square inch. Farther down, in the understory, is a mixture of palms and slimmer trees that tolerate perpetual twilight. There are palms on stilts, palms covered with spines, palms that climb like vines, palms with berries, and palms with nuts. And the forest floor is covered with saplings and seedlings that remain stunted until some tall neighbor comes crashing down and allows a flood of sunlight to pour onto the plants below.
The large trees expend an enormous amount of their productive energy in getting their green leaves as high as possible. Other plants have different strategies: climbing or perching. Myriad epiphytes —air plants—grow high in the canopy, their dust-fine seeds having lodged in the crooks of tree branches. They include ferns, orchids, peppers, and even cacti. The epiphytes are not parasites; they derive their nutrients from the rainwater coursing down the tree trunks to which they cling. Normally, rainwater alone contains insufficient nutrients to nourish a plant. But in the rain forest, as the rain splashes onto a leaf or dribbles down a stem, it absorbs organic and inorganic compounds from the surfaces of the plants or from excrement deposited by the many animal residents of the canopy. Thus, by the time a raindrop reaches one of the suspended epiphytes, it may have concentrations of nutrients such as nitrogen, phosphorus, and potassium that are anywhere from fifteen to sixty-five times higher than in normal rainwater. Some epiphytes—particularly the bromeliads, relatives of the pineapple—have evolved fleshy, waxy leaves that form a rosette which catches rainwater and organic debris that falls from the tree crowns farther up. Some of these reservoirs can hold as much as 10 gallons of water. These living cisterns have in turn become a home for frogs—and there are species that live only in certain bromeliads.
Other plants, such as the lianas that form an impassible tangle in parts of the Amazon, use larger trees as a ladder on which they climb toward the sun. Some of these coiled vines would measure up to 3,000 feet if straightened out. They have evolved some intricate strategies for reaching the sunlit heights. A botanist named Tom Ray studied the climbing behavior of a vine species with the ghoulish name Monstera gigantea; he found that as a seedling, this vine initially grows toward the darkest place within reach. Most of the time, the darkest spot in the forest is the base of the largest tree. Once the vine finds a tree, somehow it changes strategies and begins to seek sunlight.
Then there are the true parasitic plants, such as the strangler fig, which begins its life as an epiphyte high in the canopy where it first lodged as a seed excreted by a bird or monkey. The young plant unreels long, thin roots that resemble dangling bungee cords and spreads its parasol of leaves to catch the sun. If a root touches down in a spot relatively rich in leaf litter, the plant begins to thrive as nutrients get pumped up the roots. Quickly, more roots descend. Over a period of years, the thickening mass of dangling roots begins to fuse and can eventually completely encase the host tree, as if it had been dipped in cement. The mummified host dies and rots, providing a rich source of nutrients for the fig, which has made the transition from something you might see hanging in a macramé sling in a garden sho
p into a massive, 150-foot-tall giant.
To anyone from a temperate latitude, it is a bit startling to learn that a veteran tropical botanist can come upon an Amazonian tree and matter-of-factly state that he does not know its species. Yet it happens all the time, and it is a testament to the diversity of living things in a rain forest that such confusion still reigns. The Amazon is one of the few places on Earth where the description of a new species is not something to shout about. One icthyologist, Michael Goulding, has described 400 species of fish from the flooded forests around Manaus—a fifth of the 2,000 known Amazon fish species (and there may well be 3,000 or more in all). While the most diverse forests in the world’s temperate zones, in Appalachia, have no more than 25 species of tree, one 2.5-acre plot of rain forest outside Manaus was found to harbor 414 tree species. And a single tree has been shown to hold 1,500 species of insect. The Amazon basin has 20 percent of the world’s bird species. Overall, biologists have not even determined to a factor of ten how many species of plant, insect, fish, amphibian, reptile, bird, or mammal inhabit the Amazon basin. As recently as 1983, some were estimating that there are 1 million species of Amazonian plants and animals; more recent estimates top 10 or 15 million.
The Burning Season Page 4
