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Once upon a time, before the freezing embraces of the Ice Age, thick evergreen forests blanketed much of the world’s warm, humid surface. Today’s tropical rainforests—the densest and richest proliferation of plants ever known—are the survivors of these primeval jungles of ages past.
These forests, the largest of which is Brazil’s Amazon jungle, are found in a narrow belt that girdles the earth at the equator. In the tropics, constant sunlight, endless rains, and high temperatures year-round spell life. The steamy atmosphere and fast nutrient turnover have promoted favorable growth conditions and intense competition, allowing the forest flora to evolve into an extraordinary multitude of different species, exploiting to the full every conceivable niche. Tropical rainforests contain more than half of all living things known to man.
Only superficially does the rainforest resemble the fictional jungles of Tarzan. Yes, the foliage can indeed be so dense that you cannot move without a machete. But since only about 10 percent of the total sunlight manages to penetrate through the forest canopy, the undergrowth is generally correspondingly sparse, and the forest floor surprisingly open and relatively easy to move about in. (The plants array their leaves to avoid leaf shade; others are shaded purple underneath to help reflect back the light passing through the leaf; the “walking palm” literally walks across the forest floor in search of light on its stiltlike roots.)
The stagnant air is loaded with moisture. To a visitor, the tropical rainforest seems always the same: uniform heat and stifling 90 percent humidity. But this is true only near the ground. High in the tops of the trees, where the sun comes and goes, breezes blow, and moisture has a chance to be carried away, the swings in temperature between day and night are as much as 15 degrees, and the humidity may drop from 95 percent, its fairly constant nighttime level, to as low as 60 percent as the sun rises and warms the forest. Thus, within 30 vertical meters, two distinctly different climates prevail. Tropical rainforests are places of peace and renewal, like a vast vaulted cathedral—mysterious, strangely silent, and of majestic proportions.
Botanists have distinguished among 30 or so different types of rainforest. Tropical evergreen rainforest exists in areas of high rainfall (at least 200 cm) and regular high temperatures averaging no less than 25°C. In Costa Rica, the lush tropical evergreen rainforest of the Caribbean lowlands gives way on the Pacific side to a seasonally dry evergreen forest in the well-watered south.
While in temperate forests distinct species of flora congregate neatly into distinctive plant “neighborhoods” with few other species interspersed, in the rainforest you may pass one example of a particular tree species, then not see another for half a mile. In between, however, are hundreds of other species. In the rainforest, too, life is piled upon life—literally. The firm and unyielding forest floor is a “dark factory of decomposition,” where bacteria, mold, and insects work unceasingly, degrading the constant rain of leaf litter and dislodged fruits into nutrient molecules.
Fungi proliferate, too. They are key to providing the nourishment vital to the jungle’s life cycle. While a fallen leaf from a North American oak may take a year to decompose, a leaf in the tropical rainforest will fully decay within a month. The trees suck up the minerals and nutrients through a thick mat of rootlets that grow close to the surface of the inordinately thin soil. To counteract their inherent instability, many species grow side buttresses: wafer-thin flanges that radiate in a ring around the base of the tree like the tail fins of rockets.
For every tree in the jungle, there is a clinging vine fighting for a glimpse of the sun. Instead of using up valuable time and energy in building their own supports, these clutching vines and lianas rely on the straight, limbless trunks typical of rainforest tree species to provide a support in their quest for sunlight. They ride piggyback to the canopy, where they continue to snake through the treetops, sometimes reaching lengths of 300 meters. One species spirals around its host like a corkscrew; another cements itself to a tree with three-pronged tendrils.
The bully of the forest, however, is the strangler fig, which isn’t content to merely coexist. While most lianas and vines take root in the ground and grow upward, the strangler figs do the opposite. After sprouting in the forest canopy from seeds dropped by birds and bats, the strangler fig sends roots to the ground, where they dig into the soil and provide a boost of sustenance. Slowly but surely—it may take a full century—the roots grow and envelop the host tree, choking it until it dies and rots away, leaving a hollow, trellised, freestanding cylinder.
The vigorous competition for light and space has promoted the evolution of long, slender, branchless trunks, many well over 35 meters tall, and flat-topped crowns with foliage so dense that rainwater from driving tropical downpours often may not reach the ground for 10 minutes. This great vaulted canopy—the clerestory of the rainforest cathedral—is the jungle’s powerhouse, where more than 90 percent of photosynthesis takes place. Above this dense carpet of greenery rise a few scattered giants towering to heights of 70 meters or more.
The scaffolding of massive boughs is colonized at all levels by a riot of bromeliads, ferns, and other epiphytes. As they die and decay, they form compost on the branch capable of supporting larger plants that feed on the leaf mold and draw moisture by dangling their roots into the humid air. Soon every available surface is a great hanging gallery of giant elkhorns and ferns, often reaching such weights that whole tree limbs are torn away and crash down to join the decaying litter on the forest floor.
Sit still awhile and the unseen beasts and birds will get used to your presence and emerge from the shadows. Enormous morpho butterflies float by, flashing like bright neon signs. Is that vine really moving? More likely it’s a brilliantly costumed tree eyelash viper, so green it is almost iridescent, draped in sensuous coils on a branch.
Scarlet macaws and lesser parrots plunge and sway in the high branches, announcing their play-acting with an outburst of shrieks. Arboreal rodents leap and run along the branches, searching for nectar and insects, while insectivorous birds watch from their vantage points for any movement that will betray a stick insect or leaf-green tree frog to scoop up for lunch. Legions of monkeys, sloths, and fruit- and leaf-eating mammals also live in the green world of the canopy. Larger hunters live up there, too. In addition to the great eagles plunging through the canopy to grab monkeys, there are also tree-dwelling cats. These superbly athletic climbers are quite capable of catching monkeys and squirrels as they leap from branch to branch and race up trunks. There are also snakes here, some twig-thin, such as the chunk-headed snake with catlike eyes, which feasts on frogs and lizards and nesting birds.
Come twilight, the forest soaks in a brief moment of silence. Slowly, the lisping of insects begins. There is a faint rustle as nocturnal rodents come out to forage in the ground litter. All around, myriad beetles and moths take wing in the moist velvet blanket of the tropical night.
© Christopher P. Baker from Moon Costa Rica, 8th Edition