Forest, Tropical

With their towering trees and tiny orchids, great apes and minute insects, tropical forests are magnificent expressions of nature. Tropical forests may contain half the species on Earth, but many of those species will disappear because of continuous deforestation. More than half the original forests have already been lost, and therefore conserving tropical forests is an urgent concern for ecologists.

Tropical forests include many forest types. True tropical rain forest is warm and wet, occurring mainly near the equator, where the monthly temperature never dips below 18 degrees Celsius (64 degrees Fahrenheit), rainfall exceeds 1700 millimeters (66 inches) per year, and no month gets less than 100 millimeters (almost 4 inches). Away from the equator one usually finds drier tropical forests, and on mountains one finds cooler and wetter, montane tropical forests. At sites with extreme conditions such as flooding or very poor soils, specialized tropical forests, such as mangrove, occur.

Tree and Plant Species

True rain forest usually has 100 to 250 different tree species in 1 hectare (2.47 acres), including palms and stranglers (which germinate in a tree crown then grow up and down, eventually surrounding the host). These trees are mostly evergreen and come in many forms: with round, fluted, deeply furrowed, or spiny trunks; with swollen base or buttresses (flanges extending from lower trunk to the ground) or perched on stilt roots; with large leaves or tiny leaflets.

Added to this tree diversity are other plant types: shrubs, giant vines, herbs large and small, epiphytes (plants growing on other plants and not rooted in the ground), hemi-epiphytes (which begin life as epiphytes then extend roots to the ground), lichens, and mosses. With all these kinds of plants, and with trees falling and making gaps where regrowth proliferates, the three-dimensional structure of the forest is complex. Despite the fairly uniform climate, flowering, fruiting, and leaf production are somewhat seasonal.

In the other tropical forest types that are drier or cooler than true rain forest, or on extreme sites, there is more seasonal plant behavior, shorter trees, more deciduous trees, and fewer species, but plant diversity is still higher than outside the tropics.

Animals

Tropical forest animals are also diverse. In Borneo there are squirrels the size of mice and squirrels 75 centimeters (29.5 inches) long. Fourteen monkey species and 230 bird species frequent 100 hectares (247 acres) of mature forest at Cocha Cashu, Peru. Eighty-six frog and toad species and fifty-three snake species inhabited Santa Cecilia, Ecuador, before that forest was destroyed. Insects are the most diverse of all; a scientist found forty-six ant species in one large Brazilian tree, as many as inhabit all of Canada. A major goal of tropical forest scientists has been to describe and explain this tropical diversity.

The diversity of plants is not completely understood but is due to a combination of many factors: a long growing season; variety in growth form; specialization on particular light levels, soil types, or topography (such as slopes versus ridges); mutualisms with animals; variable conditions created by disturbances such as hurricanes; chance results of reproduction; and the tendency for common species to attract many animals to eat them, creating space for many less common ones.

Animal diversity stems mainly from specialization. Many animals specialize on the different resources provided by the great variety of plants and complex forest structure. For example, different plant species are food for different insect species, and flowers and fruit of many types support various kinds of nectar, pollen, fruit, and seed eaters, including insects, birds, bats, and monkeys. Birds catch insects in specialized ways that reflect forest structure and the activities of other species. For instance, different flycatcher species launch themselves from characteristically different perch heights to catch aerial prey; antwrens search the undersides of leaves; woodcreepers glean tree trunks; leaftossers scour the ground; hummingbirds steal prey from spider webs; and antbirds pluck insects fleeing swarming army ants.

Many animals survive in tropical forest through specialized mutualisms, in which animals and plants interact for mutual benefit. This happens, for example, when insects gather nectar from individual plants of one species and transfer pollen among them. These plants often have special features to attract particular insect species. Mutualism also occurs when animals eat fruits and scatter the seeds within, promoting plant reproduction. Some plants and animals truly live together. Cecropia trees house Azteca ants within hollow stems and feed them nectar; in return the Azteca exclude leaf-eating insects and vines from the tree.

The Effects of Deforestation

Humans have inhabited and used tropical forests for millennia, but widespread deforestation only began in the late twentieth century. Forest loss varies among countries and, depending on location, results from small-scale farming, ranching, logging followed by farming, fuelwood gathering, and other causes. At many places the cleared land is used briefly before soil nutrients are exhausted, and natural reforestation is hindered by the unsuitable conditions created for trees, especially by repeated burning, which also eats into remaining forest stands.

When the forest is cleared, the marvelous variety of plants and animals it contained is lost. Some can survive in small forest patches, but many cannot because light and moisture conditions are changed, critical mutualists are absent, or too few individuals of a given species are present to breed successfully. In addition to this loss of biological diversity, other consequences of tropical deforestation, depending on location, are the loss of native forest peoples, reduced rainfall, increased erosion, silting of coastal ecosystems , and possible net release of carbon dioxide and other gases that lead to global warming.

Concerned with these negative effects, local, national, and international groups are trying to stem tropical forest loss. Conservation strategies range from completely protecting remaining forest to promoting sustainable economic uses that leave the forest mostly intact to reforesting degraded lands. Sustainable means using a resource in such a way that future uses are not impaired, which requires sound knowledge of tropical forest ecology. Sustainable timber harvest is one such possibility, which can succeed only if management plans are properly implemented. The success of all conservation efforts depends on ensuring that local people benefit economically, which requires strong cooperation among peoples and nations.

see also Biodiversity; Biome; Carbon Cycle; Ethnobotany; Wood and Wood Products

Nicholas Brokaw

Bibliography

Caulfield, Catherine. In the Rainforest. Chicago: University of Chicago Press, 1984.

Forsyth, Adrian, and Ken Miyata. Tropical Nature. New York: Scribner's and Sons, 1984.

Kellman, Martin, and Rosanne Tackaberry. Topical Environments: The Functioning and Management of Tropical Ecosystems. New York: Routledge, 1997.

Kricher, John C. A Neotropical Companion: An Introduction to the Animals, Plants, and Ecosystems of the New World Tropics, 2nd ed. Princeton, NJ: Princeton University Press, 1999.

Richards, Paul W. The Tropical Rain Forest: An Ecological Study, 2nd ed. Cambridge, UK: Cambridge University Press, 1996.

Terborgh, John. Diversity and the Tropical Rain Forest. New York: W. H. Freeman and Company, 1992.

Whitmore, Timothy C. An Introduction to Tropical Rain Forests, 2nd ed. Oxford: Oxford University Press, 1998.

Tropical rain forest

The richest and most productive biological communities in the world are in the tropical forests. These forests have been reduced to less than half of their former extent by human activities and now cover only about 7% of Earth's land area. In this limited area, however, is about two-thirds of the vegetation mass and about half of all living species in the world.

The largest, lushest, and most biologically diverse of the remaining tropical moist forests are in the Amazon Basin of South America, the Congo River basin of central Africa, and the large islands of southeast Asia (Sumatra, Borneo, and Papua, New Guinea). Whereas the forests of mainland southeast Asia, western Africa, and Central America are strongly seasonal, with wet and dry seasons, the South American and central African forests are true rain forests. Rainfall is generally over 160 in (406 cm) per year and falls at a relatively even rate throughout the year. It is said that such rainforests "make their own rain," because about half the rain that falls in the forests comes from condensation of water vapor released by transpiration from the trees themselves. Rain forests at lower elevations are hot and humid year-round. At higher elevations, tropical mountains intercept moisture-laden clouds, so the forests that blanket their slopes are cool, wet, and fog-shrouded. They are aptly and poetically called "cloud forests." Tropical forests are generally very old. Unlike temperate rain forests, they have not been disturbed by glaciation or mountain-building for hundreds of millions of years. This long period of evolution under conditions of ample moisture and stable temperatures has created an incredible diversity of organisms of amazing shapes, colors, sizes, habits, and specialized adaptations.

Habitats in a tropical rain forest are stratified into three to five distinct layers from ground level to the tops of the tallest trees. Hundreds of tree species grow together in lush profusion, their crowns interlocking to form a dense, dappled canopy about 120 ft (37 m) above the forest floor. These unusually tall trees are supported by relatively thin trunks reinforced by wedge-shaped buttresses that attach to a thick mat of roots just under the soil surface. A few emergent trees rise above the seemingly solid canopy into a world of sunlight, wind, and open space. Numerous species of birds, insects, reptiles, and small mammals live exclusively in the forest canopy, never descending below the crowns of the trees.

The forest understory is composed of small trees and shrubs growing between the trunks of the major trees, as well as climbing woody vines (lianas) and many epiphytes—mainly orchids, bromeliads, and arboreal ferns—that attach themselves to the trees. Some of the larger trees may support 50–100 different species of epiphytes and an even larger population of animals that are specialized to live in the many habitats they create. These understory layers are a world of bright but filtered light abuzz with animal activity.

By contrast, the forest floor is generally dark, humid, quiet, and rather open. Few herbaceous plants can survive in the deep shade created by the layered canopy of the forest trees and their epiphytes. The most numerous animals are ants and termites that scavenge on the detritus raining down from above. A few rodent species gather fallen fruits and nuts. Rare predators such as leopards, jaguars, smaller cats, and large snakes hunt both on the ground and in the understory.

A tropical rain forest may produce as much as 90 tons
(81.6 metric tons) of biomass per acre (0.4 ha) per year, and one might think that the soil that supports this incredible growth is rich and fertile. However, the soil is old, acidic, and nutrient-poor. Ages of incessant tropical rains and high temperatures have depleted minerals, leaving an iron- and aluminum-rich podzol. Tropical forests have only about 10% of their organic material and nutrients in the soil, compared to boreal forests, which may have 90% of their organic material in litter and sediments.

The interactions of decomposers and living plant roots in the soil are, literally, the critical base that maintains the rain forest ecosystem . Tropical rain forests are able to maintain high productivity only through rapid recycling of nutrients. The constant rain of detritus and litter that falls to the ground is quickly decomposed by populations of fungi and bacteria that flourish in the warm, moist environment . Some of these decomposers have symbiotic relationships with the roots of specific trees. Trees have broad, shallow root systems to capitalize on this surface nutrient source; an individual tree might create a dense mat of superficial roots 328 ft (100 m) in diameter and 3 ft (0.9 m) thick. In this way, nutrients are absorbed quickly and almost entirely and are reused almost immediately to build fresh plant growth, the necessary base to the trophic pyramid of this incredible ecosystem.

See also Biodiversity

[William P. Cunningham Ph.D. ]

RESOURCES

BOOKS

Caufield, C. In the Rainforest. New York: Knopf, 1985.

Hecht, S., and A. Cockburn. Fate of the Forest. New York: Harper Collins, 1991.

Myers, N. The Primary Source: Tropical Forests and Our Future. New York: Norton, 1992.

Revkin, A. The Burning Season: The Murder of Chico Mendes and the Fight for the Amazon Rain Forest. New York: Houghton Mifflin, 1991.

PERIODICALS

Repetto, R. "Deforestation in the Tropics." Scientific American 262 (April 1990): 36–42.

tropical seasonal forest A tropical forest that grows in regions with a marked dry season. It is a distinct formation type, dominated by both evergreen and deciduous broad-leaved trees, flanking the rain forest in areas which have a marked dry season. The deciduous species are mostly in the canopy and become commoner as the climate gets more seasonal. There is some defoliation, especially of the biggest trees, during the dry season, the degree depending on the severity of the moisture deficit. The structure of the forest is also simpler than that of the rain forest, with fewer tree strata, and less luxuriant growths of climbing and herbaceous plants. Examples are found on all the continents with tropical territory, but are especially extensive in Central and South America, continental south-eastern Asia, and northern Australia. In Africa savannah and related types of vegetation replace much of this kind of forest. See monsoon forest.

tropical montane forest Two distinct formations, lower and upper montane forest, each of which has a characteristic structure, physiognomy, and flora; the two are usually sharply bounded. Lower tropical montane forest is tall. Upper tropical montane forest is often a forest of short trees 10–12 m high in a single stratum. When their trunks and boughs are misshapen and covered in mosses, lichens, and liverworts this is mossy forest, which coincides with belts of mist and cloud at heights of 2000–3000 m. Above the cloud zone (e.g. in New Guinea), the trees are taller and the ‘mosses’ less in evidence. On the highest tropical mountains, subalpine forest occurs as a further zone.

tropical montane forest Typically, a forest of short trees 10–12 m high in a single stratum, their trunks and boughs being mishapen and covered in mosses, lichens, and liverworts. This is the ‘mossy forest’ which coincides with the belts of mist and cloud at heights of 2000–3000 m. If the montane forest persists above the cloud zone, as it does in New Guinea, for example, then the trees are taller and the ‘mosses’ less in evidence.