Rain Forests

Rain Forests

Since rainfall controls tropical vegetation in the tropics, rain forest types may be classified with reference to local climate. These include lowland, montane , subtropical, and temperate rain forests. Their common features are at least 1,500 millimeters (approximately 33 inches) of annual rainfall and evergreen vegetation with lianas and epiphytes . Most widespread are lowland tropical rain forests, accounting for less than one-third of the tropical land surface, growing in areas receiving between 2,000 to 5,000 millimeters of annual rainfall, with relatively high and constant air temperature (annual mean ±25°C) never below freezing point. They persist in Central America, the Amazon Basin, the Congo Basin, Southeast Asia, New Guinea, and northern Australia. Their canopy is often 25 to 45 meters or higher.

In the monsoonal tropics, characterized by similar total annual rainfall, but unevenly distributed between dry and wet seasons, a related type of lowland forest is found, which becomes partly leafless during the driest months. These tropical evergreen seasonal forests occur in Central America, the northern coast of South America, Africa, India, Southeast Asia, and in some of the Pacific Islands.

Montane tropical rain forests grow in the same regions that lowland forests do, but at higher altitudes, often above 1,000 meters. Local climate is cooler (15 to 25°C), with high annual rainfall (2,000 to 4,000 millimeters or more). The canopy is often 15 to 35 meters in lower montane forest, while above 2,000 meters, in upper montane forests, it is only 10 meters or less. Fog is frequent, and in moss forests relative humidity varies little from saturation point.

Subtropical rain forests are found in the southeastern United States, southwestern South America, southern China, Japan, eastern Australia, and New Zealand, often within cooler climates (15 to 20°C) and lower rainfall (1,500 to 2,000 millimeters). The canopy generally ranges between 35 and 40 meters. Temperate rain forests occur mostly along the Pacific Coast of North America (where the canopy may be 60 meters or higher), Tasmania, and New Zealand. Although temperatures often fall below freezing point, annual rainfall remains high. In addition, wetland forests include mangrove forests, occurring in saline coastal waters, and various peat and freshwater swamp forests. The rest of this overview concentrates on tropical lowland rain forests.

Rain Forest Structure

Rain forest structure is highly complex and determined by competition for light among plant species. Isolated trees, emerging above the canopy, are often present and can be 70 to 80 meters in height. Different tree species grow following various architectural models related to bud location and branching patterns and may or may not form distinct forest layers. Lianas rooted in the ground and epiphytes (e.g., ferns, orchids, and bromeliads) growing on support branches are common in the canopy. Leaves are often medium to large in size, lustrous, and tough. Their shape is often simple, ending with a "drip tip" to shed rainfall. Compound leaves are thought to represent an adaptation to rapid upward growth or seasonal drought and occur more commonly among plants growing in light gaps, in early successional vegetation, or in tropical evergreen seasonal forests. Very little of the light falling on the canopy reaches the ground (0.5 to 2 percent of the illumination available in the canopy), so that the herb layer is much reduced but also includes some saprophytes and root-parasites. Although larger herbs from Zingiberales and Arales may occasionally form denser understory, in mature rain forests it is usually not difficult to penetrate.

This structural complexity is complicated by the temporal dynamics of the rain forest. Often, leaf fall occurs during the driest months and leaf flushing (budding and growth) during the wettest. Furthermore, rain forest trees show a variety of leafing phenologies , from continuous leafing, intermittent flushing to deciduous habits. Within the same species or individual crown, flushing may be synchronous or not. Patterns of flowering and fruiting are equally complex, with sometimes mass flowering or fruiting. Understory leaves are often long-lived, more than five years, and covered with mats of epiphytes (such as mosses, lichens, and algae).

Plant Diversity in Rain Forests

Several theories account for the higher plant and animal diversity in tropical forests compared to temperate forests. First, a greater stability may have existed in the tropics, in comparison with temperate lands, where biotas have been depleted by recent glaciations. During the Pleistocene epoch, ten thousand years ago, climatic changes transformed many rain forests into drier savanna. Some rain forests persisted as refugia (isolated refuges), later rejoining together as the climate became more favorable, increasing species richness within. Second, tropical ecosystems may provide more ecological niches than temperate ones, thereby supporting more species. Third, predation and competition in the tropics may promote higher speciation rates. Last, high species richness in the tropics may result from solar energy controlling biodiversity in near-saturated humid conditions.

The great majority of plants in rain forests consist of dicotyledonous trees. For example, the genera Ficus (Moraceae) and Piper (Piperaceae) are diverse throughout the tropics, whereas Eperua (Caesalpiniaceae) and Shorea (Dipterocarpaceae) are species-rich in Neotropical and Asian forests, respectively. Some families that are herbaceous in temperate areas develop as woody trees in rain forests (e.g., Verbenaceae, Urticaceae, and Polygalaceae). Monocotyledons are less common but include palm trees, various herbs, orchids, and grasses. Abundant woody climbers (often dicotyledons) are characteristic of rain forest vegetation. Their broad stems may cover several kilometers of canopy. Herbaceous or shrubby epiphytes, semiparasitic mistletoes, and strangling figs (Ficus ) are also species-rich.

Although tropical rain forests cover less than 6 percent of land masses, they may sustain half or more of Earth's biodiversity. For example, the Malay Peninsula contains about 7,900 plant species compared to Britain's 1,430. Further, a typical hectare of rain forest may include 150 to 200 species of trees with a diameter greater than 10 centimeters, with records of 300 species per hectare in Peruvian Amazonia. In contrast, a hectare of temperate deciduous forest might contain only one-tenth as many species. Still, many rain forest tree species are rare, with average densities of 0.3 to 0.6 trees per species and per hectare. This results in a large average distance between trees of the same species that may affect pollinating and foraging animals, as well as the plants themselves. Indeed, pests or diseases are rarely a problem in mixed rain forests, while uniform vegetation in the same area, such as plantations, is often heavily defoliated.

Contrasting strongly with mixed rain forests, monodominant rain forests are dominated by a single canopy species, such as Mora (Caesalpiniaceae) in the Neotropical region, Gilbertiodendron (Caesalpiniaceae) in Africa, and Dryobalanops (Dipterocarpacea) or Nothofagus (Fagaceae) in Australasia. These are competitively superior, shade-tolerant, slow-growing, long-lived species with large and poorly dispersed seeds.

Animal Diversity in Rain Forests

Rain forests sustain more faunal diversity than any other habitat on Earth. In particular, the Amazonian forests of Peru and Ecuador are the most diverse for mammals, birds, reptiles, amphibians, and butterflies. Arthropods are particularly diverse in rain forests since they exploit every niche from the soil to the canopy. For example, one large tree in Peru yielded 43 species of ants, equivalent to their entire British fauna, and 134 species of leaf beetles (Chrysomelidae) were collected from ten tree species in New Guinea in comparison with a total fauna of 255 British species.

The most abundant vertebrates in rain forests are frugivores, feeding on fruits and seeds. Among invertebrates (aside from earthworms in soil and epiphytes), the dominant groups rely on a variety of food ressources. These include ants (Formicidae: predators, herbivores , or fungal-feeders), rove beetles (Staphylinidae: predators, scavengers, or fungal-feeders) or weevils (Curculionidae: leaf-chewers, wood-, seed-, or flower-eaters). Other important invertebrate groups in rain forests include parasitoid wasps, moths, leaf beetles, and spiders. However, most of these species are little known and many are yet to be described.

In 1982, entomologist Terry Erwin suggested that there may be as many as 30 million species of arthropods, instead of the previously estimated 1.5 million, although this has not been substantiated. Erwin's estimates attracted considerable attention to the vast, but endangered, reservoir of genetic diversity represented by rain forest arthropods. In 1988, Erwin stated "no matter what the number we are talking about, whether 1 million or 20 million [arthropod species], it is massive destruction of the biological richness of Earth."

Rain Forest Dynamics: Regeneration

Rain forest regeneration and continuity is assured through the important processes of pollination and seed dispersal, which occur primarily through the movements of rain forest animals. Their loss in severely disturbed rain forests drastically affects regeneration capacity. Wind pollination, common in temperate regions, is rarer due to the absence of wind currents; 90 percent of rain forest plants may be insect-pollinated, with nectar the reward for pollinators. They may be strong fliers that forage over long distances, such as birds, bats, hawk moths, and large euglossine bees , which may fly up to 23 kilometers. Other short-range pollinators may include stingless, carpenter, and bumblebees, wasps, butterflies, thrips, beetles, midges, and flies. Depending on the timing of flower opening, pollinators may be either diurnal or nocturnal.

Another important aspect of pollination is fidelity to particular plant species, which ensures cross-pollination. Some pollinators are generalists (e.g., stingless bees) but restrict their visits to particular plant species. However, many rain forest plants have developed intricate relationships with their pollinators. For example, the petal tube of many flowers corresponds exactly in length and curvature to either the beaks of hummingbirds or to the tongue of certain hawk moths. Further, pollinator activities are attuned to different flowering phenologies, the most specialized of these involving figs (Ficus ) and fig wasps (Agaonidae), the former totally dependent on the latter for pollination. Usually, one particular species of fig is pollinated only by its own species of wasp.

Some rain forest plants may be dispersed by wind or gravity. However, many of them rely on animals such as ants, fish, reptiles, birds, bats, primates, deer, pigs, civets, rodents, and elephants to disperse seeds. This ensures pollination and cross-fertilization of distant tree populations to produce more vigorous and successful offspring and that seedlings have enough space and light to grow and develop. Fruits represent fleshy rewards for animals; swallowed with their seeds, the latter emerge intact in feces and ready to germinate. Animals often specialize in particular seeds or similar seed types, with larger animals often dispersing the seeds at great distance from the parent tree.

Figs are a year-round resource for rain forest frugivores and are particularly important when other fruits become scare. Fig trees are referred to as a keystone species, those that have a crucial importance in the maintenance of the rain forest ecosystem.

Insects (e.g., Bruchidae and Curculionidae), parrots, or squirrels may overcome the chemical defenses of seeds, feeding on them without dispersal. Many insects and fungi also attack the leaves and stems of seedlings. Patterns of herbivore attack below the parent trees may depend on seedling density and decrease with increasing distance from the parent. This may result from specific insect herbivores colonizing seedlings from parent trees, promoting botanical diversity by prohibiting the establishment of young trees near conspecific parents. However, this is not universal, and this model requires validation and refinement.

Rain Forest Dynamics: Succession

Natural disturbance induces a succession of vegetation. After clearance, rain forest succession may start with almost bare soil, proceed with a different kind of vegetation (called secondary forest or growth), and end with the restoration of the original, climax, vegetation. For example, the fall of a large crown of 20 meters in diameter may produce a forest gap of 400 m². Some plants will be damaged from the tree fall, but others will have improved growth opportunities, due to increased access to light. Forest gaps are common and promote local plant and animal diversity.

Secondary rain forests contain smaller trees, with many small climbers and young saplings in an understory that is often difficult to penetrate. The floral composition of these forests is different from primary rain forests. Although a few secondary species may live in natural gaps created by treefalls in primary forests, they are more abundant in secondary forests. These are dominated by a few plant species and are less species rich than primary forests. Secondary genera include Cecropia (Cecropiaceae) in the Neotropical region, Musanga (Moraceae) in Africa, and Macaranga (Euphorbiaceae) in Asia. Typically, these "pioneer species" (as opposed to the shade-tolerant species of primary rain forests) produce large quantities of small seeds carried by wind or small animals. In contrast, shade-tolerant species often bear large seeds in fleshy fruits that are dispersed by large animals. Pioneers germinate and grow rapidly (often several meters in two to three years), producing thin, short-lived, and large leaves on weak stems that break easily. Secondary vegetation is not long-lived, since species needing much sunlight to germinate and grow eventually die in the shade of their parents. These stands of pioneers are unable to regenerate under new ecological conditions, giving way to slower-growing, stronger trees that regenerate primary forest, a process that takes place over many centuries.

Herbivory and Decomposition in Rain Forests

Both herbivory and decomposition hasten the return and recycling of nutrients in the ground and promote regeneration of the forest. Most rain forest plants contain more chemical defenses than temperate plants. This may be a response to year-round high herbivore pressure, particularly from insects that represent the bulk of leaf-eating, sap-sucking, flower- and seed-eating fauna. Chemical defenses are often by-products of plant metabolism and are termed secondary metabolites, including lectins, resins, alkaloids , protease inhibitors, cyanogenic glycosides, or rare amino acids. Each plant species may contain fifty or more in its leaves, bark, or seeds. Many may be pharmacologically active, with subtle differences often due to the high genetic variation of rain forest plants. Since 99 percent of rain forest plants have not been yet chemically screened, biological prospecting for secondary metabolites was undervalued until recently—with an even greater percentage of arthropods untreated.

Herbivorous insects have developed assorted strategies to counter the plants' chemical defenses and concentrate their damage on young leaves. They may produce enzymes capable of breaking down secondary metabolites, thus becoming restricted to feeding on one or a few related plant species sharing similar chemical properties. About 9 percent of leaf area is usually lost to herbivores in tropical rain forests, a figure often considerably lower in forests growing on nutrient-poor soils. Since they invest most of their energy in growth and less in chemical defenses, herbivory on pioneer trees tends to be greater than those that are shade tolerant.

Decomposition of organic matter, performed by fungi, bacteria, and invertebrates, particularly earthworms, is rapid in rain forests. Termites are the primary decomposers of wood, often transporting rotting wood to great depths in their underground galleries. In terms of dominance, termites are ranked second to ants with up to 870 colonies per hectare, including underground and arboreal nests.

Nutrient Cycling in Rain Forests and the Consequences of Deforestation

Although most tropical rain forests grow on nutrient-poor soils, their primary production is the highest of any natural system, ranging from 300 to 900 tons of biomass per hectare. This is due to the efficient cycling of nutrients through a virtually leak-proof system, since up to 90 percent of nutrients may be stored at anytime in the vegetation.

The main source of nutrients is rainfall, which represents as much as 3 kilograms of phosphorus, 2 kilograms of iron, and 10 kilograms of nitrogen per hectare per year. The forest filters out nutrients from the water as it passes through. Epiphytes growing on leaf surfaces often fix nitrogen. At ground level, tree roots, which may extend near the soil surface 100 meters away from the tree trunks, may be three times as dense as in temperate forests and are very efficient at absorbing nutrients from the soil, whether from rainfall or from decaying organic matter. Symbiotic associations between roots and fungus or bacteria (termed mycorrhizae) are particularly efficient in recovering minerals, particularly phosphorus, from leaf litter.

Since most nutrients are held in the vegetation aboveground, clearing and burning of rain forests concentrates nutrients in the ground. Some nitrogen and sulfur are lost during burning, but large quantities of other nutrients are deposited in ash. Leaching, due to heavy rainfall, washes these nutrients far beyond the shorter roots of new grasses or shrubs. This severely disrupts the nutrient cycle, leaving barren tracts that remain unproductive or that require the ecologically unsound overapplication of fertilizers. Moreover, the clearing and removal of logs by heavy machinery result in soil compaction , water runoff, and, eventually, soil erosion. When a large area of forest is cleared, the soil becomes drier and warmer, and most of the mycorrhizae die out. Aided by nutrients, mycorrhizae, and seeds from nearby intact rain forest patches, regrowth occurs in small areas of clearance, but this is impossible for large clearings, where herbaceous vegetation colonizes infertile soils.

Indigenous People and Rain Forests

The indigenous dwellers of rain forests are dependent on them and, similarly, are endangered by habitat fragmentation and destruction. This includes several groups in Malaysia (the Orang Asli), Sarawak (the Penan), Sabah, New Guinea, the Philippines, the African Pygmy groups in Cameroon, Gabon, and Congo; and many Amerindian groups, such as the Yanomami of Brazil or Jívaro of Ecuador.

The encyclopedic knowledge of the natural world of many indigenous groups is well known and discussed by many rain forest ecologists. For example, Papua New Guineans know hundreds of plant and animal species living in their forests, and they have developed detailed nomenclatural systems in their local languages. This knowledge is not restricted to medicinal plants but also extends to the smallest of creatures. Indigenous knowledge is an inspiration for scientific research and an opportunity for inclusion of local assistants within research projects. Such knowledge also requires reward through the sharing of profits that may result from economically important discoveries.

Environmental Threats to Rain Forests

The major threats to rain forests are, in order of decreasing importance:

1. cattle ranching and farming, leading to habitat fragmentation and destruction
2. clear-cutting for timber and pulp, with similar outcomes
3. plantation cultivation , creating large areas of secondary regrowth
4. selective logging of particular tree species, leading to an irregularly structured patchwork of primary and secondary forests
5. shifting cultivation (slash- and-burn), creating small patches of secondary growth
6. natural disasters, including localized landslides and fires, leading to secondary regrowth and natural succession.

The ever-increasing and often irreversible human damage to rain forests shows no sign of slowing down. Although much controversy exists regarding rates of its loss (perhaps 50 hectares per minute) and biodiversity, it is probable that, in a few decades, large tracts of rain forests will remain only in the Guianas, upper Amazon, Congo Basin, and New Guinea. Tragically, a substantial part of Earth's biodiversity and genetic resources will be lost forever, with the potential for concomitantly disastrous effects on local and global climates. Belief that recent advances in biotechnology will remedy this situation is erroneous. The best way to slow down these alarming rates of loss is through education, conservation, and rehabilitation of the organismic components of ecology, botany, zoology, and taxonomy.

see also Biodiversity; Defenses, Chemical; Deforestation; Endangered Species; Plant Prospecting; Pollination; Rain Forest Canopy; Seed Dispersal.

Yves Basset

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