The tropical Forests
The tropical forests occur near the equator where temperature and light levels remain similar all year. Where the rainfall is also spread evenly throughout the year, tropical rainforests occur; where precipitation levels vary and there is a pronounced dry season, tropical moist forests grow, and in areas with less moisture are dry forests and savannas.
Of these forest types, the rainforests are the most well-known. Often, seasonal moist forests are also referred to as rainforests, but traditionally a rainforest does not have a pronounced dry season. The rainforests are home to more species of plants and animals than any other habitat in the world. While the location of a particular forest will determine which species can be found within (orang-utans do not live in South America; sloths do not inhabit Asia), rainforests throughout the tropics have a similar overall appearance.
Although there are more tree species here than in any other habitat, they all have a similar shape and form, contributing to the similar overall appearance of rainforests throughout the world. The number of tree species in a single hectare is a common way to assess a region's biodiversity.
The relative abundance of major types of rainforest plants - "typical" broad-leaved trees, palms, and vines - varies with location. There are three distinct areas of tropical rainforests: Southeast Asia, South and Central America, and Africa. Different species have evolved separately in each of these areas. It is not known where there is the greatest diversity of plants and animals. Since most tree species look so similar, it is hard even for trained biologists to identify them for certain. Often there are only a few people capable of distinguishing the differences between several species. The Southeast Asian forests were previously thought to be the most biodiverse regions in the world, where as many as 200 tree species could be found in a single hectare. This number has recently been overshadowed by the discovery of an incredibly diverse forest in Peru, where a single hectare contained 606 individual trees, belonging to a total of 300 different species!
Tropical rain forests can be divided into two categories, lowland and montane. Lowland rain forests are more common, but are also being destroyed at a quicker pace. This fact is a great tragedy, because the most diverse plant communities inhabit lowland forests. Montane rain forests occur at altitudes of 3,000 feet and upwards. The climate of a montane forest is a cooler, wetter one, and often shrouded in clouds of mist toward the upper layers.
Many rainforest trees are rare, because there are so many different kinds of trees that there is only room for a few individuals of each species. This is well illustrated by the Peruvian forest described on the previous page, where every other tree in a one-hectare plot belonged to a different species. Trees in the tropical forest also have relatively short life-spans. Fallen trees create small gaps in the vegetation where air and soil temperatures are higher because of increased exposure to sunlight. Specialized communities of plants grow in these microclimates, eventually shifting from bushy shrubs to full sized rainforest trees.
Under a mature rainforest canopy, there is very little undergrowth because the trees overhead block almost all of the sunlight from reaching the ground. The few low growing shrubs that do occur here are adapted for low light levels, a fact which has made many of these species popular as potted houseplants elsewhere in the world!
Some of the tallest rainforest trees reach 70 meters in height. These giants do not grow in dense patches, but are scattered through the forest, extending high above the usual canopy height of about 50 meters. The most productive and interesting part of the habitat is often high in the crowns of the trees, but the rainforest canopy is only just beginning be studied in detail. Flowers, fruits, seeds, leaves, are all produced in the highest quantities in this upper layer of the forest, so naturally this is where foraging mammals, birds, even reptiles and amphibians will be most abundant. The majority of tropical mammals are arboreal (spending most or all of their time in the trees), compared to temperate mammals, most of which are terrestrial. Beneath the rainforest canopy is an understory of young trees, usually about 15 meters tall.
Throughout the forest grow vines and epiphytes. Epiphytes are "air plants", and grow directly on the branches and trunks of trees, sending no roots down to the forest floor. Vines do grow from the ground, but use trees for support. These types of plants are extremely abundant in humid rainforests, but decrease in areas where there is a definite dry season. Vines are a major feature in some forests, competing with trees for light and nutrients, and providing animals with convenient travel routes between trees. Some vines eventually kill their "host" trees, but most are harmless, unless they grow so thickly that the trees can no longer support their weight.
The tropics are not all rainforest, although that is by far the most often discussed habitat. Vast areas are also covered by dry forests and savanna. The dry forests in Central America are dominated by Caribbean pine, and even oaks grow in some areas. In general, the Central American dry forests are not as biodiverse as those of South America. Dry forests in South America cover almost 250 million hectares, but they are threatened by logging and agriculture. Many dry forests are considered even more endangered than rainforests, but without the great variety of plants and animals, they do not attract as much attention for conservation.
Broad-leaved trees in these dry woodlands are all deciduous. Most trees do not reach heights over 7.6 meters (25 feet), compared to the trees of the nearby rainforests, which are usually over 45 meters (150 feet) tall. The soils in dry forests, like those of rainforests, are quite poor. Most dry forests grow in well-drained, sandy soils, but others occur in soggy, waterlogged areas, which at first seems to make no sense. Consider, however, that "dry" forest vegetation grows in any soil conditions that are too "extreme" for rainforests - either too dry or too wet. In fact, some areas that are covered by dry forests would be suitable areas for rainforests, if only the soil conditions were different.
The Amazon rainforest is the largest expanse of tropical forest in the world. Until recently, its size meant that humans stayed mostly around the edges, and the forest remained relatively intact. But more and more of the forest is now being destroyed, although estimates vary on exactly how much is gone and how much remains. The tropical forest contains many valuable timber species such as mahogany, and large corporations buy the land or licences to log it. Often, these companies are located far away from the forest itself - companies from Malaysia, Singapore, and other Asian countries now control large tracts of Amazonian rainforest.
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