Tropical rainforests represent the oldest major vegetation type still present on the terrestrial Earth. Like all vegetation, however, that of the rainforest continues to evolve and change, so modern tropical rainforests are not identical with rainforests of the geologic past.
Tropical rainforests grow mainly in three regions: the Malesian botanical subkingdom, which extends from Myanmar (Burma) to Fiji and includes the whole of Thailand, Malaysia, Indonesia, the Philippines, Papua New Guinea, the Solomon Islands, and Vanuatu and parts of Indochina and tropical Australia; tropical South and Central America, especially the Amazon basin; and West and Central Africa (seebiogeographic region). Smaller areas of tropical rainforest occur elsewhere in the tropics wherever climate is suitable. The principal areas of tropical deciduous forest (or monsoon forests) are in India, the Myanmar–Vietnam–southern coastal China region, and eastern Brazil, with smaller areas in South and Central America north of the Equator, the West Indies, southeastern Africa, and northern Australia.
The flowering plants (angiosperms) first evolved and diversified during the Cretaceous Period about 100 million years ago, during which time global climatic conditions were warmer and wetter than those of the present. The vegetation types that evolved were the first tropical rainforests, which blanketed most of Earth’s land surfaces at that time. Only later—during the middle of the Paleogene Period, about 40 million years ago—did cooler, drier climates develop, leading to the development across large areas of other vegetation types.
It is no surprise, therefore, to find the greatest diversity of flowering plants today in the tropical rainforests where they first evolved. Of particular interest is the fact that the majority of flowering plants displaying the most primitive characteristics are found in rainforests (especially tropical rainforests) in parts of the Southern Hemisphere, particularly South America, northern Australia and adjacent regions of Southeast Asia, and some larger South Pacific islands. Of the 13 angiosperm families generally recognized as the most primitive, all but two—Magnoliaceae and Winteraceae—are overwhelmingly tropical in their present distribution. Three families—Illiciaceae, Magnoliaceae, and Schisandraceae—are found predominantly in Northern Hemisphere rainforests. Five families—Amborellaceae, Austrobaileyaceae, Degeneriaceae, Eupomatiaceae, and Himantandraceae—are restricted to rainforests in the tropical Australasian region. Members of the Winteraceae are shared between this latter region and South America, those of the Lactoridaceae grow only on the southeast Pacific islands of Juan Fernández, members of the Canellaceae are shared between South America and Africa, and two families—Annonaceae and Myristicaceae—generally occur in tropical regions. This has led some authorities to suggest that the original cradle of angiosperm evolution might lie in Gondwanaland, a supercontinent of the Southern Hemisphere thought to have existed in the Mesozoic Era (252 to 66 million years ago) and consisted of Africa, South America, Australia, peninsular India, and Antarctica. An alternative explanation for this geographic pattern is that in the Southern Hemisphere, especially on islands, there are more refugia—i.e., isolated areas whose climates remained unaltered while those of the surrounding areas changed, enabling archaic life-forms to persist.
The first angiosperms are thought to have been massive, woody plants appropriate for a rainforest habitat. Most of the smaller, more delicate plants that are so widespread in the world today evolved later, ultimately from tropical rainforest ancestors. While it is possible that even earlier forms existed that await discovery, the oldest angiosperm fossils—leaves, wood, fruits, and flowers derived from trees—support the view that the earliest angiosperms were rainforest trees. Further evidence comes from the growth forms of the most primitive surviving angiosperms: all 13 of the most primitive angiosperm families consist of woody plants, most of which are large trees.
As the world climate cooled in the middle of the Cenozoic, it also became drier. This is because cooler temperatures led to a reduction in the rate of evaporation of water from, in particular, the surface of the oceans, which led in turn to less cloud formation and less precipitation. The entire hydrologic cycle slowed, and tropical rainforests—which depend on both warmth and consistently high rainfall—became increasingly restricted to equatorial latitudes. Within those regions rainforests were limited further to coastal and hilly areas where abundant rain still fell at all seasons. In the middle latitudes of both hemispheres, belts of atmospheric high pressure developed. Within these belts, especially in continental interiors, deserts formed (seedesert: Origin). In regions lying between the wet tropics and the deserts, climatic zones developed in which rainfall adequate for luxuriant plant growth was experienced for only a part of the year. In these areas new plant forms evolved from tropical rainforest ancestors to cope with seasonally dry weather, forming tropical deciduous forests. In the drier and more fire-prone places, savannas and tropical grasslands developed.
Retreat of the rainforests was particularly rapid during the period beginning 5,000,000 years ago leading up to and including the Pleistocene Ice Ages, or glacial intervals, that occurred between 2,600,000 and 11,700 years ago. Climates fluctuated throughout this time, forcing vegetation in all parts of the world to repeatedly migrate, by seed dispersal, to reach areas of suitable climate. Not all plants were able to do this equally well because some had less-effective means of seed dispersal than others. Many extinctions resulted. During the most extreme periods (the glacial maxima, when climates were at their coldest and, in most places, also driest), the range of tropical rainforests shrank to its smallest extent, becoming restricted to relatively small refugia. Alternating intervals of climatic amelioration led to repeated range expansion, most recently from the close of the last glacial period about 10,000 years ago. Today large areas of tropical rainforest, such as Amazonia, have developed as a result of this relatively recent expansion. Within them it is possible to recognize “hot spots” of plant and animal diversity that have been interpreted as glacial refugia.
Tropical rainforests today represent a treasure trove of biological heritage, and they also serve as sinks for more than 50 percent of all atmospheric carbon dioxide absorbed by plants annually. They not only retain many primitive plant and animal species but also are communities that exhibit unparalleled biodiversity and a great variety of ecological interactions. The tropical rainforest of Africa was the habitat in which the ancestors of humans evolved, and it is where the nearest surviving human relatives—chimpanzees and gorillas—live still. Tropical rainforests supplied a rich variety of food and other resources to indigenous peoples, who, for the most part, exploited this bounty without degrading the vegetation or reducing its range to any significant degree. However, in some regions a long history of forest burning by the inhabitants is thought to have caused extensive replacement of tropical rainforest and tropical deciduous forest with savanna.
Not until the past century, however, has widespread destruction of tropical forests occurred. Regrettably, tropical rainforests and tropical deciduous forests are now being destroyed at a rapid rate in order to provide resources such as timber and to create land that can be used for other purposes, such as cattle grazing (seedeforestation). Today tropical forests, more than any other ecosystem, are experiencing habitat alteration and species extinction on a greater scale and at a more rapid pace than at any other time in their history—at least since the major extinction event (the K–T extinction) at the end of the Cretaceous Period, some 66 million years ago (seeSidebar: Status of the World’s Tropical Forests).
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