5) The three major tropical rainforests

a. Africa: Approximately 9% of Africa’s land mass is covered with rainforests, but on this continent, the forest is almost entirely seasonal and in comparison with the Neotropics and Malesia, rainfall is relatively low, and the trees are fairly low in stature. Much of Africa had been deforested relatively recently, and has become savanna. There are four forest zones – the Guinea forest of recent origin on the west coast, the Nigerian forest, the Cameroon-Gabon forest, and the Congo forest, which is by far the largest. These forests are all species-poor in comparison to those in Asia and the Americas, and many large pantropical families (such as the palms, lianas and epiphytes) are poorly represented. There are many important timber trees but few other economically-valuable species. The African forests are relatively impoverished because they lie in areas which are somewhat dry, with stressful alternating dry and wet seasons. These forests have always been subject to intense human pressure; probably they had all been cut down in the past and so are, in actuality, secondary forests.

b. Indo-Malayan region (South and Southeast Asia): Southeast Asia is highly mountainous and insular, with an intensely humid and hot climate. The forests had not been subjected to much human pressure in the past, since the human population had been relatively low until recently. There are two main areas – the western portion, consisting of Borneo, Malaya, and Indonesia (together forming Malesia), along with southern Thailand, Burma, Cambodia, Vietnam and Laos, and the east, extending from New Guinea to the Solomon Islands and Northeastern Australia. During the glacial period, the sea level was low and much of this area was a continuous land mass. The islands and mainland of western Asia contain a variety of types of forest – lowland, highland, swamp, and peat, with rich volcanic soils in some places (Java, for instance) and infertile soils in areas away from mountain chains and alluvial plains. The lowland forests of Malesia are dominated by dipterocarps, huge canopy trees with great timber potential, which has made these the most threatened forests in the world. Of the sixteen dipterocarp genera, ten are found in Malesia, and 386 of the 550 dipterocarp species are Malesian. In contrast there is but one genus in Africa. In keeping with this, the flora of this area is the richest of any rainforests, with 25,000 -30,000 species, two-thirds of which are found only in lowland forest. There are many useful plants, such as fruit trees, spice trees, and palms.

c. Neotropics: The Neotropical forests are those of Central and South America, with Amazonia being the richest region. As many as 60% of the trees in Amazonia are leguminous (nitrogen-fixing), and there are numerous species of bromeliads (the pineapple/orchid group), epiphytes, and palms. Here, too, are found many useful plants – fruits, medicinal plants, cocoa, vanilla, rubber. There are many varieties of forest here, too, and because of the extensive tributary system associated with the Amazon River, there are both flood (varzea) and highland (terra firme) forests. The varzea forests are seasonally flooded. The climate is generally hot and humid, although there are seasonally drier areas as well.