Plants of the Tropical Rainforest

While it is dominated by woody plants, the tropical rainforest is enormously diverse. Soft-stalked plants (herbs) play a minor role among the trees, shrubsand woody climbers. One reason for the dominance of woody plants is the constancy of climate; little change in temperature and moisture through the year means that growth can be continuous. Such constancy of climate has also excluded other plant types, such as those with bulbs and tubers that help plants to resist spells of drought, from life in the rainforest. Likewise, there are no suitable temporary habitats in the dark forest undergrowth for annual plants, which grow, flower and seed quickly, passing most of the year as seeds. Unlike temperate zone woodlands in which there are only a few readily distinguishable tree species, a couple of acres of ground in the tropical rainforest may contain hundreds of plant species and scores of different trees. In fact, it may take considerable time just to find two trees belonging to the same species. Rainfall seems to be one factor behind this diversity. Generally, the wettest forests are the most diverse. The plants belong to many different families, which also increases diversity. In the northern temperate zone, the majority of the trees belong to the oak family, maple family or birch family. In the tropical rainforest, many families are represented. Some, such as the dipterocarps, which are so important in the forests of Southeast Asia, are unknown in temperate countries.

Why are tropical rainforests so diverse?

First, this is an ancientecosystem. Tropical rainforests have existed for many millions of years. During the drastic climatic fluctuations of the last million years, tropical rainforests have expanded in wet periods and shrunk in dry ones. The splitting of large areas of forest into small isolated blocks may have contributed to species richness. Two populations of a single plant species may slowly become different if separated. When conditions change and they again come into contact, interbreeding may no longer be possible, and two species are eventually established instead of one. Second, the rainforest's diversity perpetuates itself. Different varieties of epiphyte grow on the different tree. A single tree can also provide a home for different forms of epiphytes – some on the trunk, some on the main branches and others on the outer branches.

Where are the Earth's Rainforests?

Tropical rainforests makes up three main blocks – in the Americas, Southeast Asia and Australia, and Africa – which lie within a belt around the Equator. 56 percent of tropical rainforests lie in the Neotropics, mostly in the great basins of the Amazon and Orinoco rivers, extending westward to the flanksof the Andes. This is replaced by tropical seasonal forests in the northeast bulge of Brazil. 26 percent of tropical rainforests are located in the Asian tropics, centered on the Malay Archipelago, with patchy extensions into continental Asia. Africa has the third great tropical rainforest region, containing about one fifth (18 per cent) of the total area of all tropical rainforests. The African tropical rainforest is concentrated in West and Central Africa, the Congo basin and the shores of the Gulf of Guinea, but does not reach across the whole continent. Each main block of tropical rainforest has outliers, which have their own wildlife. Near the huge Amazon-Orinoco forest, there is a discrete belt of tropical rainforest along the Atlantic coastal mountains in Brazil. There is another one on the Pacific coast of Ecuador and Colombia, reaching up through Central America to south Mexico on the Caribbean shore of the American isthmus. In Africa, there are small patches of tropical rainforest on some of the mountains of East Africa, mainly in Tanzania, and an isolated forest on the west coast of Malagasy. There is also tropical rainforest on the Seychelles and the Mascarene Islands in the Indian Ocean.

Tropical rainforest merges at its edges into one of two other kinds of forest: