Abstract
Tropical rainforests are located around and near the equator, where they undergo a climatic pattern that consists of warm average temperatures and heavy rainfalls. Their current distribution over tropical America, Africa, Madagascar, Southeast Asia, and New Guinea, and more scantily in Australia, results from successive fragmentations that occurred over hundreds of millions of years. Though in perpetual regression, their cover-approximately 3 percent of the world's land surface-is estimated to be around 16.5 million square kilometers. These ecosystems comprise four distinct forest types. The first, lowland equatorial evergreen rainforests, endures continuous and high rainfalls. The second, moist deciduous and semi-evergreen seasonal forests, receives high overall rainfall, but a warm wet season alternates with a cooler dry season during which trees with null and void foliage lose their leaves. The third type, montane rainforests, is found in cooler-climate mountain areas, at altitudes ranging from 1,500 up to 3,300 meters. The fourth, flooded forests, is composed of swamps and floodplains and are permanently or seasonally subdued by flooding. Between 40 and 75 percent of all biotic species are indigenous to the tropical rainforests, which also harbor half of all the living animal and plant species on the globe and two-thirds of all flowering plants. Several million rainforest plant, invertebrate, and microorganism species are yet to be described. The architecture of a tropical rainforest consists of a succession of structured layers, each of which holds a specific biodiversity. The forest-floor stratum captures less than 2 percent of the total sunlight reaching the top of the forest cover. Firm ground floors host sparse vegetation composed only of plants that are adapted to extremely low light conditions. The flora of the understory-the next stratum, which lies above the floor stratum and receives nearly 5 percent of the sunlight-is an inextricable mix of shade-tolerant herbs, shrubs, small trees, and woody vines and shelters a rich wildlife. Next, thirty to forty-five meters above ground, the canopy is composed of a wave of treetops, which form a continuous covering over the two previous strata. This stratum contains the most profuse though also the least known biodiversity. The trunks, branches, and cavities of the tall trees accumulate incredible quantities of water and debris, and are substrates for a luxuriant flora of epiphytes, which are home to a diverse yet unexplored fauna. Finally, the emergent stratum is composed of scattered large trees that show up above the canopy. These trees are exposed to full sunlight, high temperatures, strong winds, and excessive dryness.