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  1. This post is one of my essay assignment in Botany Systematic Subject .

    Tropical rain forest contain the greatest biodiversity in the world, and estimated to contain more than half of all species of living organism ( Wilson,1988). Over 15 million species of Plant live within this biome. We can imagine how diverse is that forest, with a lot of living organisms stay and doing their life activity in this forest environment. Tropical rain forest are warm and wet forest with many tall trees. In most tropical rain forest, It rains every day. And it grow in a narrow zone near equator. Beside that, it also one of the most exciting environments to explore. The rainforest is almost like a giant creature with its own “anatomy.” In addition, tropical rainforests store huge quantities of carbon, while producing much of the world's oxygen. Some people call tropical rainforests the lungs of the planet because they make so much of the oxygen that animals breathe. Another important role tropical rainforests play is in regulating global weather. They maintain regular rainfall. They also help prevent floods, droughts, and erosion .The largest rainforest in the world is Amazon rain forest in America.
    For virtually all types of organisms, at almost any scale, diversity is higer in tropics than anywhere else in the earth. This diversity of diversities suggest that many different mechanisms influence the number of species in tropical rain forest. How diverse is rain forest? A hectare in the Appalachians may have up to 30 species of trees; in the tropics a range of 40 to 100 is common and over 300 species is not unknown. The entire La Selva site in Costa Rica has over 1,600 species of plants in 1,500 hectares; a station in the Amazon basin has over 1,800 species.

    The rain forest is nearly self-watering. Plants release water into the atmosphere through a process called transpiration. In the tropics, each canopy tree can release about 200 gallons (760 liters) of water each year. The moisture helps create the thick cloud cover that hangs over most rain forests. Even when not raining, these clouds keep the rain forest humid and warm.
    Plants in the rain forest grow very close together and contend with the constant threat of insect predators. They have adapted by making chemicals that researchers have found useful as medicines. Bioprospecting, or going into the rain forest in search of plants that can be used in foods, cosmetics, and medicines, has become big business during the past decade, and the amount that native communities are compensated for this varies from almost nothing to a share in later profits.
    The trees in tropical rain forest also have common characteristic. It is different with the trees in oher places. If we can go and observe all kinds of forest in the world, we could realized how different is the characteristic, like the temperate forest, boreal forest, etc. Tropical species frequently possess one or more of the following attributes not seen in trees of higher latitudes. First characteristic is the Buttresses. many species have broad, woody flanges at the base of the trunk. Originally believed to help support the tree, now it is believed that the buttresses channel stem flow and its dissolved nutrients to the roots. The second characteristic is Large leaves. It is Young individuals of trees destined for the B and A layers may also have large trees. When the reach the canopy new leaves will be smaller. The large leaf surface helps intercept light in the sun-dappled lower strata of the forest. Then, the last characteristic is Drip tip. Facilitate drainage of precipitation off the leaf to promote transpiration. They occur in the lower layers and among the saplings of species of the emergent layer.
    There are several factor that affecting the species diversity in Tropical rain forest, such as Massive size of trees , Large Area or total area in each region , Canopy, Climate, Heterogeneity of biological environment, The prevalence of specialized habitats, Presence of pathogens, Natural disturbances, Mountains as diversity refuges, , spatial variation in soil, endemisms, and Epiphytes plant. 
    First factor is massive size of the trees. The physical structure and complex microclimate of tropical forest are provided by the trees, which are overwhelming in their taxonomic and structural variety. Few tropical forest have the high density of massive trees that is found in the most productive forest. (Waring and Franklin 1979; Hart et al.,1989). Eventhough the size of rain forest trees and biomass of the forest are not remarkable in a global context, in term of number of spesies per unit area, the diversity of tropical rain forest is unmatched anywhere on earth. The number of tree species greater than 1o cm in diameter at breast height found in 0,1 ha can be over 200,as compared to 15-20 for the most diverse forest. 
    In terms of total number of plant species of tropical rain forest, species richness at the continental scale is correlated with total area of the rain forest in each region. Increasing size provides ample opportunity for geographic separation for groups within a species. In conjunction with this is the fact that this broad band girdling the planet has, overall, fairly constant temperature and humidity. The greatest number of plant species as well as the highest density of species per area, are found in the neotropics, which include nearly half of the total rain forest area on the globe. Africa, with the smallest total rain forest area, has the smallest total number of plant species. The relatively high number of species per unit area found in the part of indo-malaya region which is undoubtly related to the isolation of the many island and peninsular area that facilitate genetic isolation. This total regional diversity is a combination of within habitat, between-habitat, and geographical diversity. Geographycal diversity in particular , would be expected to be greater in large region than in small region.
    For the Canopy, The spreading structure of the rainforest canopy means there are more places for plants to grow and animals to live. The canopy offers many unique sources of food and shelter. For example, there are plants in the canopy called bromeliads that store water in their leaves. Frogs use these pockets of water like ponds for hunting and laying their eggs. Since all their needs are met in the canopy, some frog species spend all their lives there.
    Climate also affect the diversity in tropical rain forest. Tropical rainforests receive almost 12 hours of sunlight every day. This sunlight is converted to energy by plants through the process of photosynthesis. Since there is a lot of sunlight, there is a lot of energy locked up in the rainforest. This energy is stored in plants that are eaten by animals. Because there is a lot of food, there are many species of plants and animals. Temporal variation in rainfall is higher in the tropics than in the temperate zone, and contrast sharply with the low seasonal variation in temperature that has given the tropics their false reputation for climatic constancy. Seasonal temperature variation decrease monotonically from the boreal zone to the equator. Seasonal variation in rainfall in the tropics is equivalent to or greater than in most of the temperate zone. The high precipitation that is the signal feature of rain forest affects biological diversity by a variety of mechanisms. Wetter conditios allow plants to be more shade tolerant, so a greater number of plant functional types can survive. This phenomenon contributes to the high diversity of the understory plants found in some tropical rain forest. High precipitation also contributes directly and indirectly to the high diversity by other mechanisms. With increasing seasonal variation in precipitation , most trees in the forest drop their leaves for part of the year. Higher spatial variation in total precipitation in the tropics as compared to temperate climates is a consequences of greater evaporation from warm tropical oceans. This produce much greater maximum amounts of rainfall, as well as more extreme rainfall gradients across continental margins and mountain ranges. Spatial variation in seasonal patterns of precipitation results from both the latitudinal interactions of trade winds with topography and from the intertropical convergence zone.
    In the heterogeneity of biological environment, Evidence suggests that biodiversity has been enhanced by the enormous heterogeneity of the internal rainforest environment. The highly varied kinds of habitats available in tropical areas (differing according to altitude, rainfall, seasonality, soil type, swampiness, etc.) have led to the evolution of a myriad of plants and animals specialized for each of them. Even areas of apparently fairly uniform forest may vary in soil type, topography, or altitude and each area has its own assemblages of plant and animal species and its own ecological webs. Often habitats to which organisms have adapted are very localized, some as small as 5 to 10 km2. Therefore it is not surprising that tropical rainforests should have high diversity, since they contain so many of these specialized habitats. Amazonia, in particular, has great habitat heterogeneity because of its many large river systems, which provide seasonally-flooded forest plains with transitional forests (varzea), palm swamps where the forest is perpetually flooded, lake margins, and, between them, terra firme forests (which can themselves be divided into those on clay soils and those on sandy soils, and the latter of which can be either dry, or waterlogged, after rainfall); limestone outcrops; cloud forests; lakes, rivers, and streams. The river margins and intermittently-flooded areas allow for a variety of stages of forest succession. There are also numerous soil types, which very strongly influence the organisms which live on them and which provide great opportunities for specialist organisms. As an example, many different families of plants in the Amazon basin can live only on a certain type of clay soil almost devoid of phosphorus. These species are endemic and, curiously, many of them have adopted very large thick leaves, although they are not closely related.
    The next factor is The prevalence of specialized habitats, The fact that a great number of tropical plants which are restricted to specialized habitats (and are therefore called “habitat specialists”) has given rise to another explanation for tropical biodiversity. Many specialist plants survive only in areas of unusual habitat, which suggests that much of the speciation in the tropics (at least of plants) might have arisen through adaptation for specialized habitats. For instance, the small neotropical plant genus Phryganocydia has only three species, two of which have arisen as apparent offspring of P. corymbosa. The parent species has wind-borne seeds, but the two derivative, swamp-dwelling species have wingless seeds which are dispersed by water. Another derivative group (not yet a separate species) lives in varzea forests and has seeds with partial wings – a representative of incipient speciation. Here selection for specialized habitats is occurring.
    In the Presence of pathogens, Fairly recently it has been proposed that some of the great diversity of tree species in tropical forests might be (at least in part) the result of the activity of pathogens (van der Putten, 2000). In rainforests (and probably in some temperate forests as well), many insects and some other herbivores are adapted to survive on a single species of tree or plant. When a tree becomes infected or infested, other young trees in the vicinity of infected ones will be attacked more severely by pathogens than those farther away from the source of infestation. But if the pathogen can attack only one species of tree, trees of other species can become established near the infected individual without harm. Thus trees of any one species will not be able to survive in close proximity to each, and will become widely dispersed throughout a forest. Under these circumstances, trees of many different species will be present within a small area (unlike many temperate forests, which may consist mainly of one or a few species). Soil pathogens may play a similar role in stimulating the biodiversity of soil flora and fauna.
    Another factor is Natural disturbances. There is some evidence that natural disturbances can maintain species diversity, at least among forest plants (and, since they depend upon and are adapted to plants, animals as well). Storms and high winds are common in tropical areas, and frequently lead to considerable damage and the formation of fairly large gaps in forests. When the gap in the forest is small (as when one or a few trees fall), pioneer species will normally enter the gap and flourish, eventually being replaced by climax tree species. If the gap is larger, there may not be sufficient seeds and seedlings of pioneer species to populate the gap, and so the seedlings of other species as well can become established. Thus these gap areas will have a high diversity of plant species compared to undisturbed forest. The formation of large gaps may be essential to the maintenance of diversity in rainforests.
    In the Mountains as diversity refuges, Some mountain regions contain clusters of newer species as well as older ones, which has led to the hypothesis that mountains provide stable habitats for species – older species being maintained and new ones forming. There is evidence for tropical bird speciation in mountainous areas of east Africa (greenbuls) and the Andes (spinetails). According to this scenario, mountains act as refuges because they contain many types of habitats in which species can persist by migrating to appropriate altitudes. After organisms move into varied habitats at different altitudes and were thereby separated from each other, speciation occurs.
    Beside that, in the spatial variation in soil, processes involved in soil formation ,such as the weathering of minerals and the decomposition of organic matter are accelerated under warm, moist condition (Jenny,1941). The great variety of tropical soils result as much from the variation in climatic conditions and soil age as it does from the variation in soil parent materials. Tropical soil vary greatly in both their chemical and physical properties, ranging from extremely fertile and productive to extremely infertile. There is a little truth to the old idea that much of the tropical rain forest is underlain by latosols, red clay soil that are irreversibly baked into bricklike laterite when the forest is cut and the soil exposed to the sun. 
    Endemic species ( species with spatial ranges below a certain size or confined to a limited area) are often major contributors to high species diversity in tropical rain forest, both in terms of within-habitat diversity and small-scale geographic diversity. The tropics have an important feature in common with another environment that has high levels of endemism. Soils of the mediteranean climate regions, which have extremely high levels of endemism, are generally quite poor and there seems to be an inverse correlation between soil fertility and endemism in these region. 
    Much of the high total diversity of vascular plants in some tropical rain forest is a result of many species of epiphytic plant that live on the trunks, branches, and leaves of the trees. Epiphytes are plant with strategy that assures that they will exist in a high light environment without investing the resources to become very large. Many rain forest epiphytes have strong functional and taxonomic similarity to the plants that dominate arid environments, having high tolerance for low moisture condition. Epiphytes are totally dependent upon trees to provide the physical structure that allows them to avoid heavily shaded conditions. 
    Tropical rain forest with a lot of kind of organisms living there have contributed so many things for the human life in the world. And the diversity of the tropical rain forest was affected by some factors such as Massive size of trees , Large Area or total area in each region , Canopy, Climate, Heterogeneity of biological environment, The prevalence of specialized habitats, Presence of pathogens, Natural disturbances, Mountains as diversity refuges, spatial variation in soil, endemisms, and Epiphytes plant. The structure of this system is very important for the sustainable life for all hierarchy in tropical rain forest and being the crucial things for the organisms to exist.

    Huston, Michael A.(1994).The Coexistence of species on changing landscapes.In the Biological Diversity. New York: Cambridge University press. 669.483-555
    Ingrouillr Martin.(1992).Diversity and evolution of land plants.United Kingdom : U.S.A
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