Environmental Biology - Tropical Rain Forests

Based on Mader, Sylvia S. 1996. Biology - 5th ed. WCB and Cox, G.W.  1997.  Conservation Biology, 2nd ed. WCB.

Reading:  Cox Chapter 9.


This page contains material from the tropical rain forests unit.

Note: These notes are a skeleton, which  your instructor will embellish with anecdotes and illustrations, both verbal and visual during lectures. Therefore, reading these notes is not a substitute for coming to class!!!

Tropical Forests

There are two basic types of tropical forests: tropical rain forests and tropical deciduous forest.  Tropical rain forests are found primarily in South America, Africa, Asia and on some tropical islands.  They have a 365 day growing season, more than 200 cm rain/year, and an extremely complex canopy.  The canopy may have several layers made up of trees with various heights, as well as a profusion of epiphytes and vines.  The forest floor may be relatively open, with few plants able to survive in an area where most of the light is intercepted by the layers above.  The edges of a tropical rain forest may be a thick wall of plants; this thick growth at the light-rich forest edge is called a jungle.

The tropical rain forest is highly diverse; with many species of plants and animals.  This is due to the high productivity, the warm, moist conditions, the massive trees that provide homes for many other species including insects and epiphytes,  and so on.  According to  Cox, a plot of 50-hectares in Malaysia had over 835 species of trees (compared to less than 100 species of trees in all of North America).

The soils of the tropical rain forest are typically nutrient-poor; all of the nutrients are held in the living organisms.  Any nutrients in the soil would be swiftly leached away by the heavy rainfall.  The soils in many areas of tropical rain forests are laterite soils.  These soils are reddish in color and contain high levels of aluminum and iron; these material resist leaching.  With so few nutrients available in the soil, nutrient cycling must be highly developed, and we will consider this shortly.

Succession occurs in tropical forests.  Once cleared, a plot of land may provide for limited agriculture for a few years until the nutrients are depleted.  After the plot is abandoned, it may take over 1,000 for full recovery of the forest.

Tropical deciduous forests are like tropical rain forests, but have a pronounced dry season which causes the trees to lose their leaves.  This type of forest is found on the fringes of the tropical rain forests. As the climate gets drier, tropical deciduous forests grade into savanna, and, eventually, grasslands.

Nutrient Cycling

A discussion on tropical rainforests is a good point to deal with nutrient cycling since this phenomenon is particularly well-developed in a tropical rain forest.  Because the heavy rainfall tends to carry away nutrients, tropical rainforests have only been able to develop with the "invention" of very efficient nutrient cycling.  The warm, moist conditions in the forest are ideal for the decomposers breaking down the remains of dead organisms.  This quick decay returns the carbon and oxygen in the decomposing  material to the air, and returns nitrogen, phosphorous, calcium, and other minerals to the soil.  In the soil, the minerals are almost immediately taken up by a thick mat of plant roots and rootlike fungi.  The fungi are known as mycorrhizae (literally "fungus-roots"); many of them form symbiotic relationships with plant roots.  The mycorrhizae supply the plant with minerals and water; the plant returns sugars to the fungus.  In some cases, the association between the plant and the fungus is so close that the fungal filaments (hyphae) actually penetrate the plant roots.  In some cases, the root/fungal mat is so thick that there is direct nutrient cycling - nutrients move from dead organic matter into the roots without entering soil.  In some rain forests in Venezuela  these mats 15 to 40 cm thick and absorb 100% of the calcium and phosphorous (Stark and Jordon, 1978).  In climax forest the roots  are near the surface; in successional forests the roots run deeper.

Nutrient cycling occurs in almost every ecosystem; it's just better developed in tropical rain forests.

Stark, N.M. and C.F. Jordan. 1978. Nutrient retention by the root mat of an Amazonian rain forest. Ecology 59:434-437.

Rain Forest Destruction

Rain forests are threatened by a number of factors.  Despite the poor soil, population pressure from areas adjacent to rainforests has caused many small farmers to cross into the forests and clear small plots.  Equally important are the relatively rich investors who buy large tracts to clear and put into pasture for cattle.  Rainforests often overly rich mineral deposits and thus must be cut as a first step in mining.  Forests are also being cut, of course, for the timber that resides there.  Often forest clearing is done by burning.  This destroys the lumber, but is cheap and quick.  When the trees are felled, all of the life that depended on them is also lost.  Even where forest is left standing, it is often fragmented, and thus unsuitable for many species. Because there are so many species in the rain forest, loss of an acre of rainforest has potentially greater consequences than loss of an acre of almost any other habitat in terms of biodiversity.  Rainforests also play a major role in global weather patterns, and in the balance of CO2 in the atmosphere.  Tropical rainforest destruction puts great amounts of CO2 into the air at the same time it eliminates the trees responsible for removing CO2.  Because it increases CO2 levels, tropical rain forest destruction contributes to global warming.


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