A big reason for the lack of nutrients
in the soil is that the waste matter of the plants and animals is quickly
broken down by the large numbers of fungi and other decomposers. This process
is also accelerated by the constant warm, humid, and dim conditions inside
the canopy. Sun seeking plants will grow so competitively that they end
up blocking out most of the light and creating the completely different and
relatively stable conditions inside, safe from wind, rain, and too much sun.
Another feature of rainforest trees, though they are very tall, is that
they often have buttressing to help support their main trunks because their
roots must remain shallow to retrieve nutrients from the soil. These extra
projections of their trunks help them stay stable without being too firmly
rooted.
There are several different kinds of rainforests.
1. Tropical lowland evergreen rainforest.
This type has the highest number of different species. It is split into
the classical layers of the rainforest: emergents, canopy, understory, ground
layer. There are many trees, epiphytes and woody vines but few
herbaceous plants on the forest floor. Buttressing, cauliflory and smooth
bark are common features.
2. Tropical semi-evergreen rainforest
This has more deciduous trees in the mix. In general, the height of the
trees is a little less than in the evergreen rainforest. There are woody vines,
epiphytes (orchids, ferns), and bamboos. Though there are still
the characteristics of the evergreen forest, they are less pronounced.
3. Montane rainforest.
4. Heath forest.
These are built on soil that is of sandy origin and is generally acidic.
There are skinny climbers instead of woody ones and many epiphytes as well
as insectivorous plants.
5. Peat Swamp forest.
6. Freshwater swamp forest.
This occurs often near the Amazon river because of the annual floods. There
is generally a diversity of forest types near the water because of the increased
nutrient level in the soil from the river.
In the rainforest, there are not only the large trees that typify it
but also a number of different climbing species which take advantage of
the support from the trees to reach the much sought-after sunlight. These
include epiphytes, bromeliads, stranglers, and lianas. Epiphytes and bromeliads
can be completely airborne, gathering nutrients from debris that gathers
on them and water from the air. Stranglers will sprout on a tree limb, send
roots to the forest floor and then proceed to wrap themselves around their
host, eventually killing it entirely. Lianas are large, woody vines which
extend from the forest floor and climb upwards. There are also a large number
of herbaceous plants closer to the forest floor, ferns, palms, etc. On the
floor itself and in the soil there are tons of fungi, lichen, algae, moss,
etc.
In the succession of rainforests, when a clearing opens, the dormant
seeds of pioneer plants which have been lying in wait for just such an opportunity
sprout and take over. After a while, however, crowding
reduces the availability of sunlight and the more permanent climax species
come into play (these can live as sapling in the shade for a while before
growing to full maturity).
References:
"The Assault Continues." International Wildlife. November, 1998.
Caulfield, Catherine. In the Rainforest: Report from a Strange, Beautiful,
Imperiled World.Chicago: University of Chicago Press, 1991.
Golley, F.B., ed. Tropical Rain Forest Ecosystems: Structure and Function.
Vol. 14A.
New York: Elsevier Scientific, 1983.
Head, Suzanne, Heinzman, Robert. Lessons of the Rainforest. San Francisco:
Sierra Club Books, 1990.
Orians, Gordon H., Dirzo, Rodolfo, Cushman, J. Hall, eds. Biodiversity and
Ecosystem Processes in Tropical Forests. Ecological Studies, Vol. 122. New
York: Springer, 1996. 123 vols.
Whitemore, T.C. An Introduction to Tropical Rain Forests. Oxford: Clarendon
Press, 1990.
Pictures:
1. Picture from: http://library.thinkquest.org/27257/rainintro.html
