Rainforest canopy trees in Peru. (Photo by R. Butler)
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STRUCTURE AND CHARACTER
Tropical rainforests
across the world are quite diverse, but share several defining characteristics
including climate, precipitation, canopy structure, complex symbiotic
relationships, and diversity of species. Every rainforest does not necessarily
conform to these characteristics and most tropical rainforests do not
have clear boundaries, but may blend with adjoining mangrove forest,
moist forest, montane forest, or tropical deciduous forest.
GEOGRAPHY AND CLIMATE
Tropical rainforests lie in the "tropics," between the Tropic
of Capricorn and Tropic of Cancer. In this region sunlight strikes Earth
at roughly a 90-degree angle resulting in intense solar energy (solar
energy diminishes as you move farther north or south).
This intensity is due to the consistent day length on
the equator: 12 hours a day, 365 days per year (regions away from the
equator have days of varying length). This consistent sunlight provides
the essential energy necessary to power the forest via photosynthesis.
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Map showing world distribution of rainforests
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Because of the ample solar
energy, tropical rainforests are usually warm year round with temperatures
from about 72-93F (22-34C), although forests at higher elevations, especially
cloud forests, may be significantly cooler. The temperature may fluctuate
during the year, but in some equatorial forests the average may vary as little as 0.5F (0.3C) throughout the year. Temperatures
are generally moderated by cloud cover and high humidity.
PRECIPITATION
An important characteristic of rainforests is apparent in their name.
Rainforests lie in the intertropical convergence zone where intense
solar energy produces a convection zone of rising air that loses its
moisture through frequent rainstorms. Rainforests are subject to heavy
rainfall, at least 80 inches (2,000 mm), and in some areas over 430
inches (10,920 mm) of rain each year. In equatorial regions, rainfall
may be year round without apparent "wet" or "dry"
seasons, although many forests do have seasonal rains. Even in seasonal
forests, the period between rains is usually not long enough for the
leaf litter to dry out completely. During the parts of the year when
less rain falls, the constant cloud cover is enough to keep the air
moist and prevent plants from drying out. Some neotropical rainforests rarely
go a month during the year without at least 6" of rain. The stable
climate, with evenly spread rainfall and warmth, allows most rainforest
trees to be evergreen—keeping their leaves all year and
never dropping all their leaves in any one season.
Forests further from the equator, like those of Thailand, Sri Lanka,
and Central America, where rainy seasons are more pronounced, can only
be considered "semi-evergreen" since some species of trees
may shed all of their leaves at the beginning of the dry season. Annual
rainfall is spread evenly enough to allow heavy growth of broad-leafed
evergreen trees, or at least semi-evergreen trees.
The moisture of the rainforest from rainfall, constant cloud cover,
and transpiration (water loss through leaves), creates intense local
humidity. Each canopy tree transpires some 200 gallons (760 liters)
of water annually, translating to roughly 20,000 gallons (76,000 L)
of water transpired into the atmosphere for every acre of canopy trees.
Large rainforests (and their humidity) contribute to the formation of
rain clouds, and generate as much as 75 percent of their own
rain. The Amazon rainforest is responsible for creating as much as 50
percent of its own precipitation.
Deforestation and climate change may be affecting the water cycle in tropical rainforests. Since the mid-1990s,
rainforests around the world have experienced periods of severe drought,
including southeast Asia in 1997 and 2005 and the Amazon in 2005. Dry conditions, combined with degradation from logging and agricultural conversion, make forests more vulnerable to wildfire.
Review questions:
- Where are rainforests located?
- What are the tropics?
- Where/how does the rainforest get its energy?
- Rainforest are _________ since they are warm and have a lot of moisture in the air.
- How much rain do rainforests get?
- How do rainforests create their own rain?
- What is an evergreen tree?
- What are two things affecting the water cycle of tropical rainforests?
- Drought makes rainforests susceptible to what?
[print version | spanish | french | portuguese
| chinese | japanese]
Continued: Structure of the tropical rainforest - part II
This article was written by Rhett A. Butler [bibliographic citation for this page] and was last updated on the most recent date listed in the column on the right side.
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WEEKLY NEWSLETTER
INTERACT
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Recent news
Some toilet paper production destroys Indonesian rainforests, endangering tigers and elephants
(02/09/2012) American consumers are unwittingly contributing to the destruction of endangered rainforests in Sumatra by purchasing certain brands of toilet paper, asserts a new report published by the environmental group WWF. The report, Don't Flush Tiger Forests: Toilet Paper, U.S. Supermarkets, and the Destruction of Indonesia's Last Tiger Habitats, takes aim at two tissue brands that source fiber from Asia Pulp & Paper (APP), a paper products giant long criticized by environmentalists and scientists for its forestry practices on the Indonesian island of Sumatra. The brands — Paseo and Livi — are among the fastest growing, in terms of sales, in the United States.
Tropical ecologist: Australia must follow U.S. and EU in banning illegally logged wood
(02/09/2012) Australia should join the widening effort to stamp out illegal logging, according to testimony given this week by tropical ecologist William Laurance with James Cook University. Presenting before the Australian Senate's rural affairs committee, Laurance argued that the massive environmental and economic costs of illegal logging worldwide should press Australia to tighten regulations against importing illegally logged timber at home.
Humans drove rainforest into savannah in ancient Africa
(02/09/2012) Three thousand years ago (around 1000 BCE) several large sections of the Congo rainforest in central Africa suddenly vanished and became savannah. Scientists have long believed the loss of the forest was due to changes in the climate, however a new study in Science implicates an additional culprit: humans. The study argues that a migration of farmers into the region led to rapid land-use changes from agriculture and iron smelting, eventually causing the collapse of rainforest in places and a rise of grasslands. The study has implications for today as scientists warn that the potent combination of deforestation and climate change could flip parts of the Amazon rainforest as well into savannah.
Green groups: government moving too slowly on protecting Canada's Great Bear rainforest
(02/08/2012) Three environmental groups have submitted a letter to British Columbia Premier, Christy Clark, to ask the government to speed up the process of implementing the Great Bear Rainforest Agreement, which is meant to ensure 70 percent of old-growth forest is maintained.
Majority of protected tropical forests "empty" due to hunting
(02/08/2012) Protected areas in the world's tropical rainforests are absolutely essential, but one cannot simply set up a new refuge and believe the work is done, according to a new paper in Bioscience. Unsustainable hunting and poaching is decimating tropical forest species in the Amazon, the Congo, Southeast Asia, and Oceana, leaving behind "empty forests," places largely devoid of any mammal, bird, or reptile over a few pounds. The loss of such species impacts the whole ecosystems, as plants lose seed dispersers and the food chain is unraveled.
More rainforest news
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