STATE OF THE RAINFOREST 2014
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We find tropical rainforests in all the equatorial regions of
America, Africa, Southeast Asia and Oceania, extending
between the Tropics of Cancer, and Capricorn; north to central
Mexico and the Indian plains, south to the southern part of
Brazil and the northern tip of Australia.
Occupying only 6 % of the earth’s land surface, and in spite of
the relative poor soil, they contain a larger diversity of plants
and animals than anywhere else on earth. The rainforests
have evolved over the course of 50 to more than 100 million
years. They exhibit great variation in climate – from evergreen
rainforests where there is minimal variation in precipitation
and temperature, to rainforests with seasonal variations of drier
periods. They also vary in habitat – from swamp to dry land; and
in elevation – from lowland to montane rainforest and cloud
forest. Rainforests share, however, some general characteristics:
Stable temperature:
Around 20–25°C throughout the year.
Wet:
Annual rainfall exceeding 2,000 mm; some places can
get up to 10,000 mm (10 m) per year.
Dark:
Only 2% of the sunlight falling on the upper canopy
reaches the forest floor.
Nutrient-poor soil:
The soil is often acidic and nutrient-poor,
but there are variations.
Diverse:
Theabundanceof animal andplant species isenormous.
Asmany as 100 different tree species (types) can be found within
one square kilometre. There are several millions of different
animal species, and insects, reptiles, amphibians and mammals
are especially well represented. The total number is unknown.
Layers:
The emergent layer consists of very tall trees (up to
60–70 meters). The dense canopy layer further down absorbs
most of the sunlight, but along the trunks and stems grow
dense lianas, lichens, mosses, ferns and flowering plants. The
air in the lower layer is stagnant and humid. On the forest floor,
it is cool and shady. Between branches and twigs, seeds can
survive for years, waiting for the right conditions to re-awaken.
What is a tropical rainforest?
described in the rainforest in French Guiana, the
Pouteria maxima
.
With its thick bark and leaves, this tree is exceptionally resistant to
fire and drought and thereby has an important buffer function for
maintaining forest structure in a time of climate change.
The intricate interdependencies between species are illustrated by
the Brazil nut tree (
Bertholletia excelsa
). These trees have particularly
hard-shelled seed pods, which only the agouti – a large, forest-
dwelling rodent with particularly strong teeth – is able to break. As
the agouti stores the seeds in caches buried in the ground, some
of the seeds germinate and become the next generation of trees.
Moreover, in order to produce the nuts, the tree is dependent on a
specific species of large-bodied bees – Euglossine orchid bees. The
bees depend for it’s part on other species for their reproduction:
Unless the male bee covers itself with the scent of a specific orchid,
it won’t be able to attract the female. With such complex webs of
interdependencies, it is no wonder that it has turned out to be
impossible to grow the Brazil nut tree in plantations. But it also
implies vulnerability: take away one species, and a whole chain of
other plants and animals may be affected.
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