Environmental Engineering Reference
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temperature increase on the forest floor, killing the shade-tolerant understorey. Young
seedlings of light-demanding species become established, and the gap becomes closed by
the upward growth of these pioneers and by the crowns of trees surrounding the gap
growing into the open. When the gap is closed again, no more light-demanding seedlings
become established, but adults may persist. Seedlings of forest species will become
established, and understorey plants adapted to low light and temperatures will thrive
again.
In addition to the tropics, cyclical development has been reported from the species-
rich forests of eastern Europe. Here, as in the tropics, it is noted that trees which die are
replaced not by the same species but by different species from the forest. There may be
several causes. One is competition for light. Beneath trees that give heavy shade (e.g.
common beech, Fagus sylvatica ) the young plants of light-requiring trees like beech are
unable to mature in the shade. Another reason to explain the failure of a plant to thrive on
the same site as its parents may be the fact that different species do not have identical
nutrient requirements. If a particular species has removed nutrients from a site for a long
time, the same species may not be able to thrive on the same site.
A biological mechanism involving animals has been reported by the US ecologist
Janzen from his studies of tropical forests in Costa Rica. In explaining why individuals of
a species are scattered in the forest, with no near neighbours, he came to the conclusion
that intense predation of tree fruits when they fall to the ground by insects, especially
ants, means that a tree can reproduce only when its seed is carried far from the parent by
birds or monkeys. Seed mortality is always 100 per cent, so that regeneration in situ is
never possible.
Figure 21.6 The theory of cyclical climax as illustrated from
an area of tropical rain forest. A clearing caused by a natural
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