Environmental Engineering Reference
In-Depth Information
finish up as climax oak woodland; in Scandinavia both would finish up as coniferous
forest.
The British ecologist Arther Tansley, who coined the term 'ecosystem' (see Chapter
20), was engaged in studying the interaction of plants and soils, and also the influence of
grazing and other activities of animals on plants. He was, of course, interested in the
ways in which at any location plants, fauna, soil and climate form an interacting
ecosystem or equilibrium. Although he agreed with Clements in theory about equilibrium
and climax vegetation, he disagreed that there would be
one
climatic climax and that
plant communities behaved as organisms. He believed that the term 'organism' was best
reserved for individual plants and animals. He argued that ecological communities are
essentially complex physical-biological systems. He regarded the biosphere as a vast
number of such systems, each one tending towards its own state of maturity and
equilibrium. The time required to reach Clements's climatic climax was in practice too
long to make the concept realistic. Other environmental factors would be powerful
enough to hold a community relatively stable for considerable periods of time. Thus soil
factors of drainage or chemistry (
edaphic climax
), or topographical factors (
topographic
climax
), or human and animal activities (
biotic climax
) would prevent a true climatic
climax from forming. Within any climatic region, different plant communities could be in
relatively stable equilibrium with any one or a combination of the above factors. This is
the
polyclimax theory
of Tansley.
A third theory of climax vegetation is that of the US ecologist Whittaker, whose ideas
are similar to Tansley's. Whilst studying the vegetation patterns of the Great Smoky
Mountains, in Tennessee and North Carolina, he developed his
mosaic theory
to describe
what he called
climax pattern
. He noted that similar patterns of environmental and biotic
pressures do repeat themselves, and the vegetation is repeated like similar patterns within
a mosaic. He noted also that only 60 per cent of the vegetation could be placed in these
types, and that there was considerable gradation across community boundaries. These
'transitional' communities are termed
ecotones
.
The fourth concept of climax was first suggested by the French ecologist Aubréville
whilst studying the tropical rain forests of the then French West Africa in 1938. The
theory was revived in the 1980s, when there was renewed interest in tropical vegetation.
It is the
cyclical climax theory
, and its modern supporters argue that it is valid for many
ecosystems outside the tropics too. According to this theory, forests are regarded as areas
of cyclical succession of growth and decay. As the cycles are out of step, the forest has
the appearance of a mosaic, owing to different cycles operating side by side. There are
three key elements in the cycle. First, an
optional phase
of trees of roughly equal age is
established. Second, the optional phase deteriorates into a
decay phase
caused by the
collapse of the forest over the greater part of a particular area. Young plants are now able
to become established, but these young plants are often not the original tree species. Thus
the collapsed primary forest is succeeded by a different tree community which, when it in
turn collapses, is replaced in the third stage, the
mature
phase, by another even-aged
forest community. According to the cyclical theory, what one encounters in a primary
forest is not a constant steady state but a regularly recurring cycle. As different parts of
the landscape are at different stages of the cycle, a patchwork mosaic results. Figure 21.6
illustrates the cycle in a tropical forest when an opening in the tree canopy is caused by a
natural tree fall through disease or by ageing. After the gap has opened, light and
