Geoscience Reference
In-Depth Information
Farming and Rural Affairs (DEFRA) through its support
of low-intensity grazing in the Environmentally Sensitive
Area (ESA).
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 topograph-
ical 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 equilib-
rium 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 environ-
mental 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 vege-
tation 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 20.12
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.
CLIMAXES
The concept of climax vegetation arose at the beginning
of the twentieth century. At a time when soil scientists
were working with the idea of 'zonal soils', and clima-
tologists were defining 'climate regions', ecologists started
to conceptualize a stable type of natural vegetation which
would be in complete equilibrium with climatic and soil
conditions. The theory was put forward that, with no
human interference, the end point of succession would be
a self-sustaining and self-perpetuating community. This
community would be the one which could compete most
successfully in the prevailing soil and climatic conditions.
The US ecologist Frederick Clements proposed the term
climatic climax vegetation
, defined as 'vegetation in stable
equilibrium with climate and soil, given undisturbed
conditions and free soil drainage'. Clements is credited
with advancing this
monoclimax theory
,or
climatic climax
theory
. He developed the concept of the plant community
as an 'organism' which followed a sequence of stages as it
developed into a mature state. The mature state or
'climatic climax' would be in equilibrium with the regional
climate and the zonal soil, provided there was relatively
long-term stability. All communities would reach this end
point through plant succession, no matter what had been
the initial starting point. Thus a psammosere and a
hydrosere in a given region would ultimately reach the
same steady-state vegetation. In southern Britain both
would finish up as climax oak woodland; in Scandinavia
both would finish up as coniferous forest.
The British ecologist Arthur 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 vegeta-
tion, 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
