Environmental Engineering Reference
In-Depth Information
environmental factors. Soil conditions (pH, nutrients, soil oxygen, depth, absence of toxic
chemicals) and soil moisture (sufficient available moisture for transpiration) directly
affect plant growth, as also do human land-use practices. Other factors are important in
explaining plant distributions, although they themselves have no direct effect on plant
growth. Factors such as aspect and slope, for example, are indirect factors which are
nevertheless important because they explain, through correlations with direct growth
factors, a high proportion of plant distributions.
LIMITING FACTORS AND RANGE OF TOLERANCE
The relationship between plants and their environment forms the cornerstone of modern
ecology. In the past some biogeographers have studied a single species and the
environmental conditions that control it; this is the science of
autecology
. Others have
studied the plant communities as a whole, especially the links between organisms; this is
the science of
synecology
. The modern ecosystem approach would contain elements of
both of these; the entire community would be studied, in terms of the relations both
between the living components and between the living components and their physical
environment.
In the nineteenth century the German soil chemist Liebig formulated his famous
law
of the minimum
to express the influence of an environmental factor on plant growth. He
argued that if growth depends on several factors, what is important is that factor which is
in short supply. It is of little use, for example, if all factors are favourable but one
(perhaps a soil nutrient) is missing or in low supply. Productivity in this situation will be
zero or low, irrespective of the abundance of other factors. The law of the minimum
states: 'Growth is governed by the factor which operates at a minimum.'
In the real world environmental conditions exist as
gradients
. There may, for
example, be a pH gradient from a basic igneous rock (e.g. basalt) to an acid igneous rock
(e.g. granite). A moisture gradient may go from a wet valley bog to dry ridge crests in the
same valley. The changes in the performance of a plant species along such a trend are
called an
environmental gradient
(Figure 20.2). There will be upper and lower threshold
values on the gradient beyond which the species cannot survive. These
Figure 20.2
The range and ecological optimum of plant
species.
