Geoscience Reference
In-Depth Information
act together to produce a net effect larger than the sum
of the separate effects when the factor operates alone; this
is called a
synergistic
effect.
Figure 20.1
illustrates also that there are some
environmental factors which directly and locally affect the
plant community, whilst others seem more distant, with
only indirect effects. Thus it is possible to distinguish
between
direct
and
indirect
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.
The relationship between plants and their environment
forms the cornerstone of modern ecology. The study of a
single species and the environmental conditions that
control it is the science of
autecology
. Other studies
involve the entire plant community, stressing the links
between organisms in the science of
synecology
.Modern
ecosystem approaches contain elements of both.
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
Environmental factors controlling
vegetation
For successful growth the plant requires six essential
factors: light, heat, moisture, air, nutrients and physical
support. Plant carbohydrates are photosynthesized by
short-wave solar radiation, atmospheric carbon dioxide
and soil moisture. For growth and development this
process of
photosynthesis
must take place at a faster rate
than the rate of breakdown of carbohydrate by plant
respiration
. Next in importance to these fundamental
processes is the process of transpiration, whereby moisture
is absorbed from the soil by plant roots and transported
up the stem via the xylem tissue to leaves, where it is
evaporated into the atmosphere through leaf pores or
stomata. Transpiration is not only a cooling mechanism
for leaves exposed to solar radiation, but also the
mechanism for the majority of nutrients to be absorbed
into the plant and moved within it.
Figure 20.1
shows the position of plant communities
in relation to the different environmental factors which
control their structure, productivity and distribution.
Environmental factors are not independent variables but
are typically influencing each other, as well as the plant
community. Thus increased radiation brings an increase
in temperature which brings a decrease in soil moisture.
An increase in slope angle brings a decrease in soil depth
which brings a decrease in soil moisture. The directions
of the controls are indicated by the arrows in
Figure 20.1
.
It is a feature of vegetation that two or more factors can
Latitude
Aspect
Radiation
Precipitation
Temperature
Altitude
Soil
Moisture
Slope
Plant
Community
Soil
Figure 20.1
Environmental factors which
control the nature and
distribution of plant
communities.
Past and Present
Land Use
Geology
Direction of control
