Environmental Engineering Reference
In-Depth Information
Figure 20.1
Environmental factors which control the nature
and distribution of plant communities.
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 must take
place at a faster rate than the rate of breakdown of carbohydrate by plant
respiration
.
Second in importance to this fundamental process of
photosynthesis
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. The
environmental factors are not themselves 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 act together to produce a net effect which is 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
