Environmental Engineering Reference
In-Depth Information
control of soil erosion-including rainfall and surface runoff erosion and wind erosion; habitat, and primary
productivity. For instance, the roots and fallen leaves from the shrubs and grass on the Maousu Desert in
Northwestern China result in surface crusting of the sand dunes, which protects the desert from wind
erosion and fixes the sand dunes. In general, the ecological function of vegetation depends on the density
of vegetation cover and the vigor of vegetation.
Fig. 2.27
The mount St. Helens Volcano erupted in 1980 and destroyed the forest vegetation cover. The picture was
taken in 1993. The mountains and hills of the area were still bare 13 years after the eruption of the volcano
The ecological functions of vegetation are a function of vegetation cover, vigor, and functional index:
F
VV
[
(2.10)
g
in which
F
= functional capability of vegetation,
V
= fraction of vegetation cover,
V
g
= vigor of vegetation,
[= the functional index. The value of [ is different for different ecological functions. The functions of
wind erosion control, habitat, and primary productivity depend highly on the vigor of vegetation, and the
value of [ is large. In this case both vigor stress and mortality stress are important. For rainfall and
surface runoff erosion, however, vegetation controls the erosion mainly by its root system, in which the
vigor of vegetation plays a less important role, and the value of [ is small. In this case, the mortality
stresses are mainly considered and vigor stresses are omitted.
Vegetation is composed of trees, shrubs, and grass, which may overlap with each other. In hilly areas,
the roots of grass are not as strong as those of tree forests to resist flowing water and protect gully banks
and the overland slope from erosion. The vigor of vegetation may be represented by the density of
foliage and branches, the biomass production per unit area, or age and health of the plants. It can be
measured by the ratio of the seasonal biomass production per area to the biomass production of the
vegetation subjected to no stresses. The vigor reduces to zero if the vegetation is impaired, and dies
because of ecological stresses. Compared with vegetation cover, the vigor of vegetation is much more
dynamic and always fluctuates under the impacts of ecological stresses.
Mortality stress
is defined as the stresses directly causing mortality of vegetation. Volcano eruption,
forest fire, and logging are mortality stresses. Debris flow and landslide, in some cases, also cause
mortality.
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