Environmental Engineering Reference
In-Depth Information
Vigor stress is defined as the stresses causing only vigor reduction. Drought, pollution, grazing, insects
and diseases, windstorm, and flooding are a few examples of vigor stresses. Drought is the most
important vigor stress. If the vegetation suffers continuing drought the vigor of the vegetation may greatly
reduce or even reduce to zero. Under the action of vigor stresses, vegetation will adjust itself to fit the
stressed environment. If the vigor stresses are strong and impact the vegetation for a long time, the vigor
of the vegetation may reduce to zero. If the stresses are removed before the vegetation perishes, the
impaired vegetation can recover in a short period of time. The capability of self-healing is termed
resilience. The vegetation in warm and wet area exhibits high resilience because the precipitation,
climate, and soil favor quick vegetation recovery.
The following theorems describe vigor stresses:
(1) The vigor of vegetation is represented by the parameter V g , which varies in the range of 1 to 0.
V g = 1 implies the perfect function of vegetation in photosynthesis, erosion control, providing habitat and
primary productivity for the bio-community, wind reduction, interception of dust and raindrops, increasing
percolation into the soil, etc., and V g = 0 represents death of the trees and zero ecological functions.
(2) Vigor of vegetation reduces if vigor stresses act on the vegetation, i.e., V g < 1. The vegetation may
recover if the stresses are removed.
(3) The speed of vigor recovery depends on the structure of vegetation, or on the resilience of the
vegetation. The resilience r e is a function of the composition of the species of the vegetation, the climate,
precipitation, and the soil composition.
(4) If vigor stresses act on the vegetation for a long period of time the vigor of vegetation may reduce
to zero, which results in mortality.
Both mortality and vigor stresses are negative because they cause reduction of vigor or death of
vegetation. Reforestation and nursing enhances the cover and vigor of vegetation, and is a positive stress.
They are not included in the mortality and vigor stresses.
In some cases, the vegetation has developed under very harsh ecological conditions and the development
has taken a long period of time under no external stresses. The resilience of such a vegetation is usually
very low or zero. Any strong stress, vigor stress or mortality stress, acting on the vegetation may kill the
vegetation. Such a kind of vegetation is called vulnerable vegetation. For instance, the forest in the glacier
near Banff in Canada and the trees in Sina; Peninsula in Egypt, shown in Fig. 2.18, cannot recover if the
vegetation is destroyed by a forest fire or logging.
The grassland on the Qinghai-Tibet Plateau of China has developed over quite a long period of time
because the temperature and precipitation are low. A part of it was damaged by human harvesting of
medicinal herbs (roots of the grass). It could not recover even if the stresses are reduced or removed. In
the Maousu Desert in northwest China, people have stopped the motion of sand dunes by planting straw
and dry grass to form a framework, as shown in Fig. 2.28(a). Planting shrubs in the desert is not a
difficult job, but to assist the shrubs to survive wind storms is not so easy. Figure 2.28(b) shows the
human care for protection of shrubs to resist wind. After a long period of time the human efforts have
resulted in nice vegetation cover on the desert, as shown in Fig. 2.28(c). After a long period of
continuous effort the dunes are greened by grass and shrubs and a surface crust has formed on the desert,
which can resist against wind erosion. Nevertheless, the vegetation is vulnerable. If any stress by human
activities damages the vegetation, it cannot recover by natural processes. Human and animal traffic can
damage the crust. If the sand dunes are moved by wind, the vegetation can be quickly destroyed, as
shown in Fig. 2.28(d).
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