Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 77

damage. Compaction of soils by grazing animals can cause increased soil bulk density, reduced infiltration,
and increased runoff. Loss of capillarity reduces the ability of water to move vertically and laterally in
the soil profile. Reduced soil moisture content can reduce site capacity for lowland plant species and favor
upland species. Excessive trailing can result in gully formation and eventual channel extension and migration.
Unmanaged grazing can significantly impair vegetation. Figure 2.26(d) shows trees damaged by the goats.
Heavy husbandry impairs vegetation.
Recreation —The amount of impact caused by recreation depends on soil type, vegetation cover,
topography, and intensity of use. Various forms of foot and vehicular traffic associated with recreational
activities can damage riparian vegetation and soil structure. All-terrain vehicles, for example, can cause
increased erosion and habitat reduction. At locations, reduced infiltration due to soil compaction and
subsequent surface runoff can result in increased sediment loading to a stream (Cole and Marion, 1988).
Both concentrated and dispersed recreation can cause disturbance and ecological change. Camping, hunting,
fishing, boating, and other forms of recreation can cause disturbances to vegetation and bird colonies.
Motorcycles and horses cause far more damage to vegetation and trails than do pedestrians.

2.2.3 Classification of Stresses
The degree of vegetation development may be represented by the coverage of trees and shrubs, the
thickness of vegetation (height of trees shrubs and grasses) and the vigor of the trees, biomass per unit
area, and age and health of the plants. In the view of controlling erosion, binding soil with roots and
shelter of plants are a highly effective function of vegetation, therefore, the mass of the plant roots per
unit area and cover of vegetation are meaningful parameters. However, the mass of plant roots is difficult
to measure and the measurement may damage the plants. Thus, the density of vegetation cover (vegetation
cover for simplification) and vigor typically are applied to represent the state of vegetation development.
The vegetation cover is defined as the percentage of the area with trees and shrubs in the entire area. The
vigor of trees is dynamic responding to the impact of various stresses and is an important indicator if the
instantaneous state of the vegetation is studied.
Long-term stresses —Ecological stresses, natural or human-induced, can be classified into: ķ long-term
stresses, such as erosion, air pollution, and grazing; ĸ short-term stresses, such as drought, pests and
diseases, and acid rain; and Ĺ instant stresses, such as a volcano eruption, forest fire, logging and wind
storms. Vigor reduction and mortality of vegetation may result from short-term stress (e.g., drought) acting
on trees that have been predisposed to injury by long-term ecological stresses (e.g., air pollution).
Long-term ecological stresses, for instance, the stress resulting from air pollution can be mathematically
expressed by:

W (2.2)
In which Po 1 , Po 2 , and Po 3 are the concentrations of pollutant 1, pollutant 2, and pollutant 3; and a 1 , a 2 ,
and a 3 are impact factors of the pollutants on the vegetation. Here long-term implies the period during
which the present vegetation developed.
Short-term stress , such as drought, impacts the vegetation temporarily (one year or several years) but
more intensively.

A a Po a Po a Po

3 3 ... 11 2 2

PP

P

e

(2.3)

W

P

e

in which P is the precipitation in a year, and P e is the vegetation water demand. The vegetation water
demand can be estimated by using an ecological method or a hydrologic method. The former calculates
the water demand according to the plant species of the vegetation. The later assumes that the vegetation