Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 73

predicts changes in tree vigor. The results also suggest a mechanism by which short-term environmental
stresses affect tree physiology prior to death. Mulligan (1998) examined the impact of climatic variability
on hydrology and vegetation cover using the PATTERN ecosystem model. An examination of the
variability of soil erosion which results from this variable hydrology and vegetation cover indicates the
temporally erratic nature of erosion events, the tendency for most erosion to occur during infrequent
extreme events, and the dynamic response of erosion to climatic variability. The results also indicate the
dependence of erosion on the type of vegetation cover and vegetation response to climatic variability.
Thornes (1985) proposed a model of the geomorphologic process considering the rate of change of
vegetation. The differential equations of vegetation and erosion are coupled and the influence of wild
animals is modeled.
2.2.2 Ecological Stresses on Vegetation
Ecological stress —While the vegetation harmonizes itself with the environment, it suffers various ecological
stresses. Ecological stress is defined as any kind of disturbance, natural or non-natural, on the vegetation
development, which may change the vegetation cover or affect the evolution process of vegetation. Soil
erosion is the most important natural ecological stress and human activities are the non-natural ecological
stress. Conversely the development of vegetation cover affects the soil erosion with the interaction,
following a law of dynamics. For a watershed, vegetation and erosion may reach an equilibrium state if
the circumstances remain unchanged for a long period of time.
Generally speaking, vegetation suffers from various ecological stresses, including erosion, forest fire,
drought, windstorm, grazing, air pollution, logging, acid rain, and reforestation. Human activities, including
mining, road construction, logging, and reforestation, exhibit the most direct, and in many cases, the
strongest ecological stresses on the vegetation. Drought is the most potent natural ecological stress
reducing the vigor of vegetation, which may eventually cause mortality of vegetation because most of the
dead trees experienced vigor reduction prior to mortality. Bussotti and Ferretti (1998) studied the impact
of air pollution on a forest in Europe and found that air pollution reached the concentration likely to have
adverse effects on forest vegetation. Ozone has been proven to cause foliage injury in a variety of native
forest species in different countries. Obvious declines of vegetation related to environmental factors are:
ķ the deterioration of some coastal forests due to the action of polluted sea spray; ĸ the deterioration
of reforestation projects, especially conifers mainly due to the poor ecological compatibility between
species and site; and Ĺ the decline of deciduous oaks due to the interaction of climate stresses and pests
and diseases. Catastrophic wind causes damage and fall of trees. Clinton and Baker (2000) studied the forest
tree-fall due to storms and pit and mound topography resulting from catastrophic wind in the U.S. Wind
has caused large-scale forest disturbance.
Natural stresses —Among the many natural ecological stresses that disturb structure and functions of
vegetation are erosion, drought, storm, floods, hurricanes, tornadoes, fire, lightning, salinization of soil,
volcanic eruptions, earthquakes, insects and disease, landslides, temperature extremes, and grazing. For
instance, erosion damages the vegetation on the slope of the Yunnan-Guizhou Plateau of China (Fig. 2.25(a));
salinization of soil may kill the shrubs, grass, and trees (Fig. 2.25(b)); and an earthquake induced landslide
destroyed the vegetation on the Jiufener Mountain in Taiwan, China in 1999 (Fig. 2.25(c)); insects killed
trees (Fig. 2.25(d)); a heavy snow broke trees and almost killed a forest on the Zhangjiajie Mountain
(Fig. 2.25(e)); and a forest fire cleared the vegetation in the upper reaches of the East River watershed in
Guangdong Province in southern China (Fig. 2.25(f)). For the latter case most of the roots are still alive
and new vegetation is quickly developing.
Natural disturbances are sometimes agents of regeneration and restoration. Certain species of riparian
plants, for example, have adapted their life cycles to include the occurrence of destructive, high-energy