Environmental Engineering Reference
In-Depth Information
Vegetation succession is defined as the process of an initial pioneer suite of plants established in the
early stage of colonization of bare land, which consists mainly of herbaceous species that require high
amounts of light, being replaced gradually by a suite of plants, which consists of woods, shrubs, and
grasses that tolerate low light or closed canopy situations. There are four types of vegetation succession
associated with river management, i.e. ķ reforestation of forests removed by different stresses, such as
logging, forest fire, and volcanic eruption; ĸ colonization and development of the plant community on land
newly created by sedimentation, such as bars or islands in a river channel; Ĺ vegetation development on
blown sand dunes with humans disturbance; and ĺ vegetation development on landslide and avalanche
deposits. Figure 2.23 shows a vegetation succession from a pioneer suite consisting of lichen and grasses
(a), to a suite consisting of shrubs and grasses (b), and finally to a complex suite consisting of woods,
shrubs and grasses (c) on the Yunnan-Guizhou Plateau in southwestern China.
Riparian vegetation —While watershed vegetation affects the long-term development of the river
network, riparian vegetation exerts more direct and short term effects on the degradation and aggradation
of the river channel. Riparian vegetation may be defined as the vegetation growing on fluvial surfaces
that are inundated or saturated by the dominant or bank-full discharge (Hupp and Osterkamp, 1996).
Woody vegetation may be removed or damaged by degrading channel processes, and, in turn, may
substantially ameliorate degrading conditions and play a critical role in the initiation and character of
channel recovery processes. Most floodplains, riverine wetlands, river channel banks, and in-channel
features potentially support riparian vegetation; only terraces with flood return intervals exceeding about
3 years do not typically support riparian assemblages (Nilsson et al., 1989). Intact riparian zones are
recognized as critical features in the landscape for maintenance of biodiversity. They are 'the most
diverse, dynamic, and complex biophysical habitats on the terrestrial portion of the Earth' (Naiman et al.,
1994). In temperate areas, the riparian zone supports more species of plants than any other habitat. In
western North America, riparian zones comprise less than 1% of the total land area, yet 80% of terrestrial
vertebrate species are dependent upon them for at least part of their life cycle (Miller et al., 1995). In
North America and Europe, more than 80% of riparian corridors have disappeared in the last 200 years
(Naiman et al., 1992). Because channel incision promotes the development of terraces that are infrequently
flooded, incision potentially has a substantial role in altering, damaging, or destroying much of the
riparian zone world-wide.
Watershed vegetation —Vegetation in a watershed is the most important factor affecting surface runoff
and soil erosion. Vegetation and erosion are a couple of competing and mutually interacting aspects of a
watershed. In the northern part of the Loess Plateau of China, the vegetation hardly develops because the
extremely high rate of erosion tears away the topsoil, on which the vegetation relies. In areas with
vegetation, such as the upper reaches of the Yangtze River, erosion damages and destroys the vegetation
and scars the land surface. Erosion causes not only soil loss but also loss of water and nutrients. Kosmas
et al. (1998) found that under semi-arid climatic conditions water vapor adsorption by the soil could be more
than the water received from rainfall. They measured that from February to August. 1996, a total amount
of 226 mm of water vapor was adsorbed by the soil, while the total rainfall was only 179 mm in the same
period. Soil erosion reduces this adsorbed water, which is important for vegetation development. The total
organic carbon in soil is about 1,550 billion, plus 1,200 billion of C in oil, gas, and coal (Schapenseel and
Pfeiffer, 1998). On average, the world's rivers annually transport about 0.5 billion tons of organic carbon
to the oceans. This transport, in general, is equally distributed between dissolved and particulate fractions
of the river load (Spitzy and Ittekkot, 1991). A significant fraction of the transported carbon is finally
oxidized and emitted into the atmosphere. A principal off-site environmental impact is due to the disruption
of the global carbon cycle, resulting in an erosion-induced efflux of about 1.14 billion t/yr from soil to
the atmosphere (Lal and Kimble, 1998). The influence of the efflux on the climate is considerable.
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