Integrated River Management - River Dynamics and Integrated River Management - page 783

and step-pool system may be the best effective for controlling various types of debris flow, in which the
dams trap sediment and the step-pool system dissipate the flow energy and protect the bed against
incision.
11.3.3 Reforestation and Terrestrial Ecosystem Management With Selected Wood Species
Vegetation is the most effective form of erosion control. No man-made products can approach it in
long-term durability and effectiveness. Vegetation shields the soil surface from the impact of falling rain,
slows the velocity of runoff, holds soil particles in place, and maintains the soil's capacity to absorb water.
These plant communities in the watershed are a valuable source of energy for the biological communities,
provide physical habitat, and moderate solar energy fluxes to and from the surrounding aquatic and
terrestrial ecosystems. The characteristics of the plant communities directly influence the diversity and
integrity of the faunal communities. Plant communities that cover a large area and that are diverse in
their vertical and horizontal structural characteristics can support far more diverse faunal communities
than relatively homogenous plant communities, such as meadows. The terrestrial ecosystem and part of
the aquatic ecosystem depend on the internal complexity of vegetation, including the number of layers of
vegetation and the species comprising each layer; competitive interactions among species; and the
presence of detrital components, such as litter, downed wood, and snags. Species and age composition of
vegetation structure also can be extremely important.
However, a lot of natural vegetation has been damaged or destroyed due to agriculture, husbandry,
urbanization, and mining. The natural process of the vegetation restoration will take about 100 years in
semitropical zone (Zhuang, 1997), yet reforestation will shorten the course of restoration (Wang et al.,
2006). So planting is the most effective measure for accelerating vegetation restoration and erosion control.
Selected wood species are planted on deforested mountains, which provide habitats for native species
in the forest canopy. Both the fauna community and herbaceous vegetation with complex community
composition have developed in the understory habitats. Planting has accelerated the vegetation succession
process (Wang et al., 2006). From the 1980s, "returning farmland to forest" and other reforestation
projects were implemented to restore the ecological environment in the upper Yangtze River basin. Many
fast-growing native and alien trees were planted to increase the tree coverage and control erosion.
Nevertheless, different planted wood species provided different habitats for understory communities and
have different ecological effects, although they have the same function of erosion control.
The Xiaojiang River is a tributary of the Yangtze River. The Xiaojiang River basin has been experiencing
serious soil erosion because of deforestation and mining in the past centuries. The river basin includes
107 debris flow gullies, where numerous debris flow events occur every year. The climate in the basin
has distinct vertical zoning. The Xiaojiang River valley with elevations below 1500 m is a typical hot
and dry valley, which has an average annual precipitation of about 700 mm, brown-red soil, and savanna
shrub and grass vegetation. The surrounding area on the mountain slopes with elevations between
1500-3000 m is a temperate semiarid zone, which has an average annual precipitation of about 900 mm,
yellow-red soil, and mountainous needle and broad-leaved mixed evergreen forest vegetation. On the top
of the mountains with elevations above 3000 m is cold-temperate zone, which has average annual
precipitation of about 1200 mm, brown and meadow soil, and alpine shrub and meadow communities
(Zhang et al., 2006). It is wet from May to October during which rain falls accounting for 88% of the
annual precipitation (Du and Kang, 1990).
As discussed in Chapter 2 reforestation and multiple-drop check dams are effective for erosion control
in the basin. The rate of soil erosion may be reduced from 13,000 t/km 2 yr to nearly zero if the fraction
wood vegetation cover increases from 0.1 to 0.65. Nevertheless, the effects of wood species on the
canopy ecology are very different, which has been studied by sampling and analyses (Yang et al., 2009).