Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 65

was less than 1%. The rate of grain erosion of rocks was obtained by dividing the volume of grains over
the surface area of bare rock. The same measurement was also performed for 9 grain erosion sites in the
Xiaojiang River basin. The rate of grain erosion of bare rocks along on the Minjiang River was between
3 to 53 cm/yr, with an average rate of grain erosion of about 17 cm/yr. The rate of grain erosion in the
Xiaojiang River basin was only 1.1-4.6 cm/yr, with an average rate of about 2.8 cm/yr (Wang et al., 2010).
The rate of grain erosion in the earthquake area was much higher than in the Xiaojiang River basin
because the bare rocks in the earthquake area were fresh. The rate of grain erosion will gradually reduce
if even no control strategies are taken. Compared with the shattering erosion the rate of grain erosion was
more than 1000 times higher.
Studies have been devoted to the particle movement in the grain flow section and strategies have been
suggested to control the grain flows (Wang et al., 2007a,b,c; Xu et al., 2007). The proposed control
strategies were engineering measures for protection of highways, such as concrete sealing, protection
walls, and removing grain deposits with machines (Xu et al., 2007). These strategies are aimed at
controlling the movement of grains rather than controlling the erosion on the bare rocks. Nevertheless,
grain flow control is not essential for mountain hazard control. If the grain erosion is not controlled any
grain flow control structure will finally fail to control grain flow. Thus, essentially no control has been
achieved. Moreover, some engineering measures worsened the damage to highways (Sun et al., 2006).
The essential cause of grain erosion is devegetation and exposure of bare rocks to weathering and
temperature change. Therefore, an essential strategy to control erosion is revegetation. Several studies
have been done on the interactions between moisture, lithobiontic organisms, and rock weathering. Some
researchers simply assumed that the organic weathering replaces inorganic processes and paid attention
to the rates of bioerosion while little attention was paid to the role of erosion control played by a particular
species (Naylor et al., 2002). Only a few studies have attempted to identify bioprotection of lichens
during weathering processes recently (Carter and Viles, 2003, 2005). Yet there is little consensus over
whether rates of lichen-mediated weathering are slower than rates of a abiotic weathering of otherwise
identical rock surfaces (Lee, 2000). In fact, epilithic organisms can tremendously change microclimate.
The canopy temperature of cushion vegetation in the Alps could be 27 ć and relative humidity could be
98%, while the air temperature is 4 ć and relative humidity is only 40% (Korner 2003). Scientists also
conducted field experiments and concluded that the epilithic lichen retains moisture and reduces thermal
stress on the surface of limestone effectively (Carter and Viles, 2003).
It was found from field investigations that if a thin layer of lichen and moss grow on the rock surface,
the weathering and temperature change are mitigated and cannot directly act on the rock, and no grain
erosion occurs. The bare rock experienced grain erosion. Wind with a speed of 20 m/s from the experimental
bellows blew down more than 9 kg of grains in 10 minutes from 1m 2 of rock surface. When the same
wind acted on another rock surface, on which there was a thin layer of moss and lichen, however, no grains
were blown down. The moss and lichen layer was only about 1 mm thick, but it effectively protected the
rock from the direct action of sun and temperature change.
An experiment was done at Xiaomuling, which is a grain erosion site along the Mianyuan River.
Spores of five moss species were collected from local and neighboring areas and mixed with a clay
suspension. The clay material was collected from fine sediment deposits on the floodplains of the Mianyuan
River and the clay particles were finer than 0.01 mm. The concentration of clay suspension was 265 kg/m 3 .
The collected sporophyls were smashed with a machine. The clay suspension had a certain concentration
of the smashed sporophyls to have a sufficient amount of spores per liter of clay suspension. The
experimental plots were more than 100 m high above the Mianyuan River. Local farmers were hired to
carry the clay suspension with moss spores up to the mountain and pour down the suspension onto the
bare rock surface at several plots.