Environmental Engineering Reference
In-Depth Information
The sampled plots arelocated in a region of elevation between 1600-2200 m. This region is the main
source of solid material for debris flows, where rainstorms and severe soil erosion occurs in the wet
season. Reforestation projects have been conducted and different wood species have been planted since
the 1980s. In general one species was planted on one mountain slope and different wood species were
planted on different mountain slopes.
Samples were taken from selected sites on the reforested slopes, where four wood species were
planted: Eucalyptus spp. , Acacia mearnsii , Leucaena leucocephala and Pinus yunnanensis . The selected
sites were consistent in slope steepness, soil type and soil texture, and water content. At each site 2 tree
plots, each with an area of 10 m u 10 m, were sampled. In each tree plot 3 herb plots with an area of 1 m
u 1 m were selected randomly. The time of plantting was obtained from the records of the forest
managers, which was further confirmed with tree-rings. Table 11.5 lists the features of the selected sites
and sampled plots.
The wood species were planted in the 1980s during a national project to return farmlands to forests.
Before the reforestation project the mountain slopes had been reclaimed for agriculture for many years
and there was no native vegetation. The slope steepness was higher than 10ยบ and serious soil erosion
occurred on the slopes. Nutrient cups with one year old saplings of Eucalyptus spp. and Acacia mearnsii
were planted for quick reforestation. Leucaena leucocephala and Pinus yunnanensis were planted by
direct seeding methods. The reforested slopes were closed after plantation.
T able 11.5
Features of selected sites and sampled plots
Elevation
(m)
Azimuth
( e )
Slope
gradient ( e )
Site
Wood species
Location of plots
Tree age (yr)
E103 e 09.311' N26 e 14.737'
1635
NW 6
30
3
1
Eucalyptus spp.
E103 e 16.155' N25 e 54.110'
1914
ES 23
22
8
E103 e 16.045' N25 e 53.981'
1825
SE10
35
14
E103 e 16.095' N25 e 54.154'
1999
SE 50
10
4
2
Acacia mearnsii
E103 e 16.191' N25 e 54.329'
1957
SE59
27
7
E103 e 16.197' N25 e 54.106'
1914
SE 52
24
8
E103 e 07.994' N26 e 14.733'
1635
NE4
31
4
Leucaena
leucocephala
3
E103 e 13.228' N26 e 06.881'
1645
SE65
16
10
E103 e 12.431' N26 e 06.300'
1587
SE7
17
20
E103 e 16.097' N25 e 54.086'
1934
SE 37
17
4
4
Pinus yunnanensis
E103 e 17.633' N25 e 54.730'
2176
SE 49
40
14
E103 e 17.612' N25 e 54.827'
2152
NW 23
35
20
Figure 11.73 shows the forest cover as a function of tree age for the four species. In general the forest
cover increased with the age of the trees. Acacia mearnsii had the fastest growing speed, and the forest
cover reached 80% at an age of 7 to 8 years. Leucaena leucocephala yielded a large amount of seeds and
had a high rate of germination and regeneration (Zong et al., 2007). The two species reforested the slopes
very quickly. The forest cover with 10 year old Leucaena leucocephala had reached 80%. Nevertheless,
cover declined for 20 year old of Leucaena leucocephala mainly because the number density of the
species was too high, which led to rarefaction of the crown of trees. The forest cover of Pinus yunnanensis
increased with age at a steady but slow rate. The forest cover of 20 year old Pinus yunnanensis was about
65%, less than that of 8 year of Acacia mearnsii and 10 year Leucaena leucocephala . The forest cover of
Eucalyptus spp. was low and increased very slowly with tree age. Acacia mearnsii and Leucaena
leucocephala had the highest forest cover, and, therefore, were very effective for soil erosion control.
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