Fjölrit RALA - 05.12.1999, Blaðsíða 107

Wang Changyao etal.
105
The reasons for the relative serious desertification are the impact of human activi-
ties, with the exception of natural factors including dry climate and loose land surface
materials liable to be weathered.
Single agriculture economy and long-term over-cultivation
There was a sparse human population in the Yulin County before the Tang Dynasty,
and people were mainly engaged in animal husbandry. However, after Tang Dynasty
(618 AD) because of chaos caused by severe war, a large number of poor peasants
came here from the south of the region to farm the wasteland to meet the demand for
food supply. The vast grassland and forest land was destroyed and the land desertifí-
cation developed progressively (She Hianhei 1981). Since then, the single agricultural
economy has been carried out. As a result, the human induced vegetation destruction
is gradually increased, land desertifícation is more and more serious, and the eco-
environment has become very íragile in the region. The phenomenon does not only
violate the desertification rule in the normal geographical zone, but also in the abnor-
mal geographical zone. It illustrates that, although the natural elements have had little
change, human activities are the main reason to cause the desertifícation development.
In order to study the reason for the desertification expansion, the present land use
structure and land resources were analyzed. The results indicate that present land use
structure is irrational. Based on the analysis mentioned above, it can be concluded that
one of the most important reason for land desertification in the region is the destruc-
tion of forests and grassland farming.
Low forest coverage, overgrazing and grassland degradation
The coverage of forests and grass is very low. The statistical results show that in 1949,
the coverage of forests were only 1.8% in the region. Since 1976, large scale tree and
grass planting activities were carried out along with the implementation of “Forest
Shelter of Northem Part of China” engineering project, but the survival rate of the
trees planted is very low and the quality of forests is poor. Before 1977, the survival
rate of the trees planted was only 25% and from 1979 to 1981, the survival rate of the
trees planted only reached 50% in the Yulin County (Yang Zhongxin 1984).
Vegetation Index model using Landsat TM and field sample measurements was
generated during the grassland quality investigation. In general, the yield of fresh
grass is about 3000 kg/ha. Comparing with the 1950’s, the yield of fresh grass de-
creased by 40-60% in sand area and decreased by 30-50% in hilly area in the south-
east Yulin County. Edible grass has declined and weeds and poisonous plants have
increased, which indicates that the grassland was greatly degraded. However, artificial
grassland has developed slowly and the area of artificial grassland was less than 10%
of the total grassland areas of the county in 1987. Meanwhile, because of the irrational
management practices and overgrazing from 1949-1982, the available grassland per
sheep unit decreased gradually (Ren Zhibi 1990). Overgrazing is not only limited to
the growth of livestock and decreased grassland productivity, but also grassland deg-
radation and desertification.