Fjölrit RALA - 05.12.1999, Blaðsíða 89

RALA Reportno. 200
Rangeland degradation in Ordos Plateau, its nature and assessment
Jia Jiong
Department of Geography, Beijing Normal University, Beijing, 100875, China
Tel: 86 10 623202 76: Fax: 86 10625881 27: E-mail: jiajiong@moon.bjnet.edu.cn
ABSTRACT
The Ordos Plateau has a markedly higher percentage (more than 70%) of degraded rangelands than
the regions in the same latitude, primarily because of the frequent fluctuation of climate and the
heavier grazing pressure, combined with the exploring for fuelwood and pharmaceutical plants and
the conversion to farmland. Desert steppe in the area was examined for effects of hundreds years of
livestock grazing and other disturbances. Three permanent sites, including a series of opened plots,
light grazing plots and ungrazed plots (excluded or ‘closed’ from livestock), were designed in 1988 to
compare and examine the degradation of rangeland under different grazing pressure. Experimental
analysis showed that there was a change of vegetation biomass, botanical composition, species diver-
sity, soil moisture and other factors after different grazing treatment. Biomass of most species in-
creased in the light grazing and the ungrazed plots after two years. Total aboveground biomass ranged
from 117.6 g/m2 in the light grazing plots to 134.1 g/m2 in the closed plots, compared with 42.3 g/m2
in the opened plots. But in the seventh year aboveground biomass at the ungrazed plots b ame 13.2
g/m2 less than that in the light grazing plots. Species diversity (Shannon-Wiener Index) wí ughest in
the ungrazed plots (2.207), followed by the light grazing plots (1.892) and the opened plots s0.826) in
the seventh year. Botanical composition also changed after seven years of treatment. The semi-shrub
Artemisia songorica, dominant in opened plots, was virtually eliminated in light grazing plots where
the perennial grass Cleistogenes squarrosa became dominant. Later it was still in abundance at the
ungrazed plots, but the dominance had been replaced by another perennial grass Stipa breviflora. The
soils in the opened plots had lower moisture and higher bulk density than the other plots. Time series
analysis showed that yearly mean overgrazing rate is lower than 20%, even negative in some cases.
However, when rainfall became less than average, stocking rates were far beyond the carrying capac-
ity and vegetation was severely damaged. It is suggested that the constancy of stocking rate has been
the main reason for regional rangeland degradation. Consequently, adjustment of stocking rate ac-
cording to climate (rainfall) driven fluctuation in carrying capacity is very important for the rangeland
restoration in the area.
Key words: cumulative grazing impact, degradation, desert steppe, Ordos Plateau, rangeland, stock-
ing rate.
INTRODUCTION
Rangeland degradation may be defined as the loss of utility or potential utility or the
reduction, loss or change of features of rangeland ecosystem which cannot be replaced
(Chrisholm and Dumsday 1987). In general, rangeland degradation implies a reduc-
tion in rank or status, which includes a loss of top-soil, a change to a simple flo-
ral/fauna composition or a transition fforn one organic form to a lower organic form,
and continuous reduction of productivity/biomass of the ecosystem. Generally speak-
ing, a lower biological diversity is supposed to occur in a degraded rangeland, but
there is still much research work to be done for the issue. In the view of ecology, deg-
radation can be treated as retrogression of an ecosystem, and recovery of degraded
rangeland as secondary succession (Numata 1969). The difference between degrada-
tion and fluctuation can be found in their temporal scales and resilience (e.g. Archer
and Stokes 2000).
Once rangeland has been degraded, it is often possible to rehabilitate it and thus re-
store it to a level of utility, possibly not as good as its original state, but better than it
was in its damaged state (Ludwig and Tongway 2000; Thurow 2000). Given satisfac-
tory perception of the threat, funding, technology, and organization, degradation,