Grassland desertification by grazing and the resulting micrometeorological changes in Inner Mongolia
Introduction
Desertification is the process of deterioration in ecosystems of arid and semi-arid areas resulting from interplays of climatic variations and human activities (Dregne, 1985, UNEP, 1992). Desertification and its influence on regional environmental change have become the research focus of many scholars in the past few decades. Charney (1975) and Charney et al., 1975, Charney et al., 1977 suggested that the drought and dynamics of deserts in the Sahara be controlled by a biogeophysical feedback mechanism. Biological feedbacks play an important role in desertification worldwide (Schlesinger et al., 1990). Deforestation and resulting hydrological changes of land surface affect regional and even global climates (e.g. Shukla and Mintz, 1982, Shukla et al., 1990, Nobre et al., 1991, Wright et al., 1992). Some GCMs suggest that future global warming will likely exacerbate the degradation of semi-arid grasslands on a large scale in North America and Asia (Manabe and Wetherald, 1986). It is also true in the arid and semi-arid zones of Northern China that deforestation and afforestation affect the regional pattern of precipitation and temperature regime (Chen et al., 1992, Lin et al., 1993). For example, in the Horqin Steppe, the desertified grassland area is still expanding due to inappropriate anthropogenic activities such as prolonged overgrazing, poor farmland management and over-lumbering (Zhu and Wang, 1992). Vegetation plays very important roles in mass and heat exchange between land surfaces and the atmosphere (Grace et al., 1981). Removal or deterioration of vegetation cover affects water and heat budget of the surfaces. Therefore, in the desertification-prone areas such as the Horqin Steppe, it is requisite to conduct research on the relationship between desertification (mainly caused by degradation of vegetation) and micrometeorological changes. The primary objectives of this paper are: (1) to describe impacts of different levels of grazing intensity on the micrometeorological characteristics of grassland; and (2) to assess initiation of grassland desertification under increasing overgrazing.
Section snippets
Site descriptions and instrumentation
The studied area is located at Daliushu village (lat. 42°58′N, long. 120°43′E, 345 m above mean sea level), Naiman, Inner Mongolia, China, in a zone of continental semi-arid monsoon climate (Fig. 1). The mean meteorological conditions are summarized as follows by Liu et al. (1997): The mean annual solar radiation is 5200 MJ m–2. The total annual sunshine duration is about 2946 h. The mean annual air temperature is about 6.4°C, and the coldest and warmest monthly mean temperatures are −13.1°C of
Reflectivity (albedo)
As shown in Fig. 2 and Table 2, the reflectivity of the unfenced grassland (Plot E) in 1991 was higher in May (25%) due to lower vegetation coverage (20%), whereas it became lower in August when plants were growing densely with higher coverage (70%). After enclosure in 1992, albedo was slightly decreased at the ungrazed plot (Plot A) and the lightly grazed plot (Plot B), increased little at the moderately grazed plot (Plot C), and increased substantially at the overgrazed (Plot D). Consecutive
Albedo and grassland desertification
Reflectivity (albedo), defined as the reflected fraction of incident solar of natural surfaces, depends on many factors such as amount of cloud, sun angle, and surface features (Monteith and Unsworth, 1990). The surface albedo clearly depends on vegetation cover, and vegetation growth status (amount of aboveground biomass) as shown in Fig. 3. The fact that albedo (Ad) decreases with canopy height (Hc) (Ad=29.28×exp(–0.0091Hc), n=24, r2=0.54) as shown in Table 2 may also be used to explain the
Summary
Desertification, as a process of anthropogenic land degradation, occurs usually in the arid and semi-arid zones of the world largely because of over-utilization of natural resources and inappropriate management of environment. As an example, the Horqin Steppe, located at the northeastern China, has been suffering from desertification over the past two decades primarily because of overgrazing practice experienced in this region.
The grazing experiment conducted in Naiman of the Horqin Steppe
Acknowledgements
The authors acknowledge the National Natural Science Foundation of China (NSFC) for funding this research. They are grateful to the Sino-Japan Joint Study Program on Desertification Mechanisms under the sponsorship of the Chinese Academy of Sciences and the Science and Technology Agency of Japan. The authors also thank Prof. R.S.J. Weisburd, Institute of Biological Sciences of the University of Tsukuba for revision and helpful comments on the paper. Finally, particular thanks are due to the
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