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Sun Jimin; Ding Zhongli(Institute of Geology, Chinese Academy of Sciences, Beijing 100029)  
Desertification results not only from physical factors but also from official factors.Nevertheless, physical factors may have played a minor role in thes process,comparing with that of historical human activities. For the deserts in northern East China, the two physical factors which are responsible for desertification are sand supply and climate. Actually, enough sand supply is the basis for the large scale land desertification. Recent studies have indicated that a great amount of palaeo-eolian sands are buded by Quaternary deposits,and these palaeo-eolian sands serve as sand sources for the historical desertification in northern East China. The studied region is just located in the adjoining part of the send-arid and semi-humid climatic zones, and winds are generally strong. The climate of thes region is characterized by the covariance of strong winds with less rainfall in spring. Thus, the special climate pattrn in northern East China combining with the enough sand supply forms the basis for historical land desenification. A number of historical accounts and field investigations have indicated that there is a long history of land reclamation in northern East China. According to the historical records, there are numerous conflicts betWeen the nomadic economy from the north and the farm-based economy from the south. The result of every conflict led to environment deterioration and the migration of the farming-domestic raising boundary.When the nomadic tribes occupied these regions, stock rising became the representative living style. Because the nomadic economy was at low levels, the pastoral settlement did not cause severe environmental deterioration in these areas. However, when agricultural economy becomes the dominant living style, the dense impact of the farm-based economy can easily cause severe land degradation. For the northern East China, the most serious desertification occurs during the Qing Dynasty, when the Central Government abolished the previous prohibitions and permitted the Hans from Central China freely emigrating to the northern grasslands and reclaiming lands. Generally, the process of land desertification in northern East China is the result of reachvation of palaeo-eolian sands. Here, we choose Mu Us Desert as an example to show the process and sand dune reactivation. In the inland parts of Mu Us Desert,the desertification is largely due to the reactivation of the last giantal maximum sand dUnes when the overlying Holocene limnological layer is denuded. In the adjoining part of Mu Us Desert and the Loess Plateau (e. g. the desert-loess transitional zone),the mechanism of desertification is more complicated, not only the artificial factor but also the processes of warer erosion, winds ablation, and gravity collapse are involved in the process of sand reactivation. In the desert-loess transitional zone, the historical land desertification is mainly associated with the reactivation of sand dunes,corresponding to marine oxygen isotope Stage 2, Stage 4, and the mid-Stage 3. Other sands deposited earlier are deep buried, and have not been largely exposed to the hUman impactS, thus they are regarded as the minor scarce of sand supply for the reactivation. Moreover, our results also indicate that: (1) The present controversy on the process and mechanism of desertification in northern East China is largely due to the misunderstanding of the concept of "desertification". (2) In the area with large scale desertification, the biogeophysical feedbacks in the desert region can make the local climate changes superimposed by artificial factors. If thes kind of local climate change is regarded as "pure" regional climate information, it will lead to serious mistakes,and to the misunderstanding of the concept of "desertification".
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