中国北方半干旱和半湿润地区沙漠化的成因

通过对我国北方半干旱和半湿润地区沙质荒漠化的综合分析后认为,第四纪期间不同时间尺度沙漠化的成因是有差异的。1万年前的更新世期间,人类对生态环境的破坏作用甚微,沙漠化的出现和逆转主要受地球轨道要素制约的万年以上时间尺度的全球气候变化控制;1万年来的全新世特别是近2000年的历史时期,人类对生态环境的作用已越来越大,但沙漠化仍然主要受制于千年和百年尺度的气候波动;20世纪以来的现代时期,沙漠化过程既受数十年或数年的干湿气候波动作用,也受到人类不合理的经济活动的影响,但后者是主要的。

GENESES OF DESERTIFICATION IN SEMIARID AND SUBHUMID REGIONS OF NORTHERN CHINA

Through comprehensive analyses of the Quaternary geology, historical literatures and modern observations in northern China. the authors thought that the geneses of desertification are different in different periods and different bine scales.1 Pleistocene The widely spreaded Pleistocene stratigraphic profiles, characterized by alternately overlapped aeolian sand, loess and palaeosol, are the direct evidence of desertification of the Pleistocene in the studying regions. The stratigraphic profile located at Yulin horticultural farm at southeast fringe of Mu Us Desert clearly recorded 15 positive processes, 18 reverse processes and 16 positive-reverse conversion processes since 1.1 Ma B. P. All these processes corresponded to ten thousand to tens of thousand-scale climatic fluctuations revealed by the loess plateau and deep-sea oxygen isotope records, suggeshng that the alternations of the positive and reverse processes of the Pleistocene desertification in the study regions were mainly caused by the climatic changes which were controlled by earth orbit elements, while the influence of human activities were negligible.2 Holocene During the Holocene from 10 000a B. P. to 1900 A. D., the stratigraphic profiles suggests that there were 5 or 6 reverse processes and 6 or 7 posihve processes,with an average cycle of 1 500 a. The climatic changes indicated by the susceptibility in the same period also showed a dry-wet variations with roughly similar cycle.During the historic period, especially in the past 2 000 years the alternations of positive and reverse desertification in semiarid and subhumid regions in China was even more frequent and showed a good agreement with the warm-cold and dry-wet variations in the same period. This suggested that positive and reverse processes of desertification in the period in the studying regions were dominated by a cycle of one hundred years. Therefore the alternations of positive and reverse processes of desertification in the Holocene in the region were also controlled mainly by climate changes. During the Holocene the influence of human activities on desertification has gradually intensified and this is mainly manifested in two respects: First, social production activities destroyed vegetation and top soil, thus causing the positive processes of desertification. Second, through fencing off degraded lands, planting trees and protecting natural environment etc.The desertification in local places was reversed to some extent. However, the former was always predominant. Compared with climate changes. The influences of human activities on desertification only hold a subordinate position, in other words, hUman activities could accelerate or exacerbate the desertification processes caused by climate changes.3 At present Since 1900 A. D. the positive processes and reverse processes of desertification in the studying regions have been alternating at a shoder time-scale and reversed frequency. The development of the positive processes of desertification was also consistent with the climate changes indicated by temperature, precipitation and aridity in the same period. This shows that the climatic dry and humid degrees are the primary factors controlling the positive and reverse processes of desertification. However,only the aridification of climate is impossibe to make the desertification develop at present high velocity. Because the positive process of desertification balanced by the reverse process of desertification in wet years. Clearly, human activities play a very important role in accelerating and exacerbating the development of desertification. In the past great attention has been paid to the protection and rehabilitation of a few "points", while the anthrogenic destruction to the vast "regions" were ignored, and thus result in a situation, "points" were rehabilitated but "regions" were destroyed,and "greening couldn't keep up with desertification", as a whole, the desertification was becoming severe, although the desertified lands in local places were improved. At present the positive process