| The Qilian Mountain is characterized by a pronounced transitional region of aridzones, eastern humid regions and frigid regions of Tibet. The region is influenced by both continental climate and the climate of Tibetan Plateau. It is an important boundary of geography and climate. Due to the far distance from the ocean, the climate is comparatively arid, especially in area of low elevation where is mostly desert, grassland and Gobi. The forests are also surrounded by the arid desert, semi-desert, meadow and salina in the Qilian Mountain where the tree's growth is very sensitive to the climate changes.With the regional climate changes and the enhanced human activites, the ecological environment is seriously deteriorated in this region. The forests and grasslands are continuously degrading. The glaciers are retreating and the snow line is rising. The water resource is more and more hard up. The habitant activities and economy development are also affected profoundky. To examine the reasons and mechanism of the ecological environment changes in the Qilian Mountains, the history of the environment change in this region should be studied firstly. It is not only an important scientific object, but also a very realistic problem which needs to be solved urgently as to establish the strategy of the regional sustainable developing.Because of the accurate dating, continuous record, high resolution (annual), accurate measuring of the ring width, easy to sampling and duplicating, the tree ring data was used widely as an important archive in paleoclimate reconstruction. In the 1970s and 1990s, the discovery of thousand year long-lived trees and a number of pioneer studies demonstrated that the Qilian Mountain was a great potential area for the tree-ring research of timescale from centuries to millennia. 200 cores of six sites from the middle-west of the Qilian Mountain had been taken to develop tree-ring widthchronologies. After tree ring samples were mounted, polished, cross-dated and measured, there are three different kinds of chronology at each sampling sites were developed, including standard chronology (STD), residual chronology (RES) and arstan chronology (ARS). Based on the tree-ring records, this dissertation was focused on the reconstruction of climate changes.The correlation and response analysis showed that our samples recorded relatively strong signal of the climate. So we could get significative information of paleoclimate change. But the response of the trees to the environment change was not identical among different sampling sites. Generally speaking, it was high between the chronologies of the middle and the precipitation record of the meteorological station near the sampling site, especially extremely high in the period of August of last year to July of the current year. However, it was high between the tree-ring chronology and the temperature record of the meteorological station selected. So we chose respectively the best chronologies to reconstruct the amount of precipitation from August of last year to July of the current year for the past 500 years and the average temperature in summer for the past 200 years.The comparison among the reconstruction precipitation of the different elevation indicated that the change trend was similar at low frequency in the same period. The result of cycle analysis showed that the precipitation on different elevation exist 2-3 year and 8.14 year periods. By contrast with the history of drought and famine in the northwestern China, we could find both the reconstruction temperature and drought history existed the extreme arid period during the twenties and thirties of the 20th century.Based on the high correlation between KgmOl (the high elevation sampling site in the middle) and the temperature record of the Sunan meteorological station, we compared the chronology of the KgmOl, the curve of the temperature of Tibetan Plateau and tree-ring chronology of Liu xiaohong in this region during 1902-1980 A.D.. The result indicated that high and low value was very synchronous. They were all high values during the forties-fifties and eighties in the 20th century and were all low values during the sixties and seventies in the 20th century.The reconstructed temperature change was almost similar in middle-western of the Qilian Mountain. The main cycles were also similar, such as 2~3yr, 8 yr> 16yr> 26~ 33 yr.Comparing the reconstructed temperature of the middle-west in the Qilian Mountain and average summer temperature during 1870-2000 A.D. in China, the change trend was almost similar: it was high temperature during the twenties and thirties of the 20th century, whereafter, the temperature began to gradually descend to the minimum till the seventies of the 20th century. From the beginning of the eighties, up till now , the temperature began to its second rise as the remarkable response to the global warming of the 20th century.In addition, there was a high correlation between the KgmOl tree-ring width and PDSI of the longitude 38.75N and latitude 98.75E. The correlation value is -0.599, which far exceeded the 95% significant confidence level. However, this high negative correlation was different from the other issued result (it was almost positive correlation in arid region). To explore the reason of it, further study and experiment should be carried out in the future.
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