Millennial Moisture Variability Recorded By Tree-rings In Qilian Mountain And Its Forcing Mechanism

The Qilian Mountain is located in the interior of Eurasia and is dominated by semiarid climate. As an important geographical and climatic boundary in China, this area is particularly sensitive to global change. Due to the far distance from the ocean, the climate is comparatively arid, especially in the area with low elevation, which is mostly occupied by 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 change.With the regional climate changes and the enhanced human activities, the ecological environment is seriously deteriorated in this region over the past years. 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 profoundly. To examine the reasons and mechanism of the ecological environment changes in the Qilian Mountain, the history of environment change in this region should be studied firstly. It is not only an important scientific issue, but also urgent realistic problem to establish the strategy of the regional sustainable development.Because of the accurate dating, continuous record, high resolution (annual), accurate measuring of the ring width, easy sampling and duplicating, the tree ring data is 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. The main conclusions are followed:1. Four tree-ring series from the east part (YKS, Huangtai, Wuwei), middle part (HYG and GJG, Sunan, Zhangye) and west part (SMZ, Qifeng, Jiuquan) of Qilian Mountain were employed to analyze the moisture variability. HYG, the oldest series from the middle part of Qilian Mountain among the four tree-ring series, was used to investigate the climate change in millennium time scale, and the other tree-ring series were employed to compare with each other.All the cores in four tree-ring sample sites showed a high sensitivity as well as a well consistency among tree-ring cores. Therefore, all the cores could be used for cross-dating very well, which might reflect the influence of the same climate factor. The average correlation coefficient among cores is about 0.6. The highest correlation coefficient occurred in SMZ from the west part of Qialian Mountain(0.84), and the lowest was 0.36 from YKS in the east part of Qilian Mountain. Most of the coefficients were around 0.6 or higher. The growth of tree-rings from the west part is more sensitive than that from the east part, the main cause of which may be the water deficiency resulting from the decreasing of moisture gradually from east to west in the northwestern China.Following the criterion for developing the tree-ring chronology, we constructed the tree-ring chronology for each sample site and then identified the reliable chronology in accordance with expressed population signal (EPS) >0.85. The statistical characteristics of each chronology presented that the sensitivity of eastern chronology was lowest and the sensitivity of western chronology was highest. The result of comparison among the main statistics in common intervals also showed the similar feature. The results of correlation analysis between tree-ring chronology and climate data in the meteorological station nearby indicated that the tree-ring growth correlated with both temperature and precipitation. However, the soil moisture variability in warm season had the greatest influence on the tree-ring growth. The altitude difference between sampling site and the meteorological station may increase the correlation coefficient.2. Using the global Palmer drought severity index (PDSI) data set, the seasonal and annual PDSI mean values of 56 grid points covering 1953-2003 over northwestern China were analyzed by rotated experience orthogonal function (REOF) to evaluate the characteristics of moisture variability in the northwestern China. The results showed that the seasonal and annual mean displayed similar anomaly regions. Furthermore there were five main anomaly regions on the northwestern China based on the annual PDSI mean, which are the northern and southern parts of Xinjiang, northern part of plateau, western part of Inner Mongolia and eastern part of northwestern China. The opposite changes of dry-wet conditions were discovered after comparing the curves of feature point series and binomial fit between the eastern and western parts of northwestern China: In the western part of northwestern China influenced by westerly, a trend of wetting gradually occurred since 1980. On the contrary, a trend of drying gradually occurred in the eastern part of northwestern China mainly influenced by Asia monsoon, and especially in summer and autumn, there was a notable trend of drying over the southeastern part of northwestern China. The northwestern China is obviously affected by westerly, Asia monsoon and Tibetan Plateau. There are differences among the responses of dry-wet condition to the global warming in different regions.The comparison of the four tree-ring chronologies and other tree-rings series from the central Asian area, Asian monsoon area and the transitional zone, was then employed to analyze the moisture change in the past 150 years. The results demonstrated that those tree-ring series from the Asian monsoon area and the transitional zone preserved more signals of summer Asian monsoon in the last 150 years. However, as the sub-systems of summer Asian monsoon, East Asian monsoon and Indian monsoon influence the tree-ring sites diversely in different periods. And the long-term variations of these tree-ring series were quite different from those in the central Asian area. Except for the series from Helan Mountain, those series from Asian monsoon area and the transitional zone all showed the dry-wet-dry long term trends of moisture change in the recent 50 years. Especially, a trend of drying was apparent since the late of 1980s, which may correlate with the weakening of Asian monsoon recently. Furthermore, the trend of moisture change recorded in all series from each area was similar to the prior results from REOF analysis. The comparison of different tree-rings series captured two severe drought events successfully and suggested that the extending area and influences of the two severe drought were larger than the results documented by history writings.3. A millennium-long tree-ring-width chronology from the middle Qilian Mountains in northwestern China had been used to reconstruct March-September drought there since 775 AD. A rotated empirical orthogonal function analysis of 56 PDSI (Palmer Drought Severity Index) grid points was employed to calibrate the tree-ring chronology. Our reconstruction indicated that the change of dry and wet varied frequently in the south-middle part of northwestern China (SMNC) during the past millennium. The extreme drought and wet events were common. However, the severe drought events occurred in the whole period while severe wet events mainly occurred at the beginning and the end of the reconstruction series. The fluctuation of drought and wet presented the great difference in different periods. The reconstruction showed large amplitudes and low-frequency fluctuations in warm conditions but small amplitudes and high-frequency fluctuations in cold conditiongs. In general, the SMNC showed a warm-wet and cold-dry climate sequence in the past thousand years. Furthermore, the drought reconstruction not only revealed the long-term pattern of drought variability, but also successfully captured several extreme drought events recorded in local historical documents for the past 500 years. According to the frequency and duration of drought, the 1920-30s extreme drought event was not the only one for the past millennium. Two more drought events occurred in the periods of 840-880 AD and 1100-1127 AD may equal or severer than it. The regional drought events were in well agreement with the decline of population and the change of ancient dynasty during the past millennium in China.4. Both multi-taper spectral analysis and wavelet analysis indicated periods of drought in the reconstruction that are consistent with those associated with the Asian Summer Monsoon and perhaps even solar activity. Therefore, the Asian Summer Monsoon and solar activity were possibly the driving mechanism of the climate change in the Qilian Mountain. Compared with the proxies associated with Asian monsoon and westerlies in the past millennium, the tree-ring chronology demonstrated that the Asian monsoon had a strong influence before 1350 AD. After 1550 AD, however, a combined influence of both Asian monsoon and westerlies was apparent. Especially, the influence of westerlies on the tree-ring growth seem more obvious during the past 100 years. The correlation analysis of tree-ring series mentioned above and the global sea surface temperature in June was used to evaluate the possible forcing mechanism of climate further. The results demonstrated that, in the past 150 years, sampling site in the Tianshan Mountain was controlled by westerly while the sampling sites in Hua Mountain and Helan Mountain were influenced by East Asian monsoon. The sampling sites in Dulan and Zhiduo were mainly affected by Indian monsoon. Nevertheless, the forcing mechanism of climate in Qilian Mountain was still complex. The middle-west part of Qilian Mountain was mainly influenced by Indian summer monsoon in the past 120 years while the east part of Qilian Mountain seems correlated closely with East Asian summer monsoon.