| Ejin Banner in the lower reaches of the Heihe River, Inner Mongolia, Arid China was an vital passage of the North Silk Road in ancient times, and the cradle of civilization of Han Dynasty, Xixia Dynasty and Yuan Dynasty. Bordering upon Mongolia to the north and being important strategically, Ejin Banner, a stockbreeding region where many people from different minorities inhabit, is an important ecological county in Northern China.The natural environment is adverse and the ecosystem is extremely feeble in Ejin Banner. With the rapid growth in population and the development of irrigated farming in the middle reaches of Heihe River since 1980, the ecological environment has been increasingly deteriorated, for example, shrunken Ejin Oasis, degenerate natural vegetation, reduced biodiversity, drying Juyan Lake, declining groundwater levels, saline-alkali soil and so on. As a result, the region has turned into a cradle-land of sandstorms in China, which became an issue of universal concern. Restoring the eco-environment of the Ejin Oasis will not simply promote Sustainable development of regional economy, protect biodiversity and maintain ecological balance within the region, but play an important role in preserving ecological security in Northwest and Northern China, and in keeping the survival and integrity of oasis along Heihe River. Therefore, it seems particularly significant to have a deep insight into the process and regularity of hydroclimate changes and a thorough understanding of hydrological-climatic-ecological changes in the past, and make an interpretation of human, ecological and natural phenomenon. It is exactly a scientific subject crying out for solutions. Populus euphratica Oliv., Populus of Saliceae, one of the most widespread species in desert regions of the Middle Asia, is not merely a single arbor, a dominant species of the vegetation in Ejin oasis, and an indicator of eco-environmental degeneration, but also a natural barrier of preventing wind, fixing sand and maintaining fragile ecological balance. Now Populus euphratica in Ejin Banner is one of three major survival natural distribution zones of the world. It is of great practical significance and scientific value to study on Populus euphratica in the region.Tree-ring is the natural records of climate changes, which has many advantages, for instance, cross-dating accuracy, intense continuity, high-resolution, wide geographical distribution, easy sampling. Its theory and method have already widely used in such fields as climate, hydrology, ecology and archeology, volcanoes and so on. Recently, it has become one of the most important approaches reflecting the history of hydrological and ecological changes and reconstruction environmental evolvement in arid regions.In the thesis, we study on the responses model of radial growth of Populus euphratica species to hydroclimatic factors from tree-ring of Populus euphratica in Ejin Banner based on the theories and methods of dendrochrology. Our conclusions are as follows.1. Tree-ring samples were collected separately along the Ejin River, Northwest China, for studying the relationship between radial growth of Populus euphratica species and climatic factors. There are 287 tree-ring increment cores were retrieved from 6 sampling sites, and then they were mounted, sanded, cross-dated with skeleton plot and measured to the nearest 0.001mm with standard techniques. The quality of tree sequences was checked with the software COFECHA. Totally 18 chronologies were developed with ARSTAN program and were classified as three types, STD, RES and ARS by different detrending methods. The longest chronology series is 156 years, the shortest 87 and the average112. Then the first PC representing the greatest proportion of the total variance is gained by Principal component analysis (PCA), which is used to summarize the regional variation in radial growth patterns carried by the six site chronologies with the software PCA. The analysis results of the quality indexes of chronologies, cluster analysis and the weights of the first Principal Component (PC1) indicate that the tree ring-width chronologies at six sites is characterized by the apparent spatial change. Compared with the PC1 of 6 sites and 3 good sites, the PC1 at three sites is better.2. By correlation function and response function, the responses of radial growth of each chronology, the six chronologies and 6PC1 to 4 climatic factors of Ejin weather station and the runoff of Zhengyixia hydrological station were analyzed. The results show that the correlation between ring-width and the Zhengyixia runoff is mainly negative, the correlation between ring-width and the temperature is mainly positive in the area studied and the radial growth of trees have nothing to do with the precipitation. But the information quantity in tree ring-width chronology at every site is totally different. Radial growth of Populus euphratica assumes obvious spatial difference in the upper, middle and lower reaches of the Ejin River and is consistent with the time of the occurrence of the shallowest depth of the level of groundwater recharge. It proves that the groundwater is the main limiting factor, both direct and indirect, of the runoff by supplying with the groundwater.The hydrological factor responding to the radial growth has an obvious difference in space-time. The more obvious the correlation of ring-width of 6PC1 to hydro-climatic factors is, the more intense responses of ring-width of 6PC1 to hydro-climatic factors are. It proves that the response of the 6PC1 to the district hydrological factor is stronger than that of the single chronology.3. Except for LX chronology, the ring-width of tree is mainly correlated negatively with Zhengyixia runoff, and the negative correlation between ring-width at six sites and 6PC1 and February-April Zhengyixia runoff is more apparent, whose correlation coefficient is -0.544 (p≤0.05) respectively. It has a physiological significance. Because the all-year depth of groundwater is shallow in the area studied and the shallowest is in February-April with 2.45m,1.6m,3.55m. Not only in the rest period of February and March but also in growth period of April, has the groundwater met water demand of the growth of Populus euphratica. The over shallow groundwater supplied by runoff intimidates the growth. So the groundwater is a direct hydrological factor of affecting the radial growth of Populus euphratica. Zhengyixia runoff indirectly affects the radial growth of Populus euphratica by changes of the depth of the groundwater. The main correlation between LX chronology and Zhengyixia runoff is positive in June-August which can be explained as the time for growing and flourishing of Populus euphratica and for evaporating most severely. Though the underground level at LX site is relatively shallow, the relative elevation is the tallest, the runoff replenishment is little, and then experiences the period of minimal runoff during April-June of a year, so it is insignificant to the amount of replenishment of the groundwater. The soil layer humidity loses so greatly that the moisture the growth of Populus euphratica can make use of reduces rapidly. Therefore, the correlation between LX chronology with Zhengyixia runoff is positive in June-July. The shallow depth of groundwater is the real reason of the negative correlation between ring-width of the tree and Zhengyixia runoff. It also reflects the time space difference in responses of radial growth of Populus euphratica to the hydrology factor is limited by the hydrology factor and closely related to such small geography conditions as elevation.4. The response analyses of radial growth of Populus euphratica to climate factors reveal that the growth of Populus euphratica mainly relies on the underground water. While the water is sufficient, the growth of Populus euphratica is mostly confined by temperature factor and has nothing with the precipitation. The conclusion is consistent with that of Xinjiang's research. According to the analytic results of related periods, it is the most apparent that ring-width at the five of six chronologies and 6PC1 is correlated positively with the average temperature from February to March and that in winter(from December to February), all beyond 95% coefficient level The most obvious response to the maximum temperature is in autumn(from September to November) , while it has very little biological significance in the growth of Populus euphratica in those years. It suggests that the minimum temperature has a more significant influence on the growth of Populus euphratica than the maximum.The coefficient related to the connection between 6PC1 and average temperature from February till March and from December to February are 0.472 and 0.460(p≤0.05). Physiologically speaking, during the rest period for Populus euphratica from February to March, the demand for water is little; meanwhile the groundwater level in the region studied is shallow, so that the soil accumulates too much moisture, which proves that the relation between the ring-width of tree and runoff is mainly negative.The relatively higher temperature can improve the capacity to vaporize, benefit for the loss of soil moisture and strengthen the respiration of plants. The close relation with the minimum temperature in winter can be interpreted as the following points. On one hand, though the activities of cambium of trees in the rest period in winter come to a halt, they can synthesize organism by photosynthesis to store it in Populus euphratica cells. It is in favor of the growth of trees in next year. On the other hand, Populus euphratica in the region studied is menaced by the minimum temperature. Hence, although the trees stop growing in winter, the runoff from the October of the preceding year to the March of the year is the maximum of the whole year, the accumulation of the soil moisture results in an increase in humidity. If the minimum temperature in this period is too low, some mechanical damages, such as root broken and root displacement, thus influencing the growth of tree ring, is likely to get more severe in the following year.
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