| Poyang Lake is the largest freshwater lake in China and also the largest habitat of migratory birds, which has been received extensive international attention. The water level fluctuation of Poyang Lake is affected by the interaction of five rivers (Gan River, Xiu River, Rao River, Fu River, Xing River) flowing into Poyang Lake and exchanging water with the Yangtze River. In recent years, due to the high intensity of human activities and the tremendous impact of the impoundment of Three Gorges Reservoir, Poyang Lake has been subjected to some emerging problems, such as the increase of the total outflow of Poyang Lake and the decrease of inflow of Poyang Lake, earlier onset and extended duration of low water level condition, wetland degradation, biodiversity loss and water quality deterioration and so on, which are of widespread concerned.The research of the eco-hydrology processes linking of soil-vegetation-atmos-phere is the scientific frontier research of the wetland protection, which has an important scientific significance for the restoration of the wetland vegetation. Some emerging problems has been aroused in Poyang Lake, such as the change of lake hydrological processes and wetland degradation, eco-hydrological processes in Poyang Lake wetland was in urgent need of study to reveal the transfer mechanism of soil-vegetation-atmosphere in Poyang Lake wetland, and provide the scientific basis for reasonable formulation of Poyang Lake wetland protection measures.In this thesis, scientific problems such as Poyang Lake wetland vegetation variation and the key mechanism of eco-hydrological processes in the changing environment, the water level change and the law of wetland vegetation variation in Poyang Lake national nature reserve wetlands were further studied using data analysis and Landsat-TM remote sense images. The response of the growth of the dominant species Carex to soil water content was explored using field sampling and the barrel plant experiment, and then the relationship among soil water, vegetation water and precipitation, and water use efficiency in vegetation were studied using stable isotope method.The main results include:1. The analysis of the hydrological regime and the characteristic of wetland change in Poyang LakeThe variation of precipitation at Xingzi station, which is the typical hydrological station of Poyang Lake, was investigated by Mann-Kendall test and Standard Precipitation Index (SPI). The results indicate that the precipitation had slightly increased from1951to2006and experienced a few drought-flood variation. A serious drought occurred from1958to1969, and then extreme drought occurred from the second half of1977to the first half of1978. Finally, the trend of flood-drought abrupt alternation from1998to2006was detected. The variation of water level at Xingzi station was analyzed by Mann-Kendall test and Wavelet Analysis, the results indicate that the average water level had been decreased at the rate of0.73m/10a from1963to2009. The maximum decreaseding rate of water level occurred in the winter, while the minimum in the summer. At the scale of more than19a, the water level had significant periodic oscillation. The higher water level occurred before2002, the lower from2003to2009, and the contours of low water level in2009had not yet closed, suggesting that the decreasding of water level will continue. In the past20years, water level of Poyang Lake varied severely, and the highest water level and lowest water level decreased at the rate of1.83m/10a and0.66m/10a, respectively. Besides, the duration of high level reduced gradually while low water level condition extended.Remote sensing technology, Landsat-TM image coverring the Poyang Lake wetland, and the man-machine interactive interpretation method were used in this thesis to analyze the wetland landscape pattern after the pretreatment by applying ENVI software, such as orthorectification, registration and mask. The results indicate that water area has been decreased from fall and winter of1991to2009at a rate of2.03%/a, however, the thick grass land area has increased at a rate of1.01%/a. The characteristics of major anions and cations and their spatial distribution were investigated, the results show that the major cations and anion of lake water is Ca2+and SO24-Cl-, respectively; the pH value of Poyang Lake Reserve ranges from5.22to7.43, presenting weak acidic and alkaline; conductivity of river and ground water ranges from59.7to167.5μs/cm; the average of dissolved oxygen (DO):Xiu River> Gan River> well> ground water, however, the average of oxidation-reduction potential:ground water> Xiu River> well> Gan River. In Poyang Lake wetland, the water quality and the wetland ecosystems were destroyed by irrational human activities.2. The response of the vegetation Carex growth to the drought in the Poyang Lake wetland The Carex cinerascens which is the dominant community in Poyang lake wetland was investigated. The response of Carex to soil water stress was analyzed by designing two experimental programs. The first program is that soil water content was set into moist and drought condition. The results show that, during the experiment, the total means growth height (the measured arithmetic mean height of Carex for all of data) of the Carex that grows in moist is higher than that in drought, which is38.2cm and36.7cm respectively; the higher the volumetric soil water content, the better the growth of the Carex. Based on the first program, the second program is that several soil drought degrees were set. The results show that the mean height of the Carex’s sequence is moist>slight drought>very drought>extreme drought>mild drought. The Carex recover rapidly after water supplement with the aggravation of the drought degree. Taking the biomass (fresh weight) of Carex growth in moist condition as reference, the percentage of the slight drought’s biomass is97.26%, the mild drought99.53%and the extreme drought87.95%, respectively. The degree of Carex’s net photosynthesis in different drought degree conditions indicates very drought>extreme drought>moist>mild drought> slight drought; the change of the Carex transpiration rate is against generally the drought degree; the trend of the Carex’s water use efficiency increase as the aggravation of the drought degree.3. The characteristic of the oxygen and hydrogen stable isotope composition in soil water-vegetation water-precipitation(1) The characteristics of stable isotope in precipitation at Wucheng town in Poyang Lake were analyzed, and the local meteoric water line for Wucheng town was established, both the slope and intercept of which are greater than that of the global meteoric water line, indicating that the study area is characterized by rainy and wet climate.(2) Stable hydrogen and oxygen isotope in the soil water at three wetland sites was analyzed, the results show that, for all soil water samples, the value of8D relative to δ18O plotted above the global meteoric water line (GMWL), implying lower than the global average levels of evaporation. The stable isotope compositions of soil water were found to be mainly controlled by precipitation and evaporation, moreover, the depth of soil influenced by evaporation was about60cm. The shallow soil physical environment would be affected seriously by cover in surface soil, especially the depth of soil above40cm. The intensity of soil water evaporation varies between different surface cover. The response of stable hydrogen and oxygen isotopes in vegetation water to precipitation in the two sites (the first site and the second site) of Poyang Lake wetland were analyzed, the results show that the stable hydrogen and oxygen isotopes in vegetation water may be affected by precipitation. Further, the link of oxygen isotope among soil water, river, vegetation water and precipitation was be analyzed, the results show that vegetation water and precipitation has similar oxygen isotopes value, indicating that recent precipitation was the main water source for vegetation, not river or soil water.4. The characteristic of the carbon and nitrogen stable isotope composition in soil-vegetationThe characteristics of carbon and nitrogen isotope composition in soils were analyzed, the results show that δ13C and δ15N values of soil organic matter exhibit a weak negative correlation, indicating a variety of sources of soil organic matter in this region.813C values of surface soil were significantly lower than that of the deep soil, suggesting that isotope fractionation occurred during the decomposition process of organic matter. Carbon isotope compositions of dominant plants in the Poyang Lake wetland were analyzed, the results show that813C values of dominant plants with an average of-25.46‰, range from-14.19‰to-33.42‰.813C values of different plants existed obvious differences, therefore, their water use efficiency also had significant differences.813C value of triarrhena lutarioriparia is the highest, indicating that it has the highest water use efficiency, followed by phalaris arundinacea. Carex, reeds and artemisia selengensis had similar lower values of813C, the lowest of which was Carex, showing that water use efficiency of Carex is the lowest, and thus its competitiveness was the weakest under water limited conditions. Precipitation was not the main factor to influence or control δ13C values of the region’s vegetation. |
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