| Oasis is a landscape that is survived from the dry climate in desert and semi-desert area and under the stable water condition.It plays an essential role in the development of regional economy.However,the survival and development of oasis is absolutely and greatly dependent on water resources,which makes oasis ecosystem very fragile.The reduction as well as the irrational utilization of water resource would result easily in the degradation of the oasis ecosystem,which will threat the stability of oasis as well as the ecological and economic security.Through the on-the-spot survey on the desertification encroachment as well as researches on bio-diversity changes of artificial and natural vegetation during the deterioration processing,degradation characteristics,water environmental driven mechanism of vegetation coverage,artificial plant restoration and establishment of windbreak and sand-fixation system along the fringe of Minqin oasis have been made.The results as follows:(1)Minqin county is located in the center of deserts,there are desert and desertification lands of 1525528.3hm~2 and occupies 94.5%of total land in the county, including desert area of 803921.4 hm~2 and its occupying 49.81%of the total land. The encroaching characteristics of desertification lands are that wind-driven sand is mainly come from northern and western direction and it has basically controlled at southwestern direction(Xishawo)at fringes of the oasis.Salinity lands are concentrated at north part and Nanhu in the fringe of oasis,and Changning, Changshen and so on in the southwest of the oasis.The desertification is developing considerably day by day because of strong land reclamation at the south fringe of the oasis.(2)In the processing of vegetation deterioration at crisscross area of Minqin oasis and desert,plant species diversity is decreased with the reduction of underground water table.It is decreased tendency with the reduction of underground table at the same area or space plane distribution at different areas.Species diversity decreasing in community is the same situation with reduction of underground water table in the time sequence and space plane distribution.(3)After vegetation deterioration has happened,plant photosynthesis rate is reduced from biology and growth is also reduced.Diurnal net photosynthesis rate reaches to 10.99umol/m~2s when Haloxylon ammondendron forest is full of water supplying,its diurnal curve of photosynthesis rate is single peak and non-noon dormancy phenomenon.However,the diurnal net photosynthesis rates of Haloxylon ammondendron are just 7.37 umol/m~2s and 3.17 umol/m~2s respectively at the middle degree and serious vegetation deterioration condition,its diurnal curve is double peaks and the noon dormancy phenomenon is occurred.It also showed a characteristic that coverage of plant population and community is reduced after vegetation deterioration and dominancy of Haloxylon ammondendron population in community is increased.It reveals population of Haloxylon ammondendron has obvious competition ability under environmental degradation,it would have given us a scientific base for vegetation restoration and eco-reestablishment according to the plant competition..(4)Characteristic of water environment is special and typical in Minqin oasis.The Shiyang River is only natural river valley in Minqin oasis,it is the main source of oasis surface and underground water.Oasis vegetation changes in time and space with surface water reducing,which is the inducement factor,and it also changes with water table descending and soil water decreasing,which are the dominant factors. Low precipitation(average is 115.2mm from 1961-2002),weak precipitation intensity and much changeable ratio are the basic characters in Mingqin oasis.The vegetation deterioration mechanism is analyzed under conditions of precipitation and water table change.It shows that vegetation coverage has significant linear or non-linear exponential correlation with precipitation change and has significant logarithm negative correlation or parabola correlation with water table change. Moreover,the correlation degree of precipitation is higher obviously than that of water table,which indicates that degraded artificial and natural vegetation is strong dependence on precipitation now and has weak dependence on underground water. Also,it indicates that precipitation has stronger control capacity in time and space change of desert-oasis vegetation now.It does not only act to natural Nitraria tangutorum population but also act to artificial sand-fixed forest of Haloxylon ammodendron,both of them haven't been the steady vegetation landscape existed and developed depending on oasis water environment.Correlation between vegetation coverage and water table change represents the tendency of vegetation degradation.(5)Under the resource,environmental condition in Mingqin oasis,reestablishing techniques are proposed that are drip irrigation,mechanical measures,spring irrigation and sand crust restoration.(6)Two models of free irrigation and irrigation are composed about establishment of windbreak and sand fixation system.The free irrigation system is composed of windbreak and sand prevention belt,sand barriers + shrub sand fixing and preventing belt,sand fixing and preventing belt by shrub-grass and cultivating forest and grass belt by desert enclosure.The irrigation system is composed of windbreak and sand prevention belt,irrigated sand fixing and preventing belt,sand fixing and preventing belt by shrub-grass and cultivating forest and grass belt by desert enclosure.The free irrigation system is proposed at the area of deep water table and lack of water resource in lower reaches of Shiyang River,which is suitable in accordance with ecological and economy condition actually.And the goal of windbreak and sand fixation system will come into being in effective period.
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