Study on Groundwater Dependent Ecosystem and Ecological Mechanisms of Vegetation Succession in Desert Lake-Basin Region
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【Author in Chinese】 宋国慧；
【Author's Information】 长安大学， 地下水科学与工程， 2012， 博士
【Abstract】 Water is a valuable resource which all the living things on earth live on. As an importantpart of forming water resource—groundwater, which plays an irreplaceable role in naturalenvironment and human social activities’ sustainable development. Especially in the ariddesert area with scarce precipitation, it is an important part in maintaining ecological balanceof natural vegetation in desert region. Therefore, it is of great significance to deeply probe themechanism between the changes of groundwater ecosystem and natural vegetation’ secological successionThe vegetation’s ecological environment degenerated and land desertification intensifiedin arid desert region with dry climate, scarce precipitation, high evaporation and bad naturalenvironment. In addition to the tectonic and global climate condition changes, the reason isclosely related with the changes in groundwater system. For example, in the northwestern ariddesert region of our country, it takes on a special pattern of geographic space betweenmountains and basins from its geographic distribution characters. Among them the largeendorheic basins mainly include the Hexi Corridor of Gansu Province, Xinjiang Junggar basin,Tarim Basin, Qinghai Basin, Jilantai—Hetao Basin,ect..Due to the extremely dry climate,there appears typical desert landscape in basin’s heart. However, around the basin’s distributesome high ranges such as famous Qilian Mountains, Tianshan Mountains, Kunlun Mountains,Helan Mountains, Yinshan, and so on. And these mountains in the region become the moisturein atmosphere condensed centre in arid regions. With high precipitation providing importantrecharge sources for sand-covered alluvial plains and lacustrine basin groundwater. So,though the ecological environment is harsh in desert lake basin area, there grow many naturalvegetations in the sand–covered Salt Lake near the sand in shallow groundwater, which playan essential role in maintaining ecological environment of this area.This paper chose desert basin area as research object, relying on SEE(Society ofEntrepreneurs＆Ecology)Ecological Association Fund Project－Study on the groundwaterresources and its ecological effects in Ulan Buh Desert, taking the area of Jilan Tai lake basin covered by Ulan Buh Desert as an example, carrying out the research on desert basingroundwater ecosystem and vegetation’s ecological succession mechanism throughintegratedly using theoretical analysis, field investigation, means of indoor test. The mainresearch results:1The author put forward the definition of groundwater ecosystem, which refers to thegroundwater and groundwater dependent ecosystem, including groundwater system andgroundwater aquatic plant system. And groundwater systems mainly consist of groundwaterflow system, groundwater formation system and groundwater system biology. Groundwateraquatic plant system means the plant system which lives directly or indirectly on groundwater.2The author studied the conditions of groundwater level rising and falling, the restrictivefunction of groundwater system to desert vegetation ecological system and the adaptationmechanism of desert vegetation’s ecological system to the change of groundwater system.3From the water vertical angle, the author analyzed the transformation relationshipamong groundwater, capillary water and soil water and the contact as well as ecologicalsignificance between desert condensation and groundwater. The author also considered thatthe groundwater in desert-covered lake basin is relatively rich and the groundwater table isshallow. Meanwhile, sandy soil capillary zones generally exist on water table. The uniquedesert hydrological ecosystem in desert basin area consists of capillary fringe together withatmosphere water, plant water, soil water and groundwater. And capillary fringe plays the roleof connection in hydrological ecosystem. At the same time, it is an important link ofhydrological circle in arid region. The characteristics of groundwater dynamic and capillarywater rising decide the conditions of soil water supply in northwestern desert lake basin area,controlling the distribution pattern of desert vegetation divided by species and influencing thestability of existing vegetation and succession trend. But the dynamic of temperature field isthe key factor affecting groundwater, capillary water and soil water’s transformation.4The author clarified the definition of ecological level of groundwater in desert lakebasin and believed that it could meet the requirements of ecological environment, prevent thedegradation of vegetation, land desertification, soil salinization problems and maintain theburied depth of groundwater for azonal natural vegetation growth needed water. And it is alsoa zone to satisfy the needs of plant ecological environment and influenced by the properties of capillary soil with dynamic variations in time and space. Here the author also put forward theamount of optimal groundwater eco-environment’s groundwater depth and capillary which arefit for groundwater aquatic plants to grow. That is－the best groundwater buried depth isequal to root depth plus capillary rising height. And the amount of capillary water rise is equalto the amount of root which carries water plus the amount of soil moisture evaporation.5The main reason for vegetation degeneration is that the regional groundwater leveldeclines continuously. Besides basin phreatic water’s evaporation, the exploration ofgroundwater for Jilan Tai town’ s industry, living water and agricultural irrigation wells inChahar beach have become the main way of artificial excretion. The depth of phreatic watergradually becomes shallow ranging from0.23-9.47metres from the basin edge to the basininterior in Jilai Tai basin’s groundwater system of Ulan Buh Desert. From1984to2010, inresearch area the amount of water used in agriculture in Chahar beach oasis irrigation region,and the groundwater level continued to decline in the range of6-10metres at an average of0.27-0.45metres per year. Meanwhile, confined groundwater level fell at an annual average of0.20meters varying from1.50-2.75metres. And now the water in the north of Jilan tai SalineLake has fallen10-15metres with the artesian spring disappearing and groundwater aquaticvegetation degenerating.6The height, velocity and water volume of capillary rise are three elements with greatsignificance in vegetation ecology. During the plant growing period, if the plant roots are ableto reach capillary fringe, plant growth can be free of water stress. On the contrary, if thegroundwater falls, capillary fringe decreases and the chances for vegetation to get waterreduce, then plant growth will lead to water stress. In the research area, the height of capillaryrise of sand-covered lake red clay is3.1-4.2metres. And the theoretical maximum height ofaeolian sand soil capillary rise is0.63-1.66metres.7In Jilan Tai lake basin area of Ulan Buh Desert, natural vegetations can be divided into7groups, including Haloxylon ammodendron forest, Ammopiptanthus mongolicus shrub,White thorn Scrub, Caragana shrub, Salt claw shrub, Red sand pearl Scrub and Artemisiasphaerocephala. And among them natural Haloxylon ammodendron forest, Ammopiptanthusmongolicus shrub, Nitraria tangutorum, Kalidium, Caragana microphylla and other speciesare plant communities that are typically groundwater-dependent and azonic desert plants, which also play an important role in maintaining ecological environment in this area.8Groundwater aquatic plants appear in regular ring-belt distributions in an order ofAquatic reeds, Iris lacteals, Kalidiums, Achnatherum splendens, Ammopiptanthus mongolicas,Haloxylon ammodendrons and Nitraria tangutorums from the lake basin center to the dunefield made by outside sand. The relationship between plant growth statement and groundwaterdepth is slightly different in various geomorphic features. At the edge of Jilan tai lake basin,groundwater depth is more than3metres, and the amount of vegetation coverage is less than15%. Here with the groundwater depth increasing, the coverage of Haloxylon ammodendrongradually decreases. At the same time, in the interior of Jilan tai lake basin, with thegroundwater depth increasing, the coverage of Haloxylon ammodendron also graduallydecreases between the groundwater depth which is less than3metres and the amount ofvegetation coverage which is more than15%. But there will be no Haloxylon ammodendronsif the groundwater depth is above10metres. In the dune depressions, here in general thegroundwater depth is less than1.2metres and Haloxylon ammodendrons scarcely distributewith small coverage.9Shallow groundwater and capillary water are the main source of water for wet layer insoil. The existence of wet layer in soil is beneficial to plants to adapt themselves to waterstress in plant growth period of spring. The analysis through studying the measurements ofcontent water in soil profiles show that the soil profile from the ground below40-80meters isthe wet layer in soil contains relatively high water content. According to the statistics of airtemperature and ground temperature’s diurnal and seasonal variations as well as seasonalfrozen soil from Jilan tai meteorological station, the paper probed the mechanism of rechargewhich the movement of moisture showed to soil moisture with the effect of temperature fieldand seasonal frozen soil, considering the dynamic of temperature field is the key factor ofinfluencing shallow groundwater, capillary water and soil water’s transformation.10According to the analysis that the height of capillary rise in these two kinds of typicaldesert vegetation land in research area where underground aquatic plants-Haloxylonammodendron and Nitraria grow, it was proved that optimal groundwater depth in researcharea is1.6-4.7metres. Within the range of this groundwater depth capillary water can offeradequate water to plant growth. And if the plants grow well, the plant ecosystem will be stable.But if the groundwater depth drops to less than10metres, the shrubs in the research areawhich live on water will degrade and decline and the super-xerophytic shrubs together withgrass plant species which depend on precipitation as water resource will dominate the principal part of community.11Though the environment in desert basin area is bad, many natural vegetations still growaround the shallow groundwater and sand-covered saline lakes. The author recommendedsome concrete measures and suggestions on paying more attention to recovery of plantspecies’ selection, reasonable utilization of groundwater resources and protection ofecological environment’s vegetation restoration and reconstruction in desert area according todegraded vegetation ecological system in desert lake basin area. Seventhly, in order to protectthe ecological environment in desert area and promote coordinate development amongregional economy, society and environment, four aspects of vegetation ecosystem restorationmeasures are proposed as the followings: First of all, to build perfect sand-fixation shelterbeltsystem with Nitrarias and Haloxylon ammodendrons widely planted as well as biological andengineering measures combined.Secondly, optimizing land utilization structure, implementingthe strategy of returning the grain plots to grass and developing ecological stock raising aswell as water-saving agriculture. Thirdly, making rational use of groundwater and givingpriority to eco-environment water consumption in desert area. Finally, establishinggroundwater and vegetation’s ecological monitoring system.The expected research results will be of great theoretical conduct and practical significancefor Ulan Buh Desert ecological protection and damaged ecosystem restoration andreconstruction. Meanwhile, the research results will offer important lessons forhydro-ecosystem study of other desert lake basin areas.