Study On The Evolutionary Mechanism Of Desertification And The Plant Stress Resistance On The Southeastern Edge Of Mu Us Sandy Land

Desertification which arouses close concerns in the domestic and international community, is one of the most serious social, environmental and economic problems in contemporary world. In this study, made the spatial series of sandy grassland successional stages as the main line on the southeastern edge of Mu Us sandy land in the north of Shaanxi, through field investigation and laboratory test, started the study of the evolutionary mechanism of desertification based on the changing of soil and vegetation in nature, researched on the evolutionary mechanism of soil factors and the structure and function of the plant communities, and illustrated the function of them in keeping the stability of vegetation and ecosystem; the classification of the stress resistance types of different plants, which based on the physiological response of plants on stress, highlighted the interaction between adaptation of plants and ecosystem, provided rationale for the restoration of vegetation in different desertification stages; at the same time, using RS and GIS methods, upgraded the study of process to the pattern of processing, extracted the vegetation cover based on NDVI, accordingly, divided the desert grassland, and offered some measures for different regions.Based on this, some conclusions were put forward as follows:(1) During the process of desertification, the physical, chemical and biological properties of soil were changing regularly. With the increasing degree of desertification, soil sand particle, bulk density, C/N were increasing, while soil clay, silt, moisture, organic matter, organic carbon, total nitrogen, urease, catalase, polyphenol oxidase were decreasing; the results of correlation analysis showed, there existed close correlation among these properties; the analysis of the evolutionary law of the physical, chemical and biological properties of soil showed, with the increasing degree of desertification, the stability of soil structure and the completeness of soil function were destroyed, soil quality has being fallen, soil environment had being degraded; and the soil degradation also makes a further serious desertification.(2) During the process of desertification, the structural and functional characteristics of vegetation degraded, and the stability deteriorated steadily. With the increasing degree of desertification, vegetation communities took place retrograde succession, changed from perennial bunch grass in non-desertification stage to annual herb in severe desertification stage; vegetation cover, species density, species richness, diversity index were all decreasing, aboveground biomass was first increasing and then reducing, organic matter content of mixed plants was not changing obviously, total nitrogen content was decreasing gradually; vegetation stability index continued to diminish.(3) The analysis of the evolution of soil and vegetation in the process of desertification showed that the vegetation and soil were interdependent and cooperated with each other. The progress of vegetation succession depended on good soil environment, while the soil environment needed to be improved by the vegetation in succession. Therefore, the interaction of soil-vegetation system determined the process of desertification and its reverse; in addition, the observed and measured values of soil and vegetation all changed suddenly in the mild desertification stage, what indicated that this stage was the turning stage in which the desertification was becoming more serious or restoring, so to maintain or improve the harmony and stability of the soil-vegetation system in this stage was the key to the restoration of desertification.(4) During the process of desertification, the responses of different plants at different stages were different, the degree of damage and the ability of resistance also varied. For Stipa bungeana, the leaf water content, chlorophyll a, chlorophyll b, total chlorophyll and carotenoid decreased during the process of desertification, relative plasma membrane permeability and MDA increased constantly, the proline fluctuated and raised, SOD, CAT and POD reacted sensitively, which increased at first and then decreased rapidly, indicated that it was damaged seriously and had strict requirements for habitat; for Cleistogenes squarrosa, the physiological responses were similar to Stipa bungeana, also had weak stress resistance; for Lespedeza davurica, the leaf water content was stable originally, then reduced at IV stage, chlorophyll a, chlorophyll b, total chlorophyll and carotenoid decreased, relative plasma membrane permeability and proline increased, MDA increased firstly and then decreased and further raised, but the variance range was lower, so as SOD and CAT, POD activity was not sensitive, indicated that although the membrane system subjected to a certain degree of damage, it had self-adaptive response, the damage was to a lesser extent; for Artemisia ordosica, the leaf water content reduced at first, raised again fromâ…¡stage, chlorophyll a, chlorophyll b, total chlorophyll and carotenoid also decreased at first and then increased gradually, relative plasma membrane permeability and proline raised at first and then kept stable at III stage, MDA fluctuated slightly with little changing, SOD, CAT and POD reacted consistently, which showed the trend that increased firstly and then decreased and further raised, restrained the membrane lipid peroxidation cooperatively, membrane damage was not obvious; for Cynanchum komarovii, the physiological responses were similar to Artemisia ordosica.(5) The correlation analysis between different physiological indicators of the plant species showed, there existed close contact and interaction between their physiological indicators, but the results of correlation analysis of different plant species were various, what indicated that the resistance mechanism of different plant species which formed through different internal mechanism were different. After analyzed the complex strain relationships of various physiological indicators of the common species, chose three physiological indicators for each plant species as the basis of classification using principal component analysis, classified five common species as follows:Stipa bungeana and Cleistogenes squarrosa were sensitive and damaged types, they could make positive response to desertification stress at first, but when the desertification reached to a certain degree, they were damaged more and more seriously, irreversibly; Lespedeza davurica was active resistant types, it was damaged slightly during the desertification, and could adapt through positive coping strategies, it had not suffered irreversible damage, the stress degree was with in their tolerance threshold; Artemisia ordosica and Cynanchum komarovii were the real resistance types, they were not damaged obviously and had a good adaptability to more serious desertification.(6) Using RS and GIS methods, selected the typical area, divided the spatial pattern according to desertification degree of sandy grass. Made the study of the succession of soil and vegetation and the stress resistance of plants as the theoretical basis, based on the zoning map of different desertification degree, provided the suggestions for the management and protection of desertification:the region of non-desertification stage and potential desertification stage should be strengthened protection; the region of mild desertification stage and severe desertification stage should be improved the environment through seeding grass by artificial sown, increasing vegetation, making up green sand binder, conversing soil and water, improving the soil; the region of severe desertification should be used engineering measures through setting sand barrier to fix sand.Carried out the evolutionary mechanism of desertification and the stress resistance types of different plant species, this paper achieved innovations as follows:(1)Deeply revealed the evolutionary mechanism of desertification with analyzing the changes in soil and vegetation, and considered that the mild desertification stage was the key stage in the process of desertification.(2)Classified the stress resistance types based on the physiological indicators, identified three physiological indicators of the greatest contribution applying principal component analysis, which was a new attempt for the classification of plant groups.(3)Studied the pattern of the process, achieved the combination of theoretical research and regional governance.To sum up, the paper, in one hand, revealed the desertification evolutionary mechanism systematically through studied on the changing of soil properties, the stability, function and structure of vegetation in the process of desertification; in the other hand, classified the stress resistance types of the five common plant species according to the physiological responses, provided theoretical basis for selecting the appropriate plant species in different desertification stages; finally, supported by relevant theoretical research, explored the restore and reconstruction of different desertification regions applying technology, realized the combination of theoretical and practical significance of desertification research.