摘要
沙漠化作为当今世界社会、经济和环境的重大问题,在国内和国际社会引起了广泛的关注。本研究以陕北榆林地区毛乌素沙地东南缘沙质草地沙漠化演替阶段空间序列为主线,通过实地观测和实验分析,从自然状态下土壤、植被性质的变化展开沙漠化演变机制的研究,探讨在不同沙漠化阶段土壤因子的响应机制与植物群落结构、功能的演变机制,并阐明二者在维持植被和生态系统稳定性方面的作用；基于植物逆境生理响应的耐胁迫类型划分突出了植物种群对生态系统的适应性关系,为不同沙漠化阶段植被的修复提供理论依据；最后,运用RS和GIS手段将过程的研究提升为对过程的格局研究,基于NDVI提取植被盖度,并据此对不同沙漠化类型草地进行划分,提出不同沙漠化类型区治理和保护的建议和措施。
以此为研究主线,得出研究结论如下：
(1)沙漠化过程中,土壤的物理、化学和生物学性质均呈现出规律性响应。随沙漠化程度的加剧,土壤沙粒含量、容重、C／N增加,土壤粘粒、粉粒、含水量、有机质、有机碳、全氮、脲酶、过氧化氢酶、多酚氧化酶呈降低趋势；对土壤各指标的相关性分析表明,土壤物理、化学和生物学特性存在密切的内在相关性。对土壤物理、化学和生物学性状的演变规律进行分析发现,随沙漠化程度的加剧,土壤结构的稳定性和功能的完备性均遭到破坏,土壤质量下降,土壤环境恶化；同时,土壤退化也使得沙漠化程度进一步加剧。
(2)沙漠化过程中,植被结构和功能特征退化,植被稳定性状况不断变差。随沙漠化程度的加剧,植被群落发生了逆行演替,从非沙漠化阶段的多年生禾草类逐步演变为重度沙漠化阶段的一年生杂草类；植被盖度、密度、物种丰富度、多样性指数均呈下降趋势,地上生物量呈先升高后降低趋势；混合植物(优势种)有机质含量变化规律不明显,全氮含量在沙漠化进程中逐渐降低；植被稳定性指数不断降低,稳定性状况变差。
(3)对沙漠化过程中土壤、植被的演变特征分析发现,土壤、植被要素是相互依赖、协同演进的。植被的进展演替依赖于良好的土壤环境,而土壤环境同样需要处于进展演替中的植被来不断改善,因此,土壤——植被系统之间的相互作用决定了沙漠化的正、逆过程。沙漠化过程中,众多土壤属性指标的测定值在Ⅱ、Ⅲ阶段出现了增降幅的突变,植被属性指标的观察和测定值在Ⅲ、Ⅳ或Ⅳ、Ⅴ阶段出现了降幅的突变,表明沙漠化过程是一个渐变到突变的过程,由于植被对土壤环境的内在适应性,使得植被退化过程滞后与土壤的退化过程；分析发现,土壤、植被属性突变的关键衔接阶段为Ⅲ阶段,即轻度沙漠化阶段,说明该阶段是沙漠化严重或恢复的重要转折阶段,故维持或营造该阶段土壤——植被系统的和谐与稳定是沙漠化修复的关键。。
(4)沙漠化过程中,不同植物在不同阶段对沙漠化的响应不同,表现出的受损程度和抗逆性能也有所差异。本氏针茅(Stipa bungeana)叶片含水量、叶绿素a、叶绿素b、总叶绿素和类胡萝卜素含量均呈降低趋势,质膜相对透性和丙二醛含量持续增加,脯氨酸含量也在波动中上升,超氧化物酶、过氧化氢酶和过氧化物酶活性也反应敏感,呈先增加后迅速降低的趋势,表明其在沙漠化进程中生理受损严重,对生存环境要求严格；糙隐子草(Cleistogenes squarrosa)的生理响应与本氏针茅相似,表现出较差的耐胁迫性；达乌里胡枝子(Lespedeza davurica)叶片含水量最初基本稳定,Ⅳ阶段后降低,叶绿素a、叶绿素b、总叶绿素和类胡萝卜素含量均减少,质膜相对透性和脯氨酸含量均上升,丙二醛含量呈先增后降再增的趋势,但变幅较小,超氧化物酶、过氧化氢酶活性变化趋势与丙二醛一致,过氧化物酶活性反应不敏感,表明其膜系统受到一定程度的损伤,但自身具有适应性反应,受损程度较小；油蒿(Artemisia ordosica)叶片含水量最初有所降低,Ⅱ阶段后回升,叶绿素a、叶绿素b、总叶绿素和类胡萝卜素含量也表现出先减少后增加的趋势,质膜相对透性和脯氨酸含量先有所上升,Ⅲ阶段后基本稳定,丙二醛含量轻微波动,变化不大,超氧化物酶、过氧化氢酶和过氧化物酶活性反应一致,均呈现出先升后降再升的变化趋势,协同抑制膜质过氧化作用,膜受损并不明显；牛心朴子(Cynanchum komarovii)各生理指标的响应与油蒿基本一致。
(5)对共有种各生理指标进行相关性分析发现,植物种自身生理指标之间存在着密切的内在联系和相互作用规律,但不同植物种的相关性分析结果存在较大差异,表明不同植物种是通过不同的内部适应机理来形成抗性机制的。在分析了共有种各生理指标之间错综复杂的应变关系基础上,运用主成分分析对每种植物种提取三种负荷最大的生理指标作为其耐胁迫类型划分的依据,对五种共有植物种进行分类：本氏针茅和糙隐子草为敏感受损型,其最初都能对沙漠化胁迫做出积极的应对反应,但当胁迫达到一定程度,植物全面受损；达乌里胡枝子为积极耐受型,其在沙漠化过程中表现出轻度的受损性状,但能通过积极的应对来适应沙漠化环境,使其并未遭受不可逆转的伤害,胁迫程度在其承受阈值之内；油蒿和牛心朴子为真正的抗逆型植物,其抗性指标能协同作用,受损性状不明显,对较为严重的沙漠化具有良好的适应性。
(6)应用RS和GIS手段,选取典型区进行沙质草地沙漠化程度空间格局划分。以沙漠化土壤、植被演替分析和植物耐胁迫性研究为理论基础,根据沙漠化程度类型划分图,提出沙漠化保护和治理建议：非沙漠化阶段和潜在沙漠化阶段区域应加强保护；轻度沙漠化阶段和中度沙漠化阶段区域应注重飞播育草,增加植被,防风固沙,改善土壤；重度沙漠化阶段区域应使用工程措施设置沙障固沙。
在开展沙漠化演变机制与植被耐胁迫类型的研究中,主要从以下几个方面实现了对以往研究内容和方法的突破：
(1)从土壤、植被的共同变化深入揭示了沙漠化的演变过程,并认为轻度沙漠化阶段是沙漠化恢复和转变的关键阶段。
(2)以植物生理指标作为植物耐胁迫类型的划分依据,并应用主成分分析方法找出贡献最大的三种生理指标,是对植物群体分类的新尝试。
(3)通过对过程的格局的研究,实现了具体理论研究与不同沙漠化类型区治理的结合。
综上所述,本研究一方面通过对沙漠化过程中土壤性质、植被结构、功能和稳定性等方面的变化研究系统揭示了沙漠化的演变机制；另一方面以植物生理响应为依据进行了沙漠化共有种耐胁迫类型的划分,明确了不同植物种群对沙漠化的适应性机制,为不同沙漠化阶段适宜植物种的选取提供理论依据；最后,以相关理论研究为支撑,应用技术手段进行了不同沙漠化类型区修复与重建的探索,实现了沙漠化研究理论意义和现实意义的结合。
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 fromⅡstage, 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.
引文
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