浑善达克退化生态系统恢复研究

　　浑善达克沙地是我国四大沙地之一，地处北方干旱半干旱区，为草原区向荒漠区过渡的地带。长期以来，由于人类不合理的生产活动，加上这里脆弱的生态环境，已引起了严重的土地退化问题：流动沙丘面积由1950s年占沙地总面积的2%增加到1990s的近50%。因此浑善达克沙地成为我国研究土地退化、防治沙尘暴的重点地区，本文从自然科学和人文科学相结合的角度出发，对浑善达克沙地草地退化原因、自然恢复潜力、恢复过程、适宜物种选择，以及社区生存、生产等方面进行了综合研究，得出以下主要结论： 　　 1) 浑善达克沙地土壤种子库中含有大量的种子，在退化草地自然恢复中表现出极大的潜力。这些种子在摆脱人和牲畜干扰的前提下，可以萌发、定居并形成植物群落，使退化草地恢复;当地表达到一定程度的植被覆盖，可有效减少沙尘暴的危害。土壤种子库中的植物种类与地上植被有极显著的相似性(P<0.05)，这是地上植被形成稳定群落的基础。种子库中的物种组成影响植被恢复演替的进程;反之，恢复演替也制约着种子库组成和幼苗建立。 　　2)退化沙地草地围封后，对不同恢复阶段草地的群落学调查表明，该地区自然恢复过程大致分为3个阶段：围封2年的恢复早期，流动沙丘向半固定沙丘转变;围封3-5年的恢复中期，半固定沙丘向固定沙丘转变;围封6年后的恢复后期，为固定沙丘稳定发展阶段。根据生活型及植物种类随恢复演替的变化规律，浑善达克沙地植被演替的总体趋势可归纳为：沙米 (Agriophyllum squarrosum)+ 雾冰藜(Bassia dasyphylla)群落→黄柳(Salix gordejevii)+ 冰草(Agropyron cristatum)群落→褐沙蒿(Artemisia intramongolica) + 冰草群落→沙地榆(Ulmus pumila var. sabulosa)疏林+冰草群落。在围封禁牧下，浑善达克退化沙地草地在较短时间内实现自然恢复，因此制约退化草地恢复演替的关键因素主要是人为和牲畜的干扰，只要排除了这种干扰因素，浑善达克大面积的退化沙地草地完全能够借助自然力实现生态恢复。 　　3) 浑善达克沙地3种生境下84种植物叶片渗透势值和含水量，表现出不同功能型上的差异。总体变化趋势为： 深根系 > 浅根系;灌木 > 乔木 > 草本;分布在湿地和丘间低地的植物叶片渗透势和含水量较高，而生长在沙丘上的植物叶片渗透势较低，需要有发达的根系吸收土壤深层的水。不同植物具有独特的水分利用特性，使它们能共存于同一生态系统中。这些不同植物功能型表现出的植物水分生理生态特性，表明浑善达克天然分布的植物群落发育有完善的利用水资源的能力，能够保证在很大降水波动条件下分布有丰富的植物群落和较高的生物生产力，构成该特殊类型生态系统很强的恢复潜力。另外，浑善达克沙地沙丘的存在是该类生态系统恢复弹性较高的另一重要原因。 　　4)本地种与引入种在生理生态上表现出不同的适应能力。在相似的太阳辐射和叶片温度下，引入种旱柳的叶片水势较高，而净光合速率、气孔导度、水分利用效率则较低。这表明它的光合潜能在改变环境中没有正常发挥。同时，引入种较低的最大光化学效率进一步表明它抵抗环境胁迫的能力较低。当土壤水分可利用程度降低而导致水分竞争时，引入种很可能在竞争中被淘汰。因此，在生态恢复中，应尽量避免引入外来种，大量使用本地种。 　　5) 生态恢复不仅是自然科学问题，更重要的是社会经济问题。为了充分认识当地社区的参与对生态恢复的作用，在实验过程中，调查了当地居民自本实验开展以来的思想观念、经济收入和生产效益等变化。在生态恢复中当地社区的积极参与是保证恢复成功与否的关键因素。从发挥“自然力” 和“以人为本”的指导思想出发，在生态恢复中应注重充分利用自然的力量;在管理方面，要以解决社区居民的生产生活实际需求为目标。只有这样，才能保证生态、社会和经济可持续发展。

米脂县农地承载力系统动力学分析

在综合分析米脂县农地承载力系统结构、关系的基础上,以1998年为起始年建立系统动力学模型,根据米脂县社会经济发展现状及未来规划,设定了3种不同的方案和3种营养标准,模拟了米脂县2008-2028年农地人口承载力趋势,通过各方案比较,得出了较优的方案,最后提出了提高该地区农地承载力的政策建议。

中国粮食生产的区域演进研究

Grain is one of the primary material conditions of the human survival and the grain production concerns the stability and development of the society directly. The regional patterns influence greatly on the grain production and the rational production distribution the regional comparative advantages and promotes grain production. This thesis starts with summarizing of the characteristics of changes and the overall trend of regional pattern of grain production of our country since 1949. Then it carries on network analyses to the factors, which influences the evolvement of regional grain production patterns of our country. And finally it gives some proposals to the grain production distribution in the future. The main content includes: Firstly, Reviewing the regional evolvement of grain production in our country, and analyzing the changes of the regional pattern of grain production of our country on the provincial scale and county scale separately, since 1949, especially since the reform and opening up policy. The main grain production areas are acting an important position in ensuring the national grain security, so this thesis analyses the main matter of the main grain production areas, forecasts the grain production situation in the future, and selects the Northeastern main grain production areas as the typical area to carry on the positive research. Secondly, this thesis analyzes the origin causes from two respects of natural and social economy of the regional evolvement pattern of grain production in China. Thirdly, based on the summarizing to the status of the regional pattern of the grain production, this thesis proposes the precept of the grain production distribution in the future in our country. Therefore, the areas of three major cereal crops, rice, wheat and corn, are confirmed on the basis of the comparative advantages. Finally, this thesis puts forward the security system of guaranteeing the grain production progressing steady in China. According to the above analysis, some conclusions have been achieved as follows: (1) The grain gross production gets on extricating itself from awkward position frequently while fluctuating greatly annually since 1949 in China. (2) Since the reform, its traditional regional pattern of grain production, the most of which was concentrated in the south area, has changed rapidly. China's center of gravity of grain production has shifted from the south to the north, and on the belts of latitude, the grain production has represented a trend of focusing to the middle area in China. (3) The main grain production areas play a very important role in ensuring China's food security. With their relative severe situation of the problems of agriculture, rural area and peasant, China has carried out a series of measures, which aim at improving the food-producing conditions of the main grain production areas, and enhancing the grain yields there. Under this condition, a forecast of the producing amount of the main grain production areas under the nation's self-supplying rate of over 95% shows that the increasing provision production in these areas can meet the demand of the country. (4) The natural and social economic factors influence together on the changes of the grain production regional pattern. Along with the state system transition and progress of agricultural science and technology, the regional pattern of grain production is affected heavier by the agricultural policy and technological elements. (5) The grain production will be concentrated to the middle province in the future, which economic development level being medium-sized; According to crop allocation, although the rice superiority production area located in the South, its comparative advantage index is little in some degree. Meanwhile, the wheat and corn superiority production areas are in the North mainly and its scale superiority and production level advantage are all comparatively obviously.