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151.
铂族元素矿床特征及成因分类探讨 总被引:5,自引:0,他引:5
在收集国内外铂族元素矿床地质特征的基础上,审视了国内外不同学者对铂族元素矿床的分类方案,综合矿床容矿岩石类型、共生元素组合、矿化类型和矿床类型并结合形成环境,对铂族元素矿床进行了系统的总结和分类,划分为四类十二亚类。同时重点论述了具有开发前景的典型铂族元素矿床的基本特征、成矿物质来源和形成环境及成矿的关键问题。 相似文献
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153.
西藏冈底斯岛弧及其铜多金属矿带的基本特征与远景评估 总被引:64,自引:10,他引:64
冈底斯岛弧带属冈底斯活动大陆边缘的组成部分。带内早自中侏罗世、晚至第三纪的中酸性火山岩以及与雅鲁藏布江结合带有成因联系的Ⅰ型中酸性岛弧陆缘碱性侵入体(40-100Ma)和陆内汇聚造山引起的S型花岗岩体(15-50Ma)广泛发育。研究证实,冈底斯岛弧带属有色金属、贵金属成矿带。主要矿种的矿床类型有:海底火山热液型或火山喷流沉积型(Cu多金属)、海底热水沉积型(Cu、Pb、Zn等)、斑岩型(Cu、Mo)以及接触交代岩浆期后地下水热液充填的复合型(Cu、Pb、Zn)等。这些多期、多阶段复合叠加成矿作用和多种矿床类型的“同带共存”格局,展示了冈底斯岛弧带的巨大找矿潜力。 相似文献
154.
A Review of Forest Gap Models 总被引:25,自引:0,他引:25
Harald Bugmann 《Climatic change》2001,51(3-4):259-305
Forest gap models, initially conceived in 1969 as a special case of individual-tree based models, have become widely popular among forest ecologists for addressing a large number of applied research questions, including the impacts of global change on long-term dynamics of forest structure, biomass, and composition. However, they have been strongly criticized for a number of weaknesses inherent in the original model structure. In this paper, I review the fundamental assumptions underlying forest gap models, the structure of the parent model JABOWA, and examine these criticisms in the context of the many alternative formulations that have been developed over the past 30 years.Four assumptions originally underlie gap models: (1) The forest is abstracted as a composite of many small patches of land, where each can have a different age and successional stage; (2) patches are horizontally homogeneous, i.e., tree position within a patch is not considered; (3) the leaves of each tree are located in an indefinitely thin layer (disk) at the top of the stem; and (4) successional processes are described on each patch separately, i.e., there are no interactions between patches. These simplifications made it possible to consider mixed-species, mixed-age forests, which had been difficult previously mainly because of computing limitations.The structure of JABOWA is analysed in terms of the functional relationships used for formulating the processes of tree establishment, growth, and mortality. It is concluded that JABOWA contains a number of unrealistic assumptions that have not been questioned strongly to date. At the same time, some aspects of JABOWA that were criticized strongly in the past years are internally consistent given the objectives of this specific model.A wide variety of formulations for growth processes, establishment, and mortality factors have been developed in gap models over the past 30 years, and modern gap models include more robust parameterizations of environmental influences on tree growth and population dynamics as compared to JABOWA. Approaches taken in more recent models that led to the relaxation of one or several of the four basic assumptions are discussed. It is found that the original assumptions often have been replaced by alternatives; however, no systematic analysis of the behavioral effects of these conceptual changes has been attempted to date.The feasibility of including more physiological detail (instead of using relatively simple parameterizations) in forest gap models is discussed, and it is concluded that we often lack the data base to implement such approaches for more than a few commercially important tree species. Hence, it is important to find a compromise between using simplistic parameterizations and expanding gap models with physiology-based functions and parameters that are difficult to estimate. While the modeling of tree growth has received a lot of attention over the past years, much less effort has been spent on improving the formulations of tree establishment and mortality, although these processes are likely to be just as sensitive to global change as tree growth itself. Finally, model validation issues are discussed, and it is found that there is no single data source that can reliably be used for evaluating the behavior of forest gap models; instead, I propose a combination of sensitivity analyses, qualitative examinations of process formulations, and quantitative tests of gap models or selected submodels against various kinds of empirical data to evaluate the usefulness of these models for assessing their utility for predicting the impacts of global change on long-term forest dynamics. 相似文献
155.
中部地带是我国农业的精华地带,目前,我国农业发展中面临的部分农产品结构性剩余严重,市场竞争力弱,农业劳动生产率和土地生产力低而不稳,农民收入增长放慢,农业持续发展的后劲不足等问题在中部地带表现更为典型,探索中部地带农业现代化建设的实施途径对于研究和解决当前我国农业发展面临的诸多问题都具有重要的作用。中部地带农业现代化途径主要是:整体规划,分类推进,极点突破;改革城乡户口隔离机制,加速农村城镇化;创新土地流转机制,培育规模经济的现代农业市场主体;创新科技机制,发挥高科技对农业的带动作用;拓宽农产品加工领域加大农产品市场建设力度。 相似文献
156.
代内公平、国际贸易与可持续发展 总被引:5,自引:0,他引:5
现阶段,不同国家或区域之间商品交流的种类与数量日益增大。由于发达国家与发展中国家在国际贸易中所扮演的角色不同,因此,在等经济价值交换规律的作用下,发展中国家的环境状况逐渐恶化。文章通过构建开放经济的物质流模型,探讨了物质交换行为对系统内部的自然环境的影响,并对1987年日本的物质流动状况进行了案例研究。分析表明,发达国家的经济发展水平在很大程度上依赖于其它国家内部自然资源与环境的支撑作用。 相似文献
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158.
经济区域空间格局变化是区域经济不断发展的外在化表现,文章从区域、经济区域等基本概念入手,以经济全球化为背景,分析了市场机制作用下中国经济区域再形成过程的态势和外部环境变化,指出随着我国经济体制改革的深入进行,应积极培育合理经济区、建立合理的区际分工体系、培育区域之间合理的竞争态势,以提高我国区域竞争力和国家竞争力。 相似文献
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