克氏针茅草原的土壤种子库特征与植被恢复

近几十年来，由于过度放牧和不合理的农业开垦，内蒙古的天然草原退化严重。自2000年起，中国政府制定了“退耕还林还草”政策，并在草原地区实行了“禁牧、轮牧、休牧”等草原管理措施，期望依靠自然力来恢复退化的草原。然而，这些政策究竟能够带来什么样的后果还是一个未知数。土壤种子库和种子雨的资料对于全面理解植物种群和群落的动态具有重要价值。退化生态系统的恢复评价应该包括土壤种子库和种子雨的资料。因此，我调查了不同利用方式下克氏针茅（Stipa krylovii）草原的土壤种子库、种子雨和地上植被组成，来探讨政府的恢复政策对于该地区草原生态系统的影响。 研究地点位于中国科学院多伦恢复生态学试验示范研究站的永久实验样地，该实验样地在内蒙古自治区锡林郭勒盟的多伦县境内，该地区属于典型的农牧交错区。 采用幼苗萌发法，本研究测定了三种具有不同土地利用历史的样地的土壤种子库密度和物种组成。它们分别是：放牧样地、围封样地和弃耕样地。主要的研究结果有： （1）弃耕样地的土壤种子库密度最大，且主要是一年生和杂草类植物的种子，弃耕地由于其开垦的历史导致种子库的物种组成与放牧样地和围封样地的种子库有明显区别，其种子库缺少合适的多年生植物的种子是限制弃耕地自然恢复为天然草原的重要因素。 （2）放牧虽然会降低土壤种子库的密度，但是其中并不缺少多年生植物的种子。恢复由过度放牧引起的退化草原比恢复由农业开垦导致的退化草原要容易一些。围封对于由过度放牧引起的草原退化有明显的积极作用。 （3）采用Sorensen指数来计算地上植被与土壤种子库的相似性，这三个样地土壤种子库与地上植被的物种相似性均较高。在地上植被生物多样性的调查中，不能将草原中的一些早春植物遗漏。 采用网筛分离法，本研究测定了四种具有不同土地利用历史的样地的种子雨密度和物种组成。它们分别是：放牧样地、围封样地、割草样地和弃耕样地。主要的研究结果有： （1）不同样地间种子雨的密度差异显著（p < 0.05）。围封样地的种子雨密度最大，弃耕样地和放牧样地的种子雨密度居中，割草样地的种子雨密度最小。 （2）弃耕样地的地上植被和种子雨中均存在大量的一年生植物，这可能需要放牧或者割草等其它的一些管理措施来去除一年生植物的优势，从而加快弃耕地向天然草原的恢复速度。 （3）围封样地和割草样地的种子雨和地上植被中的物种数目均比放牧样地的要少，且围封样地中多年生非禾草类植物的比例大大增加，说明禁牧会改变草原植被的物种组成，这些究竟给草原植被的恢复带来怎样的影响，需要进一步的研究。 （4）克氏针茅草原整个取样期间种子雨的降落是连续的，因此在一些长期土壤种子库的研究中应该结合种子雨的数据，不能简单的将7月份的土壤样品作为长期土壤种子库。 采用网筛分离法，本研究测定了不同留茬高度的刈割实验样地的土壤种子库密度和物种组成。主要研究结果有： （1）无割草处理的对照样地的土壤种子库密度与其它割草处理样地的土壤种子库密度之间差异不显著。可能的原因是刈割的实验小区过小，植物种子的散布能够从不割草样地传播到割草处理的样地，或者动物的采食以及搬运降低了对照样地中种子的数量。 （2）虽然多年生禾草类植物在地上植被中占优势地位，但其在土壤种子库中的密度很小，均不到土壤种子库总量的10%。可见无性繁殖对于保持多年生禾草类植物在植被中的优势地位具有重要的意义，而相反的是非禾草类植物在土壤中保存大量的种子作为种群扩大的重要手段。

刺槐林地土壤水分与林下植物群落生物量的关系

通过样地调查,对比研究了不同林龄刺槐林地和撂荒地土壤水分年际变化特征及样地生物量特征。结果表明,刺槐林地土壤水分含量及储水量随林龄增长降低,过熟林的剖面含水率接近凋萎湿度;0—140 cm土层生长季土壤水分变异系数遵循过熟林>成熟林>幼龄林>撂荒地的规律,而140—500 cm土层则基本与上述规律相反。成过熟刺槐林下植物群落地上部生物量略高于撂荒地,土壤水分与地上部生物量仅存在微弱的负相关关系。说明刺槐生长虽然消耗了大量土壤储水,但未显著降低林下植物群落的生产力。研究表明,将刺槐作为先锋树种用于黄土高原森林草原区的植被恢复有助于迅速形成植被覆盖,发挥刺槐林的水土保持功能。同时,林下植物群落的健康发育可以保证刺槐衰退后的生态系统持续稳定地发挥其生态功能。

黄土丘陵沟壑区退耕地自然恢复植被主要物种生态位特征

为了探究加速黄土丘陵沟壑区退耕地植被恢复过程的人工物种干扰措施如物种选择与搭配的科学依据,利用TWINSPAN分类方法对33个自然恢复的退耕地样方进行分类,得出退耕地在40 a内主要经历了5个群落演替阶段,分别为猪毛蒿群落、赖草群落、达乌里胡枝子+长芒草群落、铁杆蒿群落和白羊草群落。以各个群落的综合资源环境梯度为依据,对各群落主要物种生态位特征进行了分析。通过物种的生态位宽度和生态位重叠特征分析,提出了不同演替阶段的合理物种组合。

A Research on New Method of Transportation Technics for Heavy Oil and Practice in Oil Field

介绍一种可应用于高粘度稠油管输的新工艺。即用自行研制的蒸汽引射器采用无界引射方式,将蒸汽直接注入到输油管道中,利用蒸汽释放的热量提高稠油温度降低粘度,从而达到降低稠油输送压降的目的,它比间接加热输送工艺所用的蒸汽量或耗煤量大大减少。方法在辽河油田输油管线上进行了工业现场试验,取得了很好的效果。

A Method Based on Digital Image Analysis for Measuring the Strain Field of Deformed Sheet

对薄板成形应变场传统的测量方法进行了研究,指出了其不足和误差的来源,提出了数字图像分析法测量薄板成形中的应变场,对测量原理、新的测量方法对传统方法的改进,以及如何降低误差进行了介绍,指出数字图像分析法的前景,提出了改进意见。

DPIV and Interferometry Technique for Velocity Field and Concentration Fields Measurement in Crystal Growth

The property of crystal depends seriously on the solution concentration distribution near the growth surface of a crystal. However, the concentration distributions are affected by the diffusion and convection of the solution. In the present experiment, the two methods of optical measurement are used to obtained velocity field and concentration field of NaClO₃ solution. The convection patterns in sodium chlorate (NaClO₃) crystal growth are measured by Digital Particle image Velocimetry (DPIV) technology. The 2-dimentional velocity distributions in the solution of NaClO3 are obtained from experiments. And concentration field are obtained by a Mach-Zehnder interferometer with a phase shift servo system. Interference patterns were recorded directly by a computer via a CCD camera. The evolution of velocity field and concentration field from dissolution to crystallization are visualized clearly. The structures of velocity fields were compared with that of concentration field.

Effect of non-uniform magnetic field on crystal growth by floating-zone method in microgravity

The magnetic damping effect of the non-uniform magnetic field on the floating-zone crystal growth process in microgravity is studied by numerical simulation. The results show that the non-uniform magnetic field with designed configuration can effectively reduce the flow near the free surface and then in the melt zone. At the same time, the designed magnetic field can improve the impurity concentration non-uniformity along the solidification interface. The primary principles of the magnetic field configuration design are also discussed.

The Displacement and Strain Field of Three-Dimensional Rheologic Model of Earthquake Preparation

Based on the three-dimensional elastic inclusion model proposed by Dobrovolskii, we developed a rheological inclusion model to study earthquake preparation processes. By using the Corresponding Principle in the theory of rheologic mechanics, we derived the analytic expressions of viscoelastic displacement U(r, t) , V(r, t) and W(r, t), normal strains epsilon(xx) (r, t), epsilon(yy) (r, t) and epsilon(zz) (r, t) and the bulk strain theta (r, t) at an arbitrary point (x, y, z) in three directions of X axis, Y axis and Z axis produced by a three-dimensional inclusion in the semi-infinite rheologic medium defined by the standard linear rheologic model. Subsequent to the spatial-temporal variation of bulk strain being computed on the ground produced by such a spherical rheologic inclusion, interesting results are obtained, suggesting that the bulk strain produced by a hard inclusion change with time according to three stages (alpha, beta, gamma) with different characteristics, similar to that of geodetic deformation observations, but different with the results of a soft inclusion. These theoretical results can be used to explain the characteristics of spatial-temporal evolution, patterns, quadrant-distribution of earthquake precursors, the changeability, spontaneity and complexity of short-term and imminent-term precursors. It offers a theoretical base to build physical models for earthquake precursors and to predict the earthquakes.

Numerical Simulation of Transient Thermal Field and Residual Stress in Laser Melting Process

以激光熔凝表面强韧化处理为背景,应用空间弹塑性有限单元和高精度数值算法同时考虑材料组织性能的变化模拟工件的温度场及残余应力,研究激光熔凝加工中瞬时温度场及残余应力数值模拟,同时考虑相变潜热及相变塑性的影响,用算例验证了模型的正确性,给出了不同时刻温度场分布及残余应力分布。

Divergence-free vector-field and reduction

By the Lie symmetry group, the reduction for divergence-free vector-fields (DFVs) is studied, and the following results are found. A n-dimensional DFV can be locally reduced to a (n - 1)-dimensional DFV if it admits a one-parameter symmetry group that is spatial and divergenceless. More generally, a n-dimensional DFV admitting a r-parameter, spatial, divergenceless Abelian (commutable) symmetry group can be locally reduced to a (n - r)-dimensional DFV.

Numerical Simulation of the Flow Field and Concentration Distribution in the Bulk Growth of Silicon Carbide Crystals

The physical vapor transport (PVT) method is being widely used to grow large-size single SiC crystals. The growth process is associated with heat and mass transport in the growth chamber, chemical reactions among multiple species as well as phase change at the crystal/gas interface. The current paper aims at studying and verifying the transport mechanism and growth kinetics model by demonstrating the flow field and species concentration distribution in the growth system. We have developed a coupled model, which takes into account the mass transport and growth kinetics. Numerical simulation is carried out by employing an in-house developed software based on finite volume method. The results calculated are in good agreement with the experimental observation.

Numerical simulation of magnetic field design for damping thermocapillary convection in a floating half-zone

The magnetic fields produced by electrical coils are designed for damping the the thermocapillary convection in a floating half-zone in microgravity. The fields are designed specially to reduce the flow near the free surface and then in the melt zone by adjusting the longitudinal coil positions close to the melt zone. The effects of the designed magnetic fields on reducing the flow velocity and temperature distribution non-uniformity in the melt zone are stronger than those of the case of an uniform longitudinal magnetic field obtained by numerical simulation, particularly at the melt-rod interface. It brings fundamental insights into the heat and mass transfer control at the solidification interface by the magnetic field design for crystal growth by the floating full-zone method.

Implication of scaling hierarchy associated with nonequilibrium: field and particulate

The advent of nanotechnology has necessitated a better understanding of how material microstructure changes at the atomic level would affect the macroscopic properties that control the performance. Such a challenge has uncovered many phenomena that were not previously understood and taken for granted. Among them are the basic foundation of dislocation theories which are now known to be inadequate. Simplifying assumptions invoked at the macroscale may not be applicable at the micro- and/or nanoscale. There are implications of scaling hierrachy associated with in-homegeneity and nonequilibrium. of physical systems. What is taken to be homogeneous and equilibrium at the macroscale may not be so when the physical size of the material is reduced to microns. These fundamental issues cannot be dispensed at will for the sake of convenience because they could alter the outcome of predictions. Even more unsatisfying is the lack of consistency in modeling physical systems. This could translate to the inability for identifying the relevant manufacturing parameters and rendering the end product unpractical because of high cost. Advanced composite and ceramic materials are cases in point. Discussed are potential pitfalls for applying models at both the atomic and continuum levels. No encouragement is made to unravel the truth of nature. Let it be partiuclates, a smooth continuum or a combination of both. The present trend of development in scaling tends to seek for different characteristic lengths of material microstructures with or without the influence of time effects. Much will be learned from atomistic simulation models to show how results could differ as boundary conditions and scales are changed. Quantum mechanics, continuum and cosmological models provide evidence that no general approach is in sight. Of immediate interest is perhaps the establishment of greater precision in terminology so as to better communicate results involving multiscale physical events.

The development of an inhomogeneous damage field in an elastic-plastic model

By making use of the evolution equation of the damage field as derived from the statistical mesoscopic damage theory, we have preliminarily examined the inhomogeneous damage field in an elastic-plastic model under constant-velocity tension. Three types of deformation and damage field evolution are presented. The influence of the plastic matrix is examined. It seems that matrix plasticity may defer the failure due to damage evolution. A criterion for damage localization is consistent with the numerical results.

Large strain field near a crack tip in a rubber sheet

The distribution of stress-strain near a crack tip in a rubber sheet is investigated by employing the constitutive relation given by Gao (1997). It is shown that the crack tip field is composed of two shrinking sectors and one expanding sector. The stress state near the crack tip is in uniaxial tension. The analytical solutions are obtained for both expanding and shrinking sectors.