放牧对内蒙古草原群落和土壤种子库的影响及两者关系的研究

选取内蒙古草原三种主要草原类型（草甸草原、典型草原和荒漠草原）代表性群落羊草杂类草群落、羊草群落和大针茅群落、小针茅群落，应用样线法沿水分梯度研究放牧对内蒙古草原不同植物群落功能群组成、多样性、生产力以及多样性与生产力关系的影响和放牧对土壤种子库组成、大小以及多样性的影响，在此基础上，研究土壤种子库与地上植被间的关系。主要结论如下： 1 放牧对植物群落的影响 荒漠草原的放牧演替规律为小针茅群落→猪毛菜 + 小针茅群落→猪毛菜群落;典型草原为羊草或大针茅群落→糙隐子草 + 大针茅群落或克氏针茅群落→星毛委陵菜 + 糙隐子草群落;草甸草原为羊草杂类草群落→羊草 + 贝加尔针茅群落，这是不同物种对牧压的不同适应结果造成的。 放牧使4种草原群落生活型功能群组分间发生强烈的生态替代作用，但不同的群落生态替代模式不同：放牧使小针茅群落多年生丛生禾草作用减弱，一二年生草本作用增强;羊草群落和大针茅群落多年生丛生禾草、多年生根茎禾草作用减弱，多年生杂类草作用增强;羊草杂类草群落多年生根茎禾草作用减弱，多年生丛生禾草作用增强。放牧使非旱生和C3植物作用减弱，而旱生、C4植物作用增强。 放牧对4种群落物种和功能群多样性的影响随不同的群落而表现不同：物种丰富度、物种多样性、生活型多样性 和水分生态类型多样性除羊草杂类草群落外随放牧强度的加大而降低，但适度放牧增加了羊草杂类草群落的上述多样性指标。 群落地上现存量一般随放牧强度的增大而下降，但小针茅群落反之，主要与一年生植物猪毛菜的生物量迅速增加有关。除羊草群落外，0~10 cm 地下生物量随放牧强度的变化不显著;除大针茅群落外，放牧显著降低0~30 cm 地下生物量。 放牧影响下内蒙古草原植物群落生物量随水分生态类型多样性的升高而升高，其回归方程为：Y = 809 + 774x （R2=0.84, P<0.001），其中Y代表群落地上现存量和地下生物量之和，x代表群落水分生态类型多样性。 2 放牧对土壤种子库的影响 小针茅群落、大针茅群落、羊草群落和羊草杂类草群落土壤种子库组成中均以多年生杂类草为主，分别占各自群落种子库总物种数的40%、52%、54%和67%。 生活型功能群种子库密度除羊草杂类草群落外，均以一二年生草本占优势。中度放牧升高了除小针茅群落外多年生禾草种子库密度;放牧增大了小针茅群落和羊草杂类草群落一二年生草本种子库密度;除羊草杂类草群落外，放牧对多年生杂类草种子库密度影响不大;总种子库组成中，灌木半灌木和小半灌木种子库密度不大，不随取样时间和牧压而变化。 中度放牧种子库总密度最大，小针茅群落在重度放牧最大，主要是由于猪毛菜种子库密度在重度放牧突增所致。总体上，内蒙古草原4种群落在不同取样时间不同牧压下种子库总密度波动在20.8~3819.2粒/m2。 土壤种子库物种丰富度最大值一般出现在10月份，除羊草杂类草群落外，不放牧群落较放牧群落为高，中度放牧使羊草杂类草群落土壤种子库物种丰富度增加。中度放牧增加了小针茅群落、大针茅群落7月份和羊草杂类草群落各取样时间土壤种子库物种多样性。 3 地上植被与土壤种子库的关系 土壤种子库的优势种在特定时间特定放牧强度下与地上现有植被优势种一致，但一致率仅为三次取样时间不同放牧强度下总体的23.3%。 地上植被与土壤种子库物种组成相似性指数受不同取样时间的影响，一般的10月取样最大。不同放牧强度对二者间的相似性亦有影响，中度放牧提高了小针茅群落、羊草杂类草群落各取样时间和大针茅群落、羊草群落4月份的相似性指数。隔年二次萌发法提高了二者间的相似性水平。总体上相似性指数变动在0.1~0.75之间。 地上植被现存量、总密度与各取样时间土壤种子库总密度之间不存在显著的相关性。 4 对于估计土壤种子库密度、物种组成和确定种子库与地上植被间的关系，隔年二次萌发法对于弥补直接萌发法本身所具有的不足不失为一种有益的尝试。

鄂尔多斯高原油蒿种群分布格局与群落数量特征对降水梯度的反应

近年来，种群空间分布格局日益为生态学家所重视，已成为生态学发展最快的领域和生态学理论发展的核心之一，群落的植被盖度和生物多样性是生态学研究的常用指标。植物种群分布格局及群落特征是种群和群落对环境条件长期适应和选择的结果，一方面决定于植物自身的生物学特性，另一方面与种群和群落分布的生境密切相关，对于揭示植被对环境的适应规律及它们之间的相互关系具有十分重要的意义。 本文以油蒿种群为研究对象，沿着鄂尔多斯高原从东至西的降水梯度(336～249mm)，应用传统的分布格局检验法以及点格局方法进行油蒿种群分布格局的研究；采用Gleason 指数、Shannon-Wiener 指数、Pielou 指数和Simpson 指数分析比较油蒿群落的生物多样性。从格局分析的尺度问题、严密性以及聚集程度变化趋势等几个方面，对本文采用的种群分布格局分析方法进行比较，为种群分布格局分析方法的选择提供参考。 种群分布格局分析结果表明沿着降水递减的梯度，在小尺度上油蒿种群分布格局表现为由均匀分布向随机分布转变的趋势；在大尺度上则表现为由随机分布向聚集分布转变的趋势。沿降水逐渐减弱的梯度上，油蒿种群的聚集程度逐渐增强。降水梯度对油蒿种群分布格局的影响一方面由油蒿本身的生物学特性决定，在降雨量小的地区，油蒿母株周围幼苗的存活率高，呈现聚集分布格局，而降雨量大的地区，油蒿幼苗存活概率比较平均，形成随机分布格局；其次，降雨量较大的地区，土壤水分资源较充足，油蒿个体较大，个体之间以竞争关系为主，聚集程度较低，降雨量少的地区，油蒿个体较矮小，个体之间为共同抵御恶劣生境，呈现聚集分布的格局。 群落FPC 和生物多样性指数与年平均降水量的回归分析结果显示，降水量越大，植被盖度越高，物种丰富度越大，群落物种分布均匀程度和优势度越低。充足的降雨促进油蒿群落的发育，草本层植物长势更好，生物量增加，群落的结构趋于复杂。 传统的分布格局检验方法和点格局方法在油蒿种群分布格局分析应用中得到的结果具有高度一致性，然而在实际工作中，这些方法之间具有各自的优劣和适应性。 建议在选取种群分布格局的分析方法时，要充分考虑研究目的，根据具体的物种和实验环境确定采用的方法。需在各细微尺度上做种群分布格局分析时，点格局方法优于传统的分析方法；在大范围取样及对工作效率要求较高时，方差均值比率法和聚集强度指数法更适合。由于聚集强度指数法在进行结果判定时比较模糊，建议优先选择方差均值比率法，将聚集强度指数法作为参考。降雨的减少显著改变了植物种群的空间分布格局和群落结构及物种组成，在进行生态恢复时可以参照本文的分析结果进行恢复植被的合理配置。

最近130ka来黄土物质化学风化强度的空间格局

Chemical weathering intensity of loess deposits is largely determined by three factors: chemical weathering in source regions, grain size and post-depositional weathering. The third factor is influenced by climatic conditions such as precipitation and temperature, and the dust sedimentation rate in the area of deposition. Previous studies have shown that the (CaO+MgO+Na2O)/TiO2 ratio of decarbonated residue from loess is independent of grain size changes and thus is a reliable proxy for chemical weathering. However, the validity of (CaO+MgO+Na2O)/TiO2 to describe changes in monsoon intensity requires further study. In this study, 48 sections over the last glacial-interglacial cycle on the Chinese Loess Plateau were sampled, and the major elemental concentrations of 248 decarbonated residue samples were measured to investigate the utility of the (CaO+MgO+Na2O)/TiO2 ratio as a proxy for changes in monsoon intensity. Results show that the (CaO+MgO+Na2O)/TiO2 ratio, is relatively more sensitive to climate change than other indexes independent of grain size, and is not affected substantially by sedimentation rate. Assuming the weathering regime is relatively stable in the loess source regions, the (CaO+MgO+Na2O)/TiO2 ratio is a reliable proxy for the intensity of summer monsoon. A decreasing (CaO+MgO+Na2O)/TiO2 ratio from northwest to southeast both in loess and paleosols indicates that the Chinese Loess Plateau is in the control of the East Asian summer monsoon during both interglacial and glacial times. In addition, the spatial distributions of (CaO+MgO+Na2O)/TiO2 ratios show a greater north-south gradient during interglacial periods than during glacial periods. This may suggest that the spatial precipitation gradient, controlled by the summer monsoon, is steeper during interglacials than in glacials.

Application of the Conjugate Gradient Optimization Algorithm of Neural Network on Laser Machining

在应用激光技术加工复杂曲面时,通常以采样点集为插值点来建立曲面函数,然后实现曲面上任意坐标点的精确定位。人工神经网络的BP算法能实现函数插值,但计算精度偏低,往往达不到插值精确要求,造成较大的加工误差。提出人工神经网络的共轭梯度最优化插值新算法,并通过实例仿真,证明了这种曲面精确定位方法的可行性,从而为激光加工的三维精确定位提供了一种良好解决方案。这种方法已经应用在实际中。

Strain gradient theory with couple stress for crystalline solids

A new phenomenological strain gradient theory for crystalline solid is proposed. It fits within the framework of general couple stress theory and involves a single material length scale Ics. In the present theory three rotational degrees of freedom omega (i) are introduced, which denote part of the material angular displacement theta (i) and are induced accompanying the plastic deformation. omega (i) has no direct dependence upon u(i) while theta = (1 /2) curl u. The strain energy density omega is assumed to consist of two parts: one is a function of the strain tensor epsilon (ij) and the curvature tensor chi (ij), where chi (ij) = omega (i,j); the other is a function of the relative rotation tensor alpha (ij). alpha (ij) = e(ijk) (omega (k) - theta (k)) plays the role of elastic rotation reason The anti-symmetric part of Cauchy stress tau (ij) is only the function of alpha (ij) and alpha (ij) has no effect on the symmetric part of Cauchy stress sigma (ij) and the couple stress m(ij). A minimum potential principle is developed for the strain gradient deformation theory. In the limit of vanishing l(cs), it reduces to the conventional counterparts: J(2) deformation theory. Equilibrium equations, constitutive relations and boundary conditions are given in detail. For simplicity, the elastic relation between the anti-symmetric part of Cauchy stress tau (ij), and alpha (ij) is established and only one elastic constant exists between the two tensors. Combining the same hardening law as that used in previously by other groups, the present theory is used to investigate two typical examples, i.e., thin metallic wire torsion and ultra-thin metallic beam bend, the analytical results agree well with the experiment results. While considering the, stretching gradient, a new hardening law is presented and used to analyze the two typical problems. The flow theory version of the present theory is also given.

Effect of Microtopography, Slope Length and Gradient, and Vegetative Cover on Overland Flow Through Simulation

Overland flow on a hillslope is significantly influenced by its microtopography, slope length and gradient, and vegetative cover. A 1D kinematic wave model in conjunction with a revised form of the Green-Ampt infiltration equation was employed to evaluate the effect of these surface conditions. The effect of these conditions was treated through the resistance parameter in the kinematic wave model. The resistance in this paper was considered to be made up of grain resistance, form resistance, and wave resistance. It was found that irregular slopes with microtopography eroded more easily than did regular slopes. The effect of the slope gradient on flow velocity and flow shear stress could be negative or positive. With increasing slope gradient, the flow velocity and shear stress first increased to a peak value, then decreased again, suggesting that there exists a critical slope gradient for flow velocity and shear stress. The vegetative cover was found to protect soil from erosion primarily by enhancing erosion-resisting capacity rather than by decreasing the eroding capability of overland flow.

The Flow Theory of Mechanism-Based Strain Gradient Plasticity

The flow theory of mechanism-based strain gradient (MSG) plasticity is established in this paper following the same multiscale, hierarchical framework for the deformation theory of MSG plasticity in order to connect with the Taylor model in dislocation mechanics. We have used the flow theory of MSG plasticity to study micro-indentation hardness experiments. The difference between deformation and flow theories is vanishingly small, and both agree well with experimental hardness data. We have also used the flow theory of MSG plasticity to investigate stress fields around a stationary mode-I crack tip as well as around a steady state, quasi-statically growing crack tip. At a distance to crack tip much larger than dislocation spacings such that continuum plasticity still applies, the stress level around a stationary crack tip in MSG plasticity is significantly higher than that in classical plasticity. The same conclusion is also established for a steady state, quasi-statically growing crack tip, though only the flow theory can be used because of unloading during crack propagation. This significant stress increase due to strain gradient effect provides a means to explain the experimentally observed cleavage fracture in ductile materials [J. Mater. Res. 9 (1994) 1734, Scripta Metall. Mater. 31 (1994) 1037; Interface Sci. 3(1996) 169].

Stress Wave Propagation in a Gradient Elastic Medium

The gradient elastic constitutive equation incorporating the second gradient of the strains is used to determine the monochromatic elastic plane wave propagation in a gradient infinite medium and thin rod. The equation of motion, together with the internal material length, has been derived. Various dispersion relations have been determined. We present explicit expressions for the relationship between various wave speeds, wavenumber and internal material length.

A new hardening law for strain gradient plasticity

A new hardening law of the strain gradient theory is proposed in this paper, which retains the essential structure of the incremental version of conventional J(2) deformation theory and obeys thermodynamic restrictions. The key feature of the new proposal is that the term of strain gradient plasticity is represented as an internal variable to increase the tangent modulus. This feature which is in contrast to several proposed theories, allows the problem of incremental equilibrium equations to be stated without higher-order stress, higher-order strain rates or extra boundary conditions. The general idea is presented and compared with the theory given by Fleck and Hutchinson (Adv. in Appl. Mech. (1997) 295). The new hardening law is demonstrated by two experimental tests i.e. thin wire torsion and ultra-thin beam bending tests. The present theoretical results agree well with the experiment results.

Deflections and Curvatures of a Film–Substrate Structure with the Presence of Gradient Stress in MEMS Applications

The close form solutions of deflections and curvatures for a film–substrate composite structure with the presence of gradient stress are derived. With the definition of more precise kinematic assumption, the effect of axial loading due to residual gradient stress is incorporated in the governing equation. The curvature of film–substrate with the presence of gradient stress is shown to be nonuniform when the axial loading is nonzero. When the axial loading is zero, the curvature expressions of some structures derived in this paper recover the previous ones which assume the uniform curvature. Because residual gradient stress results in both moment and axial loading inside the film–substrate composite structure, measuring both the deflection and curvature is proposed as a safe way to uniquely determine the residual stress state inside a film–substrate composite structure with the presence of gradient stress.

Analysis of residual stress gradient in MEMS multi-layer structure

Residual stress and its gradient through the thickness are among the most important properties of as-deposited films. Recently, a new mechanism based on a revised Thomas-Fermi-Dirac (TFD) model was proposed for the origin of intrinsic stress in solid film

Influences of strain rate and temperature variation on material intrinsic length of plasticity strain gradient theory

Cowper-Symonds and Johnson-Cook dynamic constitutive relations are used to study the influence of both strain rate effect and temperature variation on the material intrinsic length scale in strain gradient plasticity. The material intrinsic length scale decreases with increasing strain rates, and this length scale increases with temperature.