青藏高原东缘高山林线乔木种群生态特征研究

青藏高原东部分布着世界最高的林线，该区域也是由欧亚北温带物种形成的林线的南界。在大面积野外踏勘的基础上，选择青藏高原东部具有典型高山林线分布的三个地点（滇西北白马雪山、川西北鹧鸪山及岷江源地区）作为研究区，从种群的结构、生存特征、分布格局及分形特征等方面对青藏高原东缘高山林线乔木种群生态学特征进行了研究，并在此基础上探讨了人类活动对林线种群生态特征及林线格局的影响。结果表明，林线区乔木树种多以单种群形式存在，林线区群落结构简单，乔木层多为单一树种组成，其生长型较之郁闭林发生了急剧的变化：树木高度急剧下降，而发展多茎多分枝的生长型。生长型的转变是高山林线乔木对恶劣自然条件的形态适应。 研究发现，在青藏高原东缘，阴坡林线乔木主要是冷杉（Abies spp.），阳坡主要由圆柏（Sabina spp.）组成，少数地方还有云杉（Picea spp.）。阴坡乔木种群结构多表现为增长型，幼苗和幼树在种群中占较大比重，种群潜在自然更新能力较强，但幼龄个体死亡率非常高，存活曲线多接近Deevey-Ⅲ型；阳坡乔木种群幼苗个体数极少，幼树相对增加。野外调查表明，人为活动较频繁的阳坡林线区幼苗数量极少甚至缺失，而受人为活动干扰较小的样地中幼苗和幼树数量明显增多，从一个侧面说明放牧等人类活动可能对林线种群的更新带来较大影响，而对卡卡沟围栏内外的样地分析也进一步证明了这一结果。 所研究林线乔木种群各龄级的空间格局在不同尺度上表现为聚集、随机和均匀分布，以聚集分布为主；各龄级在不同尺度上表现出显著的相关性，幼苗通常与另外两个龄级的关联性较密切。各龄级间显著的相关性表明不同龄级个体在空间交错分布，有利于对各种资源的充分利用，对种群的生存和发展非常有利，反映了高山生态系统恶劣生境中种群的一种适应对策。 林线乔木种群各龄级分布格局的计盒维数有差别，林线种群的计盒维数总是小于郁闭林种群的计盒维数。另外，郁闭林各龄级计盒维数通常也高于林线各龄级，表明不同海拔或者不同群落类型中的乔木树种具有不同的水平空间占据能力。林线区种群分布格局的计盒维数都很低，占据现实水平空间的程度较低，具有相对较高的生态间隙维，其潜在占据空间的能力较高，群落还可提供给种群的最大空间限度较大，但实际上由于受群落中种内、种间的竞争及林线区恶劣的生态环境条件的限制，其潜在空间占据能力可能难以表现出来。 青藏高原东缘高海拔地带以季节性游牧为主要的资源利用和生产方式，阳坡森林郁闭度低于阴坡，灌丛数量和种类较阴坡少，融雪早且积雪时间短，所以阳坡包括高山林线区成为当地牧民游牧路线的必经之地。牲畜的践踏、啃食使得幼龄乔木树种个体数量大大减少，严重阻碍了林线乔木种群的自然更新，同时种群占据空间的能力也明显降低。因此可以认为，在青藏高原东部地区，山地游牧等人为干扰叠加于恶劣的自然条件，阳坡林线的自然更新潜力受到抑制，其生存状态较之阴坡林线显著恶化，并可使阳坡林线高度逐渐降低。高山林线区森林一旦破坏在短时间内很难有效更新和恢复，因此，对于处于恶劣高山生境中的乔木种群应加强保护，同时适度控制人为干扰强度和幅度以减少其直接和间接破坏，防止阳坡林线退化并促进高山生态系统的自然恢复。 Eastern Qinghai-Tibetan Plateau has the highest timberline of the world. On the basis of field surveys and literature reviews, three typical alpine timberlines were chosen for in-depth studies, i.e., Baima Snow Mountain in northwest Yunnan, Zhegu Mountain and the waterhead area of Minjiang River in west Sichuan. Using the methodologis of population ecology, we analyzed the population structure, survival characteristics, spatial point patterns and fractal dimensions of the timberline tree populations and discussed the impacts of grazing on the structure and spatial pattern of alpine timberline. Compared with closed forests, the community structure of timberline is simpler, usually with one or two species constituting the tree layer. Differences also exist in the growth forms: the trees were significantly shorter with more stems and branches, reflecting morphological adaptation of trees to the severe conditions at timberline. In the eastern Qinghai-Tibetan Plateau, Abies spp. often formed alpine timberline in the north-facing slope while Sabina spp. and sometimes Picea spp. in the south- facing slope. The population structures of north-facing slope showed an increasing trend, with numerous seedlings and saplings. However, the survival curves tend to follow Deevy-III because of high dead ratio of young individuals. There are only few seedlings in the south-facing slope with heavy grazing, demonstrating that human disturbance may prevent regeneration at alpine timberline, which was confirmed by comparisons between fenced enclosures and control plots in the Kaka Valley. Depending on the spatial scales on consideration, the individuals of different age-classes showed clumping, random or even distribution, but mostly with clumping distribution. At all scales, individuals in different age-classes were all significantly correlated with each other while the seedlings were usually more correlated to two other age classes. This high degree of correlation among different age classes indicates that individuals of different age classes are spatially interlocked with each other, which helps sufficient utilization of various resources and is conducive to the survival and development of population. It is another adaptation strategy for trees at the severe environment. The spatial patterns of different age classes had different box dimension. In general, the box dimensions of total individuals and each age class at timberline are always smaller than that of closed forests, suggesting that space occupation capacity is not the same for populations at different altitude or in different communities. Populations on both the south- and the north-facing slopes had a very low box dimensions (far away from the max., 2), however, the lower the box dimension, the bigger the potential space provided by community. In fact, because of inner- and inter- competition as well as the severe conditions at timberline, this kind of potential ability can hardly be realized. Mountain pastoralism is the major type of as well as the only most effective way of resource uses in the high elevation regions of the eastern Qinghai-Tibetan Plateau. Due to lower canopy cover, less bushes and short snow-cover time, south-facing slopes became the favorite pastures. Damages from livestock through tramping, browsing and others have greatly reduced the number of young individuals. As a result, the potential of timberline trees to regenerate and their ability to occupy more space are greatly inhitibted. We conclude that human disturbances (mountain pastoralism) as well as harsh environmental conditions co-worked to inhibit the regeneration of tree populations in the south-facing slope and made south slopes more difficult than the north-facing slopes for trees to survive and develop, resulting a gradual retreat of timberline in the north-facing slopes. Forests at alpine timberline are susceptible to disturbance and difficult to regenerate and restore once damaged and controlling human disturbances is important for protecting the forest ecosystems at the timberline area.

Isoscalar Giant Monopole Resonance in Relativistic Continuum Random Phase Approximation

The isoscalar giant monopole resonance (ISGMR) in nuclei is studied in the framework of a fully consistent relativistic continuum random phase approximation (RCRPA). In this method the contribution of the continuum spectrum to nuclear excitations is treated exactly by the single particle Green's function technique. The negative energy states in the Dirac sea are also included in the single particle Green's function in the no-sea approximation. The single particle Green's function is calculated numerically by a proper product of the regular and irregular solutions of the Dirac equation. The strength distributions in the RCRPA calculations, the inverse energy-weighted sum rule m(-1) and the centroid energy of the ISGMR in Sn-120 and Pb-208 are analysed. Numerical results of the RCRPA are checked with the constrained relativistic mean field model and relativistic random phase approximation with a discretized spectrum in the continuum. Good agreement between them is achieved.

Experimental study of two-proton correlated emission from S-29 excited states

An experiment of a S-29 beam bombarding a Au-197 target at an energy of 49.2 MeV/u has been performed to study the two-proton correlated emission from S-29 excited states. Complete-kinematics measurements were carried out in the experiment. The relative momentum, opening angle, and relative energy of two protons, as well as the invariant mass of the final system, were deduced by relativistic-kinematics reconstruction. The Si-27-p-p coincident events were picked out under strict conditions and the phenomenon of p-p correlations was observed among these events. The mechanisms of two-proton emission were analyzed in a simple schematic model, in which the extreme decay modes like He-2 cluster emission, three-body phase-space decay, and two-body sequential emission were taken into account. Associated with the Monte Carlo simulations, the present results show that two protons emitted from the excited states between 9.6 MeV and 10.4 MeV exhibit the features of He-2 cluster decay with a branching ratio of 29(-11)(+10)%.

Fano effect on shot noise through a Kondo-correlated quantum dot

Transport in a semiopen Kondo- correlated quantum dot is mediated through more than one quantum state. Using the Keldysh technique and the equation of motion method, we study the shot noise S for a wide range of source- drain voltages V-sd within a model incorporating the additional states as a background continuum, demonstrating the importance of the Fano interference. In the absence of the interference, the noise is revealed to be a probe of the second moment of the local density of states, and our theory reproduces the well- known peak structure around the Kondo temperature in the S-V-sd curve. More significantly, it is found that taking account of the background transmission, the voltage dependence of the noise exhibits rich peak- dip line shapes, indicating the presence of the Fano effect. We further demonstrate that due to its two- particle nature, the noise is more sensitive to the quantum interference effect than the simple current.

Range-based attacks on links in random scale-free networks

Range and load play key roles in the problem of attacks on links in random scale-free (RSF) networks. In this paper we obtain the approximate relation between range and load in RSF networks by the generating function theory, and then give an estimation about the impact of attacks on the efficiency of the network. The results show that short-range attacks are more destructive for RSF networks, and are confirmed numerically.

SEU ground and flight data in static random access memories

This paper presents the vulnerabilities of single event effects (SEEs) simulated by heavy ions on ground and observed oil SJ-5 research satellite in space for static random access memories (SRAMs). A single event upset (SEU) prediction code has been used to estimate the proton-induced upset rates based oil the ground test curve of SEU cross-section versus heavy ion linear energy transfer (LET). The result agrees with that of the flight data.

Importance of self-consistency in relativistic continuum random-phase approximation calculations

A fully consistent relativistic continuum random phase approximation (RCRPA) is constructed, where the contribution of the continuum spectrum to nuclear excitations is treated exactly by the single-particle Green's function technique. The full consistency of the calculations is achieved that the same effective Lagrangian is adopted for the ground state and the excited states. The negative energy states in the Dirac sea are also included in the single-particle Green's function in the no-sea approximation. The currents from the vector meson and photon exchanges and the Coulomb interaction in RCRPA are treated exactly. The spin-orbit interaction is included naturally in the relativistic frame. Numerical results of the RCRPA are checked with the constrained relativistic mean-field theory. We study the effects of the inconsistency, particularly the currents and Coulomb interaction in various collective multipole excitations.

Experimental study of the two-proton correlated emission from the excited states of Ne-17,Ne-18 and S-28,S-29

A series of experiments have been performed by complete kinematics measurements to study two-proton (2p) correlated emission from the excited states of Ne-17,Ne-18 and S-28,S-29 via the Coulomb excitation by bombarding on Au-197 target. 2p and residua coincident events were picked Out under strict conditions. Visible p-p correlations were observed. It is shown that 2p can be emitted from the high-lying excited states. 2p halo may lead to 2p emission with large spectroscopy factor for the states close to or beyond the threshold.

Relativistic Continuum Random Phase Approximation and Applications II. Applications

The fully consistent relativistic continuum random phase approximation (RCRPA) has been constructed in the momentum representation in the first part of this paper. In this part we describe the numerical details for solving the Bethe-Salpeter equation. The numerical results are checked by the inverse energy weighted sum rules in the isoscalar giant monopole resonance, which are obtained from the constraint relativistic mean field theory and also calculated with the integration of the RCRPA strengths. Good agreement between the misachieved. We study the effects of the self-consistency violation, particularly the currents and Coulomb interaction to various collective multipole excitations. Using the fully consistent RCRPA method, we investigate the properties of isoscalar and isovector collective multipole excitations for some stable and exotic from light to heavy nuclei. The properties of the resonances, such as the centroid energies and strength distributions are compared with the experimental data as well as with results calculated in other models.

Relativistic Continuum Random Phase Approximation and Applications I. Formalism

A fully consistent relativistic continuum random phase approximation (RCRPA) is constructed in terms of the Green's function technique. In this method the contribution of the continuum spectrum to nuclear excitations is treated exactly by the single particle Green's function, which includes also the negative states in the Dirac sea in the nose aapproximation. The theoretical formalism of RCRPA and numerical details are presented. The single particle Green's function is calculated numerically by a proper product of regular and irregular solutions of the Dirac equation. The numerical details and the formalism of RCRPA in the momentum representation are presented.

SiO2 nanoparticles as scatterers for random organic laser action in red fluorescent dye 4-(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran doped polystyrene films

Random multimode lasers are achieved in 4-(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) doped polystyrene thin films by introducing silicon dioxide (SiO2) nanoparticles as scatterers. The devices emit a resonance multimode peak at a center wavelength of 640 nm with a mode linewidth less than 0.87 nm. The threshold excitation intensity is as low as 0.25 mJ pulse(-1) cm(-2). It can be seen that the microscopic random resonance cavities can be formed by multiple scattering of SiO2 nanoparticles.

ZnO Nanorods as Scatterers for Random Lasing Emission from Dye Doped Polymer Films

A random lasing emission from 4-(dicy-anomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) doped polystyrene (PS) thin films was realized by the scattering role of ZnO nanorods. The device was fabricated by spin-coating DCJTB doped PS on ZnO nanorods. The ZnO nanorods were grown on indium-tin-oxide (ITO) glass substrate by hydrothermal synthesis method. It can be seen that the device emits a resonance multimode peak at center wavelength of 630 nm with a mode line-width of less than 0.23 nm and exhibits threshold excitation intensity as low as 0.375 mJ pulse(-1) cm(-2). The agreement of the dependence of threshold pumped intensity on the excitation area with the random laser theory indicates that the lasing emission realized here is random laser. Our results demonstrate that the nanostructured ZnO nanorods are promising candidate as alternative sources of coherent light emission to realize organic lasers.