基于生态系统服务理论的草地可持续利用评价

草地退化不仅仅是今天中国才发生的事情。历史上曾经发生“黑尘暴”的美国North Dakota州，现在仍然有62.7％土地被用作耕作，林地所占面积仅仅占1％。私有制和种植业较畜牧业的高的利润率，是该州种植业比重高于畜牧业的重要原因。草原保护项目（CRP）得到了美国政府财政的大力支持， 在North Dakota州采用了以草地恢复为主的措施。美国发达的教育体系带来的高流动率，使得North Dakota州自30年代后居住人口稳定在60－70万成为可能，从而避免了我国出现的草原地区不断增加的人口压力。由此可见：草地作为重要的自然资源，不可能完全保护起来;没有其他配套措施，私有制可能会带来新一轮垦殖;草地畜牧业仍然是畜牧业的重要组成部分，畜牧业发展需要结合种植业尤其需要与饲料生产相结合。 依据生态系统服务的理念，首次发展了生态系统服务指数（ESI），试图通过对生态系统服务功能的综合考虑，提出科学的适宜放牧率的评价方法。本文利用美国北达科他州立大学中部草原研究站17年长期放牧试验数据，选取植物多样性Shannon-Wiener指数、地上净初级生产力、土壤表层含水量和单位面积家畜增重等四个指标，通过对不同指标分别赋予不同的权重，计算不同管理目标下ESI及其稳定性，并对单目标管理与多目标管理进行了比较研究。结果表明，对于北美混合普列里(Prairie)草地，围封不利用或建立自然保护区，虽然生态系统比较稳定，但既不能有效的提高植物多样性、初级生产力和土壤水分含量等生态功能，又没有畜产品产出;而在重牧或极重牧处理下虽然获得了较大的畜产品生产，但导致了草地生态系统的退化和较大的系统不稳定性。因此，这两种管理方式在实践中都是不可取的。应用生态系统服务指数综合考虑，认为应该权衡各项生态功能和生产功能，此时轻牧或中牧是最适宜的。因此，ESI的建立避免了单项指标的评价偏差，使得适宜放牧率的确定更加合理。 利用前面构建的多目标权重评价体系，结合内蒙古草原生态系统定位站放牧试验样地的数据，探讨了锡林郭勒盟定位站附近的适宜放牧率为2.67羊/ha，低于单目标条件下的每公顷绵羊4羊/ha。同时在经济学和生态系统系统服务理论的基础上，依据谢高地等人（2001年）的研究，并提出了环境税和生态补偿的标准，分别是每公顷15元和90元。

生态补偿机制研究：以山东省为例

生态补偿机制因为能有效解决环境保护领域的外部性问题而日益引起关注。它是一套对造成自然环境破坏的行为进行处罚、对生态环境保护的行为实行奖励的制度。生态补偿机制不但能为生态保护筹措资金，而且能统筹协调各区域和各行业的关系，从而达到经济发展与保护生态平衡协调、区域协调发展和可持续发展的最终目标。 首先进行了生态补偿的理论研究。分析了生态补偿的概念、经济学原理、生态补偿网络的构成、以及目前在确定生态补偿的补偿依据和补偿额度方面所面临的问题和争议。引进生态系统服务功能评价的原理与方法，探讨将生态系统服务功能评价应用于生态补偿的可行性，并提出了生态补偿的组成部分和一般机制。 区域补偿是一类重要的生态补类型。山东省是一个面积较大、经济较发达、但各地发展还不够平衡的省份，在山东开展生态补偿具有重要意义。以山东省为研究案例，研究了在山东省开展区域生态补偿的可行性和实施办法，并根据山东省各地的生态系统服务功能提出优先补偿地区。提出了综合考虑各区域的单位面积非市场生态系统服务功能价值和单位面积GDP的生态补偿决策因子，并对山东不同生态功能区和17市的生态补偿决策因子分别进行了计算。从研究的结果来看，山东西部地区由于经济较落后和生态系统服务功能中的非市场价值较大而拥有相对较大的生态补偿决策因子，应该优先给与区域生态补偿，以促进山东整个区域的协调发展。 生态系统类型转换的补偿是生态补偿的重要组成部分。在山东省南四湖地区开展退耕还湿生态工程对保障南水北调东线工程水质安全具有重要意义，但要在当地开展大规模退耕还湿，合理的生态补偿机制至关重要。以南四湖地区退耕还湿地生态工程的生态效益和生态补偿机制为研究案例，探讨了生态系统类型转换的生态补偿方法。研究发现南四湖退耕还湿后生态系统服务功能大为增强，但当地农民的收入会受到显著负面影响，因此应该给与一定的资金补偿以提高农民参与退耕还湿的积极性和促进整个区域的可持续发展。另外，生态补偿的方式应该多样化和结合当地实际，除资金补偿以外，南四湖地区还应该重点采取技术补偿的方式，由政府组织专家给当地居民提供湿地恢复和湿地植物资源开发的技术。资金补偿和技术补偿对以提高农民退耕还湿的积极性和湿地系统的长期稳定运行具有重要意义。 最后对生态补偿的实施和保障机制进行了理论探讨。结合山东的研究案例和参考国内外经验，讨论和提出了生态补偿的主管部门、补偿费用征收机制、补偿费用使用的机制、生态补偿费用使用的监督机制等，并对生态补偿机制存在的问题进行了分析。根据研究结果对山东实施生态补偿的优先领域进行了讨论并提出了建议。 总之，通过生态补偿机制的理论研究和在山东开展生态补偿的案例研究，提出了生态补偿的一般机制和保障措施，以及对山东省开展生态补偿的建议，研究结果对促进生态补偿和可持续发展具有重要意义。

吴起县退耕还林(草)政策实施情况调查研究

吴起县退耕还林(草)政策经过8 a的实施取得了显著成效。项目实施区生态环境得到明显改善,农业综合生产能力得到很大的提高,农民通过退耕补助得以直接增收,为形成面对农户的生态补偿制度探索了经验。然而政策实施过程中仍然存在着诸如退耕地块分布和林分结构不合理,草畜业发展滞后,牧草资源利用率不高,退耕政策不够灵活,补偿标准缺乏弹性等问题。建议在以后退耕政策中注重加强规划指导,合理确定退耕范围,在生态补偿制度框架下优化退耕补助标准,通过深化土地产权改革,促进退耕土地合理流转和规模化经营,继续推动农业后续产业发展,巩固退耕成果。

Performance analysis of bit error rate for free space optical communication with tip-tilt compensation based on gamma-gamma distribution

In this paper, the gamma-gamma probability distribution is used to model turbulent channels. The bit error rate (BER) performance of free space optical (FSO) communication systems employing on-off keying (OOK) or subcarrier binary phase-shift keying (BPSK) modulation format is derived. A tip-tilt adaptive optics system is also incorporated with a FSO system using the above modulation formats. The tip-tilt compensation can alleviate effects of atmospheric turbulence and thereby improve the BER performance. The improvement is different for different turbulence strengths and modulation formats. In addition, the BER performance of communication systems employing subcarrier BPSK modulation is much better than that of compatible systems employing OOK modulation with or without tip-tilt compensation.

Dispersion analysis and compensation of collinear optical parametric chirped pulse amplification systems

In an optical parametric chirped pulse amplification (OPCPA) laser system, residual phase dispersion should be compensated as much as possible to shorten the amplified pulses and improve the pulse contrast ratio. Expressions of orders of the induced phases in collinear optical parametric amplification (OPA) processes are presented at the central signal wavelength to depict a clear physics picture and to simplify the design of phase compensation. As examples, we simulate two OPCPA systems to compensate for the phases up to the partial fourth-order terms, and obtain flat phase spectra of 200-nm bandwidth at 1064 nm and 90-nm at 800 nm.

Pulse compression by filamentation in argon with an acoustic optical programmable dispersive filter for predispersion compensation

We have experimentally demonstrated pulses 0.4 mJ in duration smaller than 12 fs; with an excellent spatial beam profile by self-guided propagation in argon. The original 52 fs pulses from the chirped pulsed amplification laser system are first precompressed to 32 fs by inserting an acoustic optical programmable dispersive filter instrument into the laser system for spectrum reshaping and dispersion compensation, and the pulse spectrum is subsequently broadened by filamentation in an argon cell. By using chirped mirrors for post-dispersion compensation, the pulses are successfully compressed to smaller than 12 fs.

Splitting of femtosecond laser pulses by using a Dammann grating and compensation gratings

When a Dammann grating is used to split a beam of femtosecond laser pulses into multiple equal-intensity beams, chromatic dispersion will occur in beams of each order of diffraction and with different scale of angular dispersion because the incident ultrashort pulse contains a broad range of spectral bandwidths. We propose a novel method in which the angular dispersion can be compensated by positioning an m-time-density grating to collimate the mth-order beam that has been split, producing an array of beams that are free of angular dispersion. The increased width of the compensated output pulses and the spectral walk-off effect are discussed. We have verified this approach theoretically and validated it through experiments. It should be highly interesting in practical applications of splitting femtosecond laser pulses for pulse-width measurement, pump-probe measurement, and micromachining at multiple points. (c) 2005 Optical Society of America.

Analysis of dispersion compensation for position-dependence in externally modulated CATV lightwave systems by using chirped fiber grating

The dispersion compensation characteristics of the chirped fiber grating (CFG) for different dispersion compensation positions are analyzed in externally modulated cable television (CATV) lightwave system and the analytic expression of the composite second order (CSO) distortion is derived. The analyses give a reasonable explanation for the position-dependent effect of CFG dispersion compensator, which was found in practical systems. Moreover, the theoretical result is also verified by an experiment. It is believed that the theory will be helpful in designing optical CATV fiber links with nodes at proper positions both for intensity amplification and dispersion compensation.

Analysis of dispersion compensation by chirped fiber grating in directly and externally modulated CATV systems

The dispersion compensation effect of the chirped fiber grating (CFG) is analyzed theoretically, and analytic expressions are derived for composite second-order (CSO) distortion in analog modulated sub-carrier multiplexed (AM-SCM) cable television (CATV) systems with externally and directly modulated transmitters. Simulations are given for the two kinds of modulations and for standard single mode fiber and non-zero dispersion shift fiber (NZDSF) systems. The results show that CFG could be used as a dispersion compensator in directly modulated systems, but its dispersion coefficient should be adjusted much more precisely than the externally modulated system. The requirements for the NZDSF system could be loosened much. It is proposed that directly modulated source may be used as a transmitter in CATV systems combined with tunable CFG dispersion compensator being adjusted precisely, which may be more cost-effective than externally modulation technology. (c) 2006 Elsevier GmbH. All rights reserved.

Digital holographic aberration compensation in electron holography

This paper proposes a new digital method to compensate for the aberration of an electron objective lens in electron holography. In this method, the object wavefront in the exit pupil plane is numerically reconstructed from a digitized electron hologram, and is corrected by multiplying it with the conjugated phase-error function. Then, an aberration-free image can be obtained by calculating the Fresnel integral of this corrected wavefront. In comparison with traditional methods, this method is much more convenient and accurate. Some verifying experiments are also presented in this paper. (C) 2003 Society of Photo-optical Instrumentation Engineers.

Compensation for self-focusing by use of cascade quadratic nonlinearity

We demonstrate theoretically and experimentally compensation for positive Kerr phase shifts with negative phases generated by cascade quadratic processes. Experiments show correction of small-scale self-focusing and whole-beam self-focusing in the spatial domain and self-phase modulation in the temporal domain. (C) 2001 Optical Society of America.

Newly designed multilayer thin film mirror for dispersion compensation in Ti:sapphire femtosecond lasers

There are two different effects to generate group delay dispersion by multilayer thin film mirrors: chirper effect and Gires-Tournois effect. Both effects are employed to introduce desired dispersion in the designed mirror. Thus the designed mirror provides large dispersion throughout broad waveband. Such mirror can be used for dispersion compensation in Ti:sapphire femtosecond lasers. Most group delay dispersion of a 5-mm Ti:sapphire crystal can be compensated perfectly with only four bounces of the designed mirror.

Compensation of reflectance response deviations of guided-mode resonant filters induced by overetching fabrication

Unless the fabrication error control is well treated, it easily causes overetched fabrication errors, which causes the resonant peak value deviation during the fabrication process of guided-mode resonant filters (GMRFs). Hence, the fabrication error control becomes a key point for improving the performance of GMRF. We find that, within the range of the groove depth from 93 to 105 nm, the relationship between the overetched error and the resonant peak value deviation is nearly linear, which means that we can compensate the reflectance response deviation and reduce the resonant peak value deviation by the method of covering the layer film on the GMRF. Simulation results show that the deviation is compensated perfectly by this way. (C) 2008 Optical Society of America