基于XDSL串扰噪声的应对技术分析研究

在DSL运行环境中，由于电缆内部线对间的非屏蔽而存在电磁耦合导致的串扰。串扰噪声造成XDSL速率下降甚至重训练，影响业务质量。本文对串扰导致的性能损失和导致重训练的原因进行分析并且就解决串扰导致稳定性问题的方法进行探讨。     

Future prospects for the rare, late-flowering Gentianella germanica and Gentianopsis ciliata in Dutch nutrient-poor calcareous grasslands

We discuss the population biology of two calcareous grassland gentians, Gentianella germanica and Gentianopsis ciliata, in relation to the habitat management currently practiced in The Netherlands. There, at the margin of their range, both species are rare. Gentianella germanica persists on six remaining locations, whereas Gentianopsis ciliata, with two populations, is nearly extinct. Gentianella germanica is a strict biennial, Gentianopsis ciliata an iteroparous perennial. Both species depend on insects for seed production and suffer from low insect visitation. Pollination experiments in one Dutch population demonstrated that Gentianopsis ciliata is self-compatible, but hardly sets any seed under natural conditions due to pollen limitation. The low reproductive success of both species is partly due to the low pollinator densities at their late flowering time, partly caused by the small population size of the gentians themselves, and partly a result of mowing too early. The latter has destroyed the seed crop of several subsequent years in one population of Gentianopsis ciliata and some of Gentianella germanica. In Gentianella germanica, the early mowing and low insect visitation seems to have resulted in selection of less herkogamous and consequently more autofertile individuals. The perspectives for Gentianopsis ciliata are currently extremely poor in The Netherlands. Under the present circumstances, extinction will most likely occur within 10-20 years. Population reinforcement (seeding, artificial cross-pollination with nearby populations) should be considered if we want to conserve this species. For both gentians, but also of other flagship species of nutrient-poor calcareous grasslands, the total grassland area needs to be enlarged and must constitute an interconnected network of reserves. The traditional management method, sheep grazing, is to be preferred over mowing, but only if the flock visits each grassland patch or reserve for only a short time of less than a day, and with intervals of more than 2 weeks between visits. If mowing is preferred for other reasons, it should be done rotationally, and not before October.     

基于GIS的贺兰山两侧沙漠边缘带近20a来土地景观格局变化

毛乌素沙地与腾格里沙漠位于贺兰山东西两侧,是中国北方重要的沙漠之一。20世纪80年代至21世纪初,受北方气候变化与沙漠边缘地带人类活动强度加大的影响。两大沙漠边缘地带所属的陕西、甘肃、以及宁夏各县土地利用/土地覆盖发生了显著的变化。同时,由于受到自然地带差异以及其它因素的影响,两大沙漠边缘带土地利用/土地覆盖变化表现出不同的特征。试用景观生态学中景观格局分析方法对这种差异进行描述,通过对两大沙漠边缘带土地利用/土地覆盖格局变化对比研究,力图揭示我国北方干旱区与半干旱区沙漠边缘生态脆弱带生态环境对气候与人类活动变化表现出的不同响应方式。     

水轮机汽蚀安全裕量问题探讨

水轮机运行完全避免汽蚀破坏是困难的。问题在于设法减轻汽蚀机件的破坏作用,同时应避免水轮机在汽蚀发展到使效率降低和机组强烈振动的情况下运行。汽蚀安全裕量便成为一个最为重要的可变参数,水轮机汽蚀安全裕量的选用,关乎水轮机装置允许吸出高度确定,直接影响水能资源的合理利用和工程建设的投资大小,而且也影响到机组运行寿命和维修管理费用等。因此,进一步对汽蚀安全裕量的取值问题进行探讨,不但是学术问题,而且具有一定的经济价值和实践意义。     

Leaf margin phenotype-specific restriction-site-associated DNA-derived markers for pineapple (Ananas comosus L.)

To explore genome-wide DNA polymorphisms and identify DNA markers for leaf margin phenotypes, a restriction-site-associated DNA sequencing analysis was employed to analyze three bulked DNAs of F1 progeny from a cross between a ‘piping-leaf-type’ cultivar, ‘Yugafu’, and a ‘spiny-tip-leaf-type’ variety, ‘Yonekura’. The parents were both Ananas comosus var. comosus. From the analysis, piping-leaf and spiny-tip-leaf gene-specific restriction-site-associated DNA sequencing tags were obtained and designated as PLSTs and STLSTs, respectively. The five PLSTs and two STSLTs were successfully converted to cleaved amplified polymorphic sequence (CAPS) or simple sequence repeat (SSR) markers using the sequence differences between alleles. Based on the genotyping of the F1 with two SSR and three CAPS markers, the five PLST markers were mapped in the vicinity of the P locus, with the closest marker, PLST1_SSR, being located 1.5 cM from the P locus. The two CAPS markers from STLST1 and STLST3 perfectly assessed the ‘spiny-leaf type’ as homozygotes of the recessive s allele of the S gene. The recombination value between the S locus and STLST loci was 2.4, and STLSTs were located 2.2 cM from the S locus. SSR and CAPS markers are applicable to marker-assisted selection of leaf margin phenotypes in pineapple breeding.     

The relationship of environmental factors to spatial and temporal variation of fish assemblages in a floodplain river in Texas, USA

Abstract –  Spatial and temporal variation of species–environment relationships were evaluated for shallow-margin and deep-water fish assemblages in the Brazos River, a large floodplain river in Texas, USA. Total variation among the deep-water assemblages (11 species, 86% turnover across gill net samples) was greater than for shallow-margins (38 species, 64% turnover across seine samples). For both shallow-margin and deep-water assemblages, variation was greater among sites than between winter and summer seasons. Shallow-margin assemblage structure was related to depth, velocity and substrate, whereas for deep-water assemblages river discharge, temperature and velocity were important. Season itself accounted for little of the variation among either shallow (6.7%) or deep-water (2.3%) assemblages. Overall temporal patterns of shallow-margin samples appeared to show responses to juvenile recruitment, spates and migration of coastal fishes, whereas for deep-water samples, patterns related to use of reproductive habitats, juvenile recruitment and seasonal activity levels. Brazos River assemblages were less variable overall in comparison with studies along similar length of reach in headwater streams and wadeable rivers. The residual variation in species distribution (54% for shallow-margin and 67% for deepwater) that was not explained by instream variables and season suggests a greater influence of biotic interactions in rivers, particularly those across the spatially dynamic interface of main channel habitats and shallow river margins.     

卸油管路校核计算理论的补充分析和修正

铁路轻油罐车的卸油管路系统，在夏天作业时常发生气阻气蚀现象，根据这一事实，调查了我国诸多油库，从大量的生产数据中发现，理论计算结果与实际生产状况存在着较大的差距，进而分析了恶劣工况下促使气阻的气蚀提前形成的各种原因，如烃类汽化特征、溶解气和夹带气泡、气泡窝存、罐内液体温度不均匀、管路漏失等加以考虑，并对过去的计算理论作了补充。为了增加管路系统的剩余压力，确保卸油安全，还提出了向罐车内输入压缩空气，     

The challenge of conserving grassland insects at the margins of their range in Europe

Insects have experienced higher rates of decline than other popular taxa in the calcareous grasslands of Europe. This paper reviews reasons why insects are so vulnerable to environmental change, particularly at the edges of their range. Case studies of the ant Myrmica sabuleti and the butterflies Polyommatus bellargus and Thymelicus acteon are used to illustrate the mechanisms responsible for this vulnerability and the need for different management regimes to maintain species’ niches in different parts of their range.     

Factors affecting plant species distribution in hedgerows of southern Quebec

Woody linear features of farmlands have received much attention as pools of diversity in European countries, but they have, in comparison, been little considered in eastern North American ecological studies. Here, we evaluate the relative contribution of environmental conditions, history of management practices within hedgerows, and adjacent land-use on herb and shrub species distribution in woody hedgerows of agricultural landscapes of southern Quebec, Canada. We pay particular attention to the distribution of forest herbs and weed species and identify the main factors that explain their presence. We also evaluate the amount of spatial structuring in the species data and explanatory variables.Our results show significant differences in species composition related to the unique influence of all sets of factors considered. Adjacent land-use (from intensive crop to fallow land) remains the best predictor of overall species distribution, followed by environmental conditions and history of management practices. The probability of finding forest herbs in hedgerows increases as intensity of adjacent agricultural practices (from intensive crop to fallow land) decreases. Potential weed species, on the other hand, are more likely to be found within narrow hedgerows adjacent to the more intensively managed fields. We discuss the results in terms of the interactions between the different determinants of plant diversity in hedgerows and demonstrate the necessity to distinguish between multiple structuring factors to devise proper conservation strategies. Such strategies will have to take into account the ecological as well as the agricultural exploitation context in which linear features are found to be effective.