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复杂地形草地植被碳储量遥感估算研究进展 总被引:1,自引:0,他引:1
草地生态系统是我国最大的陆地生态系统,其植被碳储量的准确评估对维护国家生态安全和指导畜牧发展有重要作用。植被生物量和草地面积是草地植被碳储量估算的关键,随着遥感技术的发展,两者估算精度和效率显著提高,先后发展出多种草地生物量遥感估算模型和土地覆被产品,并已在平坦地区取的较好估算结果。然而,复杂地形区迥异于平地的几何形态和水热分布所产生的不均一的生态系统结构和功能,给草地生物量和草地面积的遥感估算带来诸多困难,影响对草地植被碳储量的准确判定。本文在回顾国内外草地植被碳储量遥感估算方法与所需关键参数的基础上,对遥感估算复杂地形草地植被碳储量过程中所面临“遥感影像地形效应的去除和尺度选择”、“植被指数与地形指标的选取”、“过程模型植被生长参数的率定”、“草地面积估算”以及“气象数据与复杂地形上微气候的匹配”等问题进行了总结并提出相应的解决思路,以期为草地植被碳储量遥感估算模型的合理构建以及估算精度的提高提供参考。 相似文献
94.
Positional identity in the visual system affects the topographic projection of the retina onto its central targets. In this review we discuss gradients and positional information in the retina, when and how they arise, and their functional significance in development. When the axons of retinal ganglion cells leave the eye, they navigate through territory in the central nervous system that is rich in positional information. We review studies that explore the navigational cues that the growth cones of retinal axons use to orient towards their target and organize themselves as they make this journey. Finally, these axons arrive at their central targets and make a precise topographic map of visual space that is crucial for adaptive visual behavior. In the last section of this review, we examine the topographic cues in the tectum, what they are, when, and how they arise, and how retinal axons respond to them. We also touch on the role of neural activity in the refinement of this topography. © 1993 John Wiley & Sons, Inc. 相似文献
95.
Toward a more accurate time scale for the human mitochondrial DNA tree 总被引:11,自引:0,他引:11
Masami Hasegawa Anna Di Rienzo Thomas D. Kocher Allan C. Wilson 《Journal of molecular evolution》1993,37(4):347-354
Several estimates of the time of occurrence of the most recent common mitochondrial DNA (mtDNA) ancestor of modern humans have been made. Estimates derived from noncoding regions based on a model that classifies sites into two categories (variable and invariable) have been consistently older than those derived from the third positions of codons. This discrepancy can be attributed to a violation of the assumption of rate homogeneity among variable sites when analyzing the noncoding regions. Additional data from the partial control region sequences allow us to take into account some of this further heterogeneity. By assigning the sites to three classes (highly variable, moderately variable, and invariable) and by assuming that the last common mtDNA ancestor of humans and chimpanzees lived 4 million years ago, the most recent common mtDNA ancestor of humans is estimated to have occurred 211,000 ±111,000 years ago (±1 SE), consistent with the estimate, 101,000 ± 52,000 years, made from third positions of codons and also with those proposed previously. We used the same technique to estimate when a putative expansion of modern humans out of Africa took place and estimated a time of 89,000 ± 69,000 years ago. Even though the standard errors of these estimates are large, they allow us to reject the multiregional hypothesis of modern human origin.Deceased July 21, 1991
Correspondence to: M. Hasegawa 相似文献
96.
The compositional properties of human genes 总被引:8,自引:0,他引:8
Brahim Aïssani Giuseppe D'Onofrio Dominique Mouchiroud Katheleen Gardiner Christian Gautier Giorgio Bernardi 《Journal of molecular evolution》1991,32(6):493-503
Summary The present work represents the first attempt to study in greater detail previously proposed compositional correlations in genomes, based on a body of additional data relating to gene localizations as well as to extended flanking sequences extracted from gene banks. We have investigated the correlations that exist between (1) the GC levels of exons of human genes, and (2) the GC levels of either intergenic sequences or introns associated with the genes under consideration. In both cases, linear relationships with slopes close to unity were found. The similarity of the linear relationships indicates similar GC levels in intergenic sequences and introns located in the same isochores. Moreover, both intergenic sequences and introns showed GC levels 5–10% lower than the corresponding exons. The above findings considerably strengthen the previously drawn conclusion that coding and noncoding sequences (both inter- and intragenic) from the same isochores of the human genome are compositionally correlated. In addition, we find linear correlations between the GC levels of codon positions and of the intergenic sequences or introns associated with the corresponding genes, as well as among the GC levels of codon positions of genes. 相似文献
97.
Jos M. Oliva-Lozano Daniel Rojas-Valverde Carlos D. Gmez-Carmona Víctor Fortes Jos Pino-Ortega 《Biology of sport / Institute of Sport》2020,37(4):429
This study aimed to describe the worst-case scenarios (WCS) of professional soccer players by playing position in different durations and analyse WCS considering different contextual variables (match half, match location and match outcome). A longitudinal study was conducted in a professional soccer team. Data were collected from different WCS durations in the total distance (TD), high-speed running distance (HSRD), and sprinting distance (SPD). A mixed analysis of variance was performed to compare different WCS durations between playing positions and contextual variables, making pairwise comparisons by Bonferroni post hoc test. Positional differences were found for TD (p < 0.01, ωp2 = 0.02), HSRD (p < 0.01, ωp2 = 0.01) and SPD (p < 0.01, ωp2 = 0.02). There was a significant interaction when comparing WCS by match half in TD (F = 6.1, p < 0.01, ωp2 = 0.07) but no significant differences in HSRD (p = 0.403, ωp2 = 0) or SPD (p = 0.376, ωp2 = 0). A significant interaction was identified when comparing WCS by match location in TD (F = 51.5, p < 0.01, ωp2 = 0.14), HSRD (F = 19.15, p < 0.01, ωp2 = 0.05) and SPD (F = 8.95, p < 0.01, ωp2 = 0.01) as well as WCS by match outcome in TD (F = 36.4, p < 0.01, ωp2 = 0.08), HSRD (F = 13.6, p < 0.01, ωp2 = 0.04) and SPD (F = 7.4, p < 0.01, ωp2 = 0.02). Positional differences exist in TD, HSRD, and SPD in match-play WCS, and contextual variables such as match half, match location and match outcome have a significant impact on the WCS of professional soccer players. 相似文献
98.
The visual system is beautifully crafted to transmit information of the external world to visual processing and cognitive centers in the brain. For visual information to be relayed to the brain, a series of axon pathfinding events must take place to ensure that the axons of retinal ganglion cells, the only neuronal cell type in the retina that sends axons out of the retina, find their way out of the eye to connect with targets in the brain. In the past few decades, the power of molecular and genetic tools, including the generation of genetically manipulated mouse lines, have multiplied our knowledge about the molecular mechanisms involved in the sculpting of the visual system. Here, we review major advances in our understanding of the mechanisms controlling the differentiation of RGCs, guidance of their axons from the retina to the primary visual centers, and the refinement processes essential for the establishment of topographic maps and eye-specific axon segregation. Human disorders, such as albinism and achiasmia, that impair RGC axon growth and guidance and, thus, the establishment of a fully functioning visual system will also be discussed. 相似文献
99.
Geostatistical analysis of spatial variations in leaf traits of woody plants in Tiantong,Zhejiang Province 下载免费PDF全文
《植物生态学报》2016,40(1):48
100.