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41.
42.
Michael T. Stoddard Christopher M. McGlone Peter Z. Fulé Daniel C. Laughlin Mark L. Daniels 《西北部美国博物学家》2011,71(2)
Dense ponderosa pine forests in the southwestern United States inhibit understory production and diversity and are susceptible to high-severity wildfire. Restoration treatments involving overstory thinning and prescribed burning are being implemented to increase understory productivity and diversity and to reduce the risk of severe wildfire. However, disturbances associated with treatments may favor invasion of nonnative species, and the severity of the disturbance may be related to the level of nonnative species establishment. We examined understory community composition, species richness, and plant cover responses to 3 stand-scale replicates of 4 different tree-thinning intensities. Restoration treatments altered the composition of the understory community regardless of thinning intensity. Understory richness and cover were highly variable among experimental blocks, but we observed strong trends of increasing richness and cover in the treated stands. Immediately following restoration treatments, nonnative species cover comprised 6% of the total cover where treatment-induced disturbances were the greatest. However, the initial increase in nonnative species did not persist and was reduced by half 6 years after treatment. Plant community composition was still in flux by the sixth year after treatment, indicating that continued monitoring is necessary for evaluating whether restoration targets are maintained over time. Los densos bosques de pino ponderosa en el suroeste de los E.U.A. inhiben la producción y diversidad del sotobosque y son propensos a incendios severos. El raleo del dosel y la quema controlada se están implementando como tratamientos de restauración para incrementar la productividad y diversidad del sotobosque y para reducir el peligro de incendios severos. Sin embargo, las perturbaciones asociadas con estos tratamientos podrían favorecer la invasión por especies no nativas, y la severidad de la perturbación puede estar relacionada con el nivel de establecimiento de estas especies. Examinamos la reacción del sotobosque con respecto a la composición de sus comunidades, a la riqueza de especies y a la cobertura de la vegetación, a tres réplicas a nivel de rodal de cuatro distintas intensidades de raleo. Los tratamientos de restauración cambiaron la composición de la comunidad del sotobosque independientemente de la intensidad del raleo. La riqueza y la cobertura del sotobosque variaron mucho entre las áreas estudiadas, pero observamos marcadas tendencias de mayor riqueza y cobertura en los rodales que recibieron el tratamiento. Inmediatamente después de los tratamientos de restauración, la cobertura de especies invasoras constituía el 6% de la cobertura total donde las perturbaciones producidas por los tratamientos fueron más grandes. No obstante, el aumento inicial en la cobertura de especies invasoras no persistió, y se redujo a la mitad en un lapso de seis años después del tratamiento. La composición de las comunidades de plantas aun seguía cambiando en el sexto año después del tratamiento, lo cual indica que será preciso el monitoreo continuo para determinar si se lograron las metas de restauración. 相似文献
43.
The effect of TiO2, ZrO2 and Na3AlF6 ultrafine powders on the fine structure and the phase composition of Fe-14Mn-1.2C steel was investigated. The introduction of the ultrafine powders into the melt influenced the grain size, the quantity, and the character of distribution of nonmetallic inclusions in the railroad frogs. The microstructure of castings was improved significantly because of the refinement of the grain structure and an increase of the grain-boundary area. After the modifying mixture was introduced into the melt, either the microtwins of one or two intersecting systems or the precipitations of ε-martensite of different types, or simultaneously the microtwins and wafers of ε-martensite, were present in each grain. 相似文献
44.
科学钻探是现代地球科学研究中一个不可或缺的工具,因为它是直接获得地球深部过程信息的唯一来源,对于我们更好地认识了解我们赖以生存的星球、解决紧迫的社会经济问题具有关键性的作用。国际大陆科学钻探计划(ICDP)始于1996年,2005年在德国波茨坦召开了第二届国际大陆钻探会议。 相似文献
45.
休斯飞机公司研制的“幼畜”导弹AGM-65A~G,是一系列很成功的武器。这种系列导弹的品种还在增多,性能还在改善。今天的电子部件大概每隔三年要更新换代一次。“幼畜”导弹本身的改进、其它武器借用“幼畜”的技术以及研制崭新的空对地导弹,看来有着无限广阔的前景。 可望最先实现的两项重大进展将是:(1)导弹在发射后自动锁定目标,从而能扩大导弹的可能射程;(2)多枚导弹在很短时间内的快速连射,将变得越来越容易。下一 相似文献
46.
Mark Last 《国外科技新书评介》2005,57(7):17-18
本书是《机器感知与人工智能》丛书的第57卷。主要内容为现代信息系统存储的时间数据中预处理、挖掘与解释中的某些新进展,并将时间维数加到数据库中产生时间序列数据库(TSBD)。书中介绍了数据挖掘与知识发现中的新的状况与挑战。其中包括寻找时间序列数据的有效表达方法,测量时间序列的相似性,检测时间序列的改变点以及时间序列的分类与聚变。 相似文献
47.
A new feathered maniraptoran dinosaur fossil that fills a morphological gap in avian origin 总被引:3,自引:0,他引:3
Recent fossil discoveries have substantially reduced the morphological gap between non-avian and avian dinosaurs, yet avians
including Archaeopteryx differ from non-avian theropods in their limb proportions. In particular, avians have proportionally longer and more robust
forelimbs that are capable of supporting a large aerodynamic surface. Here we report on a new maniraptoran dinosaur, Anchiornis huxleyi gen. et sp. nov., based on a specimen collected from lacustrine deposits of uncertain age in western Liaoning, China. With
an estimated mass of 110 grams, Anchiornis is the smallest known non-avian theropod dinosaur. It exhibits some wrist features indicative of high mobility, presaging
the wing-folding mechanisms seen in more derived birds and suggesting rapid evolution of the carpus. Otherwise, Anchiornis is intermediate in general morphology between non-avian and avian dinosaurs, particularly with regard to relative forelimb
length and thickness, and represents a transitional step toward the avian condition. In contrast with some recent comprehensive
phylogenetic analyses, our phylogenetic analysis incorporates subtle morphological variations and recovers a conventional
result supporting the monophyly of Avialae.
Supported by Hundred Talents Project of the Chinese Academy of Sciences, National Natural Science Foundation of China (Grant
Nos. 40125006, 40472018), and National Basic Research Program of China (Grant No. 2006CB806400) 相似文献
48.
Geochemical data from ancient sedimentary successions provide evidence for the progressive evolution of Earth's atmosphere and oceans. Key stages in increasing oxygenation are postulated for the Palaeoproterozoic era (~2.3?billion years ago, Gyr ago) and the late Proterozoic eon (about 0.8?Gyr ago), with the latter implicated in the subsequent metazoan evolutionary expansion. In support of this rise in oxygen concentrations, a large database shows a marked change in the bacterially mediated fractionation of seawater sulphate to sulphide of Δ(34)S?25‰ before 1?Gyr to ≥50‰ after 0.64?Gyr. This change in Δ(34)S has been interpreted to represent the evolution from single-step bacterial sulphate reduction to a combination of bacterial sulphate reduction and sulphide oxidation, largely bacterially mediated. This evolution is seen as marking the rise in atmospheric oxygen concentrations and the evolution of non-photosynthetic sulphide-oxidizing bacteria. Here we report Δ(34)S values exceeding 50‰ from a terrestrial Mesoproterozoic (1.18?Gyr old) succession in Scotland, a time period that is at present poorly characterized. This level of fractionation implies disproportionation in the sulphur cycle, probably involving sulphide-oxidizing bacteria, that is not evident from Δ(34)S data in the marine record. Disproportionation in both red beds and lacustrine black shales at our study site suggests that the Mesoproterozoic terrestrial environment was sufficiently oxygenated to support a biota that was adapted to an oxygen-rich atmosphere, but had also penetrated into subsurface sediment. 相似文献
49.
近代科学技术的历史已发展到了一个非常惊人的程度,这也应归功于二十世纪前三十年的一些革命性的发现,这些发现和产生它们的思想,以及随后由此产生的技术,便形成了人们的遗产,这些遗产已经成为我们现代科学技术的基础。本文拟集中精力简述几个主要问题。为此,详细地叙述这些遗产是很有必要的。上世纪末已创建了所谓的经典力学,或称之为牛顿力学。当时有些极有权威的物理学家认为:经典力学即牛顿力学将不会再有多大的发展了。十九世纪获得的两项伟大科学成就——詹姆斯·克莱克·麦克斯韦尔的电动力学,以及以原子结构理论为依据的路特伟格·波尔曼对热力学的解释。两者之间的统一在十九世纪末已被人们公认了。它们和艾萨克·牛顿力学一起构成了自然科学基础知识中最重要的知识框架。这就是当时的物理学。同时,工业革命给十八世纪和十九世纪时期的西方世界带来了巨大的发展,这一发展最初是以牛顿力学和后来的麦克斯韦尔电动力学以及波尔曼的热力学为基础。尽管已有这些成就,但 相似文献
50.