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A nonlinear finite element analysis of 4 RC deep flexural simply supported beams with the program VT2 is presented. Crack development and failure modes , longitudinal reinforcement strain distributions and load -deflection curves were compared as a result in order to evaluate the validity of the disturbed stress field model to analyse such deep simply supported beam. 相似文献
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Seasonal pattern of total soil respiration in undisturbed and disturbed ecosystems of Central Himalaya 总被引：1，自引：0，他引：1
Summary The rates of CO2 efflux were measured by an alkali absorption method (using 20 ml 0.5 N NaOH) from soils in four undisturbed sites [two evergreen oak forests, Quercus floribunda Lindl. (tilonj oak), Quercus leucotrichophora A Camus (banj oak), and two evergreen conifer forests, Cedrus deodara Loud. (deodar forest) and Pinus roxburghii Sarg. (chir pine forest)] and three disturbed sites. The sites were located between elevations of 1850 and 2360 m in the Central Himalaya. The seasonal pattern of soil respiration was similar in all the sites with a maximum during the rainy season, intermediate rates during the summer season and the lowest level of activity in winter. The rate of CO2 efflux was higher in broadleaf than in conifer forests, and it was lowest in the disturbed sites. Among the edaphic conditions, soil moisture, N, organic C, pH, soil porosity, and root biomass positively affected total soil respiration. The proportion of root respiration to total soil respiration was higher in the disturbed sites than the undisturbed sites in winter. Conditions in the winter season were less favourable for microbial respiration than for root respiration. 相似文献
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神府矿区弃土弃渣体侵蚀特征及预测 总被引：7，自引：0，他引：7
神府矿区在煤炭开采过程中形成了弃渣体、弃土体及扰动土体等不同下垫面类型,其物质组成、结构性及产流产沙规律与撂荒地有极大差异,且已产生严重的水土流失。采用野外模拟降雨试验方法,对比研究了弃土弃渣体、扰动土体与撂荒地侵蚀特征的差异。结果表明,(1)撂荒地侵蚀速率随降雨历时先下降后逐渐稳定,扰动土体侵蚀速率呈增大后趋于稳定;35°弃土体侵蚀速率增至峰值后减小并逐渐稳定,40°时则一直处于波动状态;石多砂少和砂多石少弃渣体在产流初期侵蚀波动剧烈,并伴有泥石流现象,15 min后基本稳定。(2)同一下垫面侵蚀速率随雨强增大而增大,而各坡度间侵蚀速率差异不显著(p0.05);雨强相同时各下垫面、坡度间侵蚀速率差异均显著(p0.05),复杂疏松的物质组成是弃土弃渣体及扰动土体侵蚀特殊于撂荒地的根本原因,相同降雨条件下石多砂少弃渣体、砂多石少弃渣体、弃土体及扰动土体侵蚀速率分别是撂荒地的6.51倍~14.25倍、57.91倍~239.2倍、43.60倍~180.1倍和2.27倍~3.06倍。(3)弃土弃渣体及扰动土体侵蚀速率与中值粒径、分形维数、降雨强度、坡度及径流参数呈幂函数关系。研究结果对矿区弃土弃渣侵蚀模型建立与生态环境建设有重要的科学意义。 相似文献
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高寒草地不同扰动生境土壤微生物氮素生理群数量特征研究 总被引：12，自引：2，他引：12
采用平板表面涂抹法和稀释法对天祝高寒草地5种不同干扰生境（围栏内、围栏外、多年生草地、一年生草地和鼠丘地）土壤微生物氮素生理群（氨化细菌、硝化细菌、自生固氮菌和反硝化细菌）数量分布特征进行了测定，结果表明：1）同一扰动生境，不同空间层次草地土壤微生物各氮素生理群数量变化较大，0-20cm数量大于20-40cm，一般前者为后者的1．44-7．32倍；且不同氮索生理群之间数量差异很大，氨化细菌〉自生固氮菌〉反硝化细菌〉硝化细菌；2）不同扰动生境，相同空间层次草地土壤微生物各氮素生理群数量差异也较大，变幅在1．27—9．29倍；且不同扰动生境对各氮素生理群的影响不同，对硝化细菌数量影响最大，反硝化细菌和氨化细菌次之，自生固氮菌较小；3）不同生境土壤微生物氮素生理群数量以一年生草地总数量最大，多年生和围栏内草地次之，鼠丘地和围栏外草地最少；4）不同扰动生境。不同空间层次，虽然不同生理群微生物数量各有特点，但总趋势为：0—20cm，7月份数量较小，8月份上升至最大，9月份下降，20-40cm数量变化趋势与0—20cm相似，但变幅显著变小。 相似文献
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通过对向家坝工程扰动区内的木蓝(Indigofera tinctoria)、盐肤木(Rhus chinensis)、海桐(Pittosporum tobira)等3种生态恢复先锋灌木的光合与水分生理特性的分析测定,结果表明:海桐、盐肤木及木蓝的光饱和点在1 000～1 200μmol.m-2.s-1,光补偿点均低于30μmol.m-2.s-1,各物种的表观量子效率均在0.05～0.06间,表明3种灌木均为喜阳植物,对弱光利用能力均比较强。在光强为1 000μmol.m-2.s-1的光照条件下,3种灌木净光合速率差异不显著,均表现出较强的净光合能力;海桐和木蓝的蒸腾速率显著低于盐肤木,但水分利用率显著高于盐肤木。 相似文献
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[目的]对贵阳市边郊不同人为干扰迹地土壤种子库进行研究。[方法]利用种子萌发试验,从土壤种子库数量、物种组成、物种多样性等方面,对贵阳市乌当区马背山喀斯特次生林不同干扰迹地(弃耕地、火烧地和碎石堆积地)土壤种子库进行研究。[结果]土壤种子库中,弃耕地、火烧地和碎石堆积地的种子密度分别为7 919±616、2 791±350和4 204±372粒/m2,物种组成以演替早期阶段的草本植物为主。碎石堆积地土壤种子库的Margalef丰富度指数最大,火烧地最小;弃耕地土壤种子库的Shanon-Wiener多样性指数最大,火烧地最小;碎石堆积地的Pielou均匀度指数最大,弃耕地最小,表明火烧地土壤种子库的种子数量和种类组成最少,弃耕地优势种明显。弃耕地和火烧地土壤种子库的相似性较高。在0～15 cm土层内,弃耕地和火烧地土壤种子密度随土层深度的增加而减少,碎石堆积地则随土层深度的增加先减小后增大。[结论]该研究为退化喀斯特植被的恢复与重建提供了理论参考。 相似文献
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Small mammals in a mosaic of forest remnants and anthropogenic habitats—evaluating matrix quality in an Atlantic forest landscape 总被引：1，自引：0，他引：1
The matrix of altered habitats that surrounds remnants in human dominated landscapes has been considered homogeneous and inhospitable.
Recent studies, however, have shown the crucial role of the matrix in maintaining diversity in fragmented landscapes, acting
as a mosaic of units with varying permeability to different species. Inclusion of matrix quality parameters is especially
urgent in managing fragmented landscapes in the tropics where agriculture frontiers are still expanding. Using standardized
surveys in 23 sites in an Atlantic forest landscape, we evaluated matrix use by small mammals, the most diverse ecological
group of mammals in the Neotropics, and tested the hypothesis that endemic species are the most affected by the conversion
of original forest into anthropogenic habitats. By comparing species distribution among forest remnants and the predominant
adjacent habitats (native vegetation in initial stages of regeneration, eucalyptus plantations, areas of agriculture and rural
areas with buildings), we found a strong dissimilarity in small mammal assemblages between native vegetation (including initial
stages) and anthropogenic habitats, with only two species being able to use all habitats. Endemic small mammals tended to
occupy native vegetation, whereas invading species from other countries or open biomes tended to occupy areas of non-native
vegetation. Our results highlight that future destruction of native vegetation will favor invading or generalist species which
could dominate highly disturbed landscapes, and that some matrix habitats, such as regenerating native vegetation, should
be managed to increase connectivity among populations of endemic species. 相似文献
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采用样带网格调查法与α多样性指数分析法,对小兴安岭天然、采伐、火烧、造林森林沼泽过渡带群落、草本层、灌木层和乔木层的物种丰富度、多样性指数及其沿过渡带环境梯度的变化规律进行研究,以便评价不同恢复途径对森林沼泽过渡带植物多样性的影响效果,探讨退化森林沼泽生态系统恢复与重建的有效途径。结果表明:天然、采伐、火烧、造林森林沼泽过渡带群落物种丰富度依次分布在25～37、16～39、21～29和19～36之间,干扰使其群落物种丰富度依次下降了37.4%、21.1%和11.3%;天然、采伐、火烧、造林森林沼泽过渡带群落物种多样性指数依次分布在1.5931～1.9488、0.4607～1.3674、0.6365～1.2235和1.4000～1.9575之间,干扰使其群落物种多样性依次下降了60.5%、48.9%和14.6%;天然、采伐、火烧、造林森林沼泽过渡带草本层、灌木层和乔木层物种多样性指数依次分布在1.5762～1.8157、0～1.2673、0.2895～1.1720,0.3008～0.9927、0.5539～1.2327、0.4034～0.7522,0.6322～1.0937、0.6365～1.3514、0～0.1223,1.3157～1.8565、0.2712～0.8909、0.1175～0.6559之间,干扰森林沼泽过渡带草本层、灌木层和乔木层物种多样性分别较天然过渡带有所减少或增加(-67.7%、 26.3%、-28.6%,-51.5%、 67.2%、-88.9%,-19.0%、-18.4%、-23.1%)。总体上看天然森林沼泽过渡带物种多样性最高,人工造林途径与之较接近,而采伐、火烧干扰后自然演替恢复效果相对较差。 相似文献
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