首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1031篇
  免费   154篇
  国内免费   506篇
  2024年   15篇
  2023年   48篇
  2022年   40篇
  2021年   86篇
  2020年   79篇
  2019年   88篇
  2018年   70篇
  2017年   79篇
  2016年   69篇
  2015年   73篇
  2014年   65篇
  2013年   94篇
  2012年   69篇
  2011年   65篇
  2010年   56篇
  2009年   86篇
  2008年   80篇
  2007年   87篇
  2006年   60篇
  2005年   49篇
  2004年   58篇
  2003年   42篇
  2002年   24篇
  2001年   22篇
  2000年   22篇
  1999年   15篇
  1998年   19篇
  1997年   16篇
  1996年   14篇
  1995年   18篇
  1994年   11篇
  1993年   6篇
  1992年   11篇
  1991年   10篇
  1990年   8篇
  1989年   7篇
  1988年   8篇
  1987年   4篇
  1986年   7篇
  1985年   4篇
  1983年   1篇
  1982年   2篇
  1981年   1篇
  1980年   1篇
  1979年   1篇
  1978年   1篇
排序方式: 共有1691条查询结果,搜索用时 661 毫秒
831.
The water-retaining capacity, percolation and evaporation of stony soil in Liupan Mountains, China, were measured in order to understand the effect of rock fragments on soil hydrological processes. The results indicated that the effective water-retaining capacity of soil is positively related with the volumetric content of rock fragments, but there is no relation between saturated water-retaining capacity and rock fragment content. For the soil layers within 0–40 cm, the steady infiltration rate increases with increasing volumetric content of rock fragments until it reaches the range of 15%–20%, and then it decreases when the rock fragment content further increases. For the soil layers below 40 cm, the steady infiltration rate always increases with increasing rock fragment content. The soil evaporation rate decreases with increasing volumetric content of rock fragments when it varies in the range of 0–20%, while the soil evaporation rate keeps basically stable when the rock fragment content is higher than 20%. The soil evaporation rate shows a rising tendency with increasing size of rock fragments.  相似文献   
832.
Late Emsian-Early Eifelian radiolarians from the southern Ural Mountains are recorded and described for the first time. They come from the Kamennaya Gora section on the Tanalyk River near the town of Buribay, Bashkiria. The new Late Emsian-Early Eifelian radiolarian assemblage consists of 20 species of 11 genera of spherical, discoidal, spindle-shaped, pylomate, and spiny radiolarians. Four new species, i.e., Entactinia rostriformis sp. nov., Palaeodiscaleksus tumefactus sp. nov., Primaritripus buribayensis sp. nov., and P. chuvashovi sp. nov., are described. The diagnosis of the genus Primaritripus Afanasieva, 2000 is amended. The fact that the oryctocenosis is dominated by discoidal radiolarians suggests that, in the Late Emsian-Early Eifelian, the area under study was occupied by a relatively shallow marine basin. Based on radiolarians, a new Upper Emsian-Lower Eifelian biostraton, beds with Primaritripus buribayensis-Entactinia rostriformis, is established. The history of the study of Early Devonian radiolarians is considered.  相似文献   
833.
小陇山油松林乔木层生物量相容性线性模型   总被引:3,自引:0,他引:3  
以甘肃小陇山的油松林分为对象,应用图形分析、相关分析和回归分析法,建立了基于森林资源调查资料的林分生物量相容性线性模型.结果表明,小陇山油松林分各组分生物量(W)与林分活立木蓄积量(V)、与林分平均胸径-平均树高-林分密度(-D2-Hρ)均存在极其显著的线性相关关系.采用代数和法,建立了W=a bV、W=a b(-D2-Hρ)2种形式的林分乔木层生物量相容性线性模型,以95%的可靠性估计,2种形式模型的预估精度均在95%以上.理论上证明了W=a 6V这种建立在林分水平上的生物量线性估计模型可直接推广应用到区域尺度.  相似文献   
834.
Abstract Despite increased knowledge about environmental toxins and changes in lead use (i.e., the mandated use of nonlead paint, gasoline, and shotgun pellets used for hunting waterfowl on federal lands), lead poisoning continues to occur in terrestrial birds. The degree of exposure and its demographic effect, however, continue to be described, emphasizing the growing concern over lead exposure. We examined 302 blood samples from common ravens (Corvus corax) scavenging on hunter-killed large ungulates and their offal piles to determine if lead rifle-bullet residuum was a point source for lead ingestion in ravens. We took blood samples during a 15-month period spanning 2 hunting seasons. Of the ravens tested during the hunting season, 47% exhibited elevated blood lead levels (≥ 10 μg/dL) whereas 2% tested during the nonhunting season exhibited elevated levels. Females had significantly higher blood lead levels than did males. Our results confirm that ravens are ingesting lead during the hunting season and are likely exposed to lead from rifle-shot big-game offal piles.  相似文献   
835.
A new species of Centaurea, Centaurea sicana , endemic to Sicily, is described. This new taxon is close to Centaurea parlatoris and Centaurea giardinae (Sect. Dissectae), the former occurring at Palermo and in the Madonie Mountains, while the latter occurs on the Etna volcano. The new species has so far been found only on the Sicani Mountains, a limestone mountain range in western Sicily, while the related species occur in the surrounding mountains. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 157 , 785–788.  相似文献   
836.
Saintpaulia (H. Wendl.) is a forest-dependent, endangered plant genus endemic to Tanzania and Kenya. The taxonomy of Saintpaulia from the Usambara Mountains and adjacent lowlands in north-eastern Tanzania is problematic because of the morphological similarity of the species and the presence of considerable intraspecific variation. Conventional molecular phylogenetic methods have failed to reveal the genetic structure of this Saintpaulia complex. In this study, we assessed the genetic composition of 12 Saintpaulia populations, representing four different morphotypes, from the East Usambara Mountains using inter-simple sequence repeat (ISSR) markers. Relatively high genetic diversities were observed within populations (mean h  = 0.320), indicating their adaptive potential. Little genetic differentiation amongst populations (mean F st  = 0.063) and the genetic divergence of the rosulate and trailing morphotypes support the hypothesis of ongoing divergent evolution within the East Usambara metapopulation(s) of Saintpaulia .  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 157 , 347–355.  相似文献   
837.
Good JM  Sullivan J 《Molecular ecology》2001,10(11):2683-2695
The northern Rocky Mountains have experienced a complex history of geological events and environmental fluctuation, including Pleistocene glaciation. To provide an initial assessment of the genetic impact of this history on the regional biota we estimated phylogenetic relationships within Tamias ruficaudus, a regional endemic, from cytochrome b sequence variation using parsimony, maximum likelihood, and nested clade analysis. Analyses of sequence variation in 187 individuals from 43 localities across the distribution of T. ruficaudus indicate a history of vicariance events and range fluctuation consistent with successive periods of extensive Pleistocene glaciation in the northern Rocky Mountains. Intraspecific divergence levels (c. 4.7% uncorrected) and phylogenetic structure are consistent with a genealogical vicariance initiated prior to the Late Pleistocene, whereas nested clade analyses indicate more recent population history structured by both fragmentation and range expansion. A comparison of sequence variation with bacular morphology indicates that the two genetically and morphologically differentiated entities exhibit a zone of differential character introgression. Sequence data support a multiple refugia hypothesis and provide a phylogeographical case study for the ongoing synthesis of regional biogeography for northern Rocky Mountain endemics.  相似文献   
838.
We assessed the geneticdiversity consequences of applying ecologicalreserve design guidelines to four federally-listedglobally-rare plant species. Consequences weremeasured using two metrics: proportion of allalleles and of common alleles included inreserves. Common alleles were defined as those alleleshaving a frequency of 0.05 in at least onepopulation. Four conservation professionalsapplied ecological reserve guidelines to choosespecific populations of each species forinclusion in reserves of size 1 to N – 1, whereN is the total number of populations of each species.Information regarding genetic diversity was notused in selecting populations. The resulting reservedesigns were compared to random designs, andthe agreement among experts was assessed usingKendall's coefficient of concordance.Application of ecological reserve design guidelines provedmostly ineffective in capturing more geneticdiversity than is captured selectingpopulations randomly. Meeting establishedtargets for genetic diversity, such as one advocated by theCenter for Plant Conservation, required largernumbers of populations than are suggested to besufficient. Relative performance of expertdesigns differed among species and wasdependent on whether the proportion of allalleles or of common alleles was used as a measure ofdiversity. Furthermore there was no significantconcordance among experts in order in whichpopulations were incorporated into reserves asexperts differed in priority they placed onindividual guidelines.  相似文献   
839.
孙航 《云南植物研究》2002,24(3):273-288
横断山-喜马拉雅植物区系的开端是在晚白垩纪和早古近纪(早第三纪)。古植物资料在古近纪初期横断山-喜马拉雅植物区系是同古地中海沿岸一致的以照叶林为主的暖湿植物区系。古近纪后期和新近纪(新第三纪)以后古地中海气候逐步旱化。原来的暖湿植物区系在地中海地区逐步消失,而在横断山及喜马拉雅和东亚其他地区得以保存和发展,现代横断山及东喜马拉雅的亚热带森林即是其后裔。古近纪中期以后由于古地中海的逐步退却,气候变得干旱,原暖湿植物区系逐步被现代旱生的地中海植物区系所取代。新近纪以后,旱生的现代地中海植物区系由于喜马拉雅和横断山的隆起而转向适应高山环境,逐步分化形成了现代的中国-喜马拉雅成分。横断山-喜马拉雅地区硬中高山栎林的起源;铁筷子属,绿绒蒿属,芒苞草属,假百合属及马桑属的地中海,喜马拉雅-横断山间断分布的形成便是古地中海植物区系残遗的体现;黄花木属,独一味属等众多中国喜马拉雅成分就是古地中海祖先的后裔。这些代表类群的分析研究表明现代的喜马拉雅-横断山的高山植物区系以及中国-喜马拉雅成分中有相当的一部分是起源于新生代旱生的地中海植物区系。  相似文献   
840.
We studied the autumn food habits of the Asiatic black bear (Ursus thibetanus) from 1993 to 1997, based on 202 fecal samples in the Chichibu Mountains, central Japan. Nuts occupied the highest proportions of autumn foods (59.9–85.8% important values). Although the proportion of nuts of Quercus crispula, Fagus crenata and Fagus japonica varied greatly between the years, acorns of Q.crispula were most prevalent in four of the five years. We also determined the relative nut production of these three species by counting the number of nuts or cupules on the ground. Black bears consumed the nuts according to their relative availabilities. Nuts of Q.crispula appeared to be the most important food because: (i) these nuts were eaten in the highest proportion in four of the five study years; and (ii) even in poor years the bears consumed acorns of Q.crispula, whereas nuts of Fagus spp. were not consumed. We discuss the significance of alternative foods for black bears in relation to food tree diversity in the forest and the necessity for long-term studies examining the food habits of Asiatic black bears.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号