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   328篇
  免费   42篇
  国内免费   51篇
  421篇
  2024年   1篇
  2023年   7篇
  2022年   14篇
  2021年   18篇
  2020年   14篇
  2019年   19篇
  2018年   17篇
  2017年   13篇
  2016年   11篇
  2015年   11篇
  2014年   20篇
  2013年   15篇
  2012年   12篇
  2011年   8篇
  2010年   13篇
  2009年   13篇
  2008年   14篇
  2007年   25篇
  2006年   16篇
  2005年   9篇
  2004年   12篇
  2003年   19篇
  2002年   18篇
  2001年   21篇
  2000年   11篇
  1999年   12篇
  1998年   8篇
  1997年   3篇
  1996年   8篇
  1995年   5篇
  1994年   3篇
  1993年   3篇
  1992年   6篇
  1991年   7篇
  1990年   3篇
  1989年   1篇
  1987年   4篇
  1985年   1篇
  1984年   1篇
  1982年   2篇
  1980年   2篇
  1976年   1篇
排序方式: 共有421条查询结果,搜索用时 15 毫秒
91.
92.
93.
The horse reference genome from the Thoroughbred mare Twilight has been available for a decade and, together with advances in genomics technologies, has led to unparalleled developments in equine genomics. At the core of this progress is the continuing improvement of the quality, contiguity and completeness of the reference genome, and its functional annotation. Recent achievements include the release of the next version of the reference genome (EquCab3.0) and generation of a reference sequence for the Y chromosome. Horse satellite‐free centromeres provide unique models for mammalian centromere research. Despite extremely low genetic diversity of the Y chromosome, it has been possible to trace patrilines of breeds and pedigrees and show that Y variation was lost in the past approximately 2300 years owing to selective breeding. The high‐quality reference genome has led to the development of three different SNP arrays and WGSs of almost 2000 modern individual horses. The collection of WGS of hundreds of ancient horses is unique and not available for any other domestic species. These tools and resources have led to global population studies dissecting the natural history of the species and genetic makeup and ancestry of modern breeds. Most importantly, the available tools and resources, together with the discovery of functional elements, are dissecting molecular causes of a growing number of Mendelian and complex traits. The improved understanding of molecular underpinnings of various traits continues to benefit the health and performance of the horse whereas also serving as a model for complex disease across species.  相似文献   
94.
Biofuel production from oilseed trees in small‐scale agroforestry systems is considered as a strategy for energy security, rural development and ecosystem services provision in low‐income countries. However, the economic potential of these systems remains unclear, as profitability studies commonly ignore key methodological issues such as quantitative uncertainty analysis, full accounting for opportunity costs, and inclusion of all value chain actors. This study addresses these methodological shortcomings and develops a framework for quantifying the long‐term financial performance of agroforestry‐based biofuel value chains. The framework is applied to a case in South India, to calculate profitability of pongamia (Millettia pinnata) cultivation and processing. The results show that pongamia cultivation has limited financial potential, and is only profitable in small‐scale settings, in the middle to long term and for a subset of farmers. If biodiesel is envisaged as the end product, the value chain requires substantial fiscal and marketing support to be economically viable. For current prices, financial performance is much higher if the seed oil is marketed instead of processed to biodiesel. Increased mechanization, increased yields and optimized agroforestry set‐ups might improve financial outcomes and reduce risks for both farmers and processors. These findings are case‐specific, while the developed framework opens the door to comprehensive investigation of the financial performance of other oilseed tree species and in other regions.  相似文献   
95.
Trends in, and potential causes of, insect pest problems of the Tasmanian blue gum, Eucalyptus globulus globulus, in south‐western Australia are reviewed. Historical evidence suggests that insect pest problems of E. g. globulus in south‐western Australia have greatly increased in the last 10 years, which corresponds to a time of rapid expansion of the blue gum industry in the region. Current major establishment pests include the African black beetle, Heteronychus arator, spring beetles, Liparetrus spp. and Heteronyx spp., and the wingless grasshopper, Phaulacridium vittatum. Current major pests of established trees are the Eucalyptus weevil, Gonipterus scutellatus, and chrysomelid beetles, Chrysophtharta spp. and Cadmus excrementarius. The occurrence of these insects on an introduced eucalypt is not unexpected because insect‐rich native eucalypt forests dominate the landscape where E. g. globulus plantations are grown. Insect damage may also be exacerbated because E. g. globulus is grown as a monoculture.  相似文献   
96.
生态系统工程与现代混农林业生产体系   总被引:11,自引:0,他引:11  
当今世界面临着人口剧增,能源短缺和环境恶化等危机。在第三世界人口飞速增长,耕地需求扩大,毁林种粮,弃林从牧的问题十分严重,引起土壤肥力衰退,水土流失,气候失调,环境恶化,从而阻碍了农牧业的发展。为了保护森林,维护环境,解决农林、牧林业之间的矛盾,有效的途径就是将农、林、牧业有机地结合起来,建立林-农复合生态系统。因而发展农林业在国际上受到充分重视,早于1977年就成立了国际农林业研究中心(ICRAF),推动农林业的研究与生产。我国对农林业的研究与推广近年来也有长足的发展。预计农林业将成为现代农业和林业的发展趋势之一。为此,我们请南京林业大学熊文愈教授和姜志林教授负责组稿、审稿和修改论文,共汇集农林业的论文16篇。这些论文部分反映了当前我国农林业的理论研究工作和实际生产经验。现经本刊编辑,计划作为农林业专题在本刊分四期刊出,以引起广大读者对农林业的重视。在此我们对熊文愈和姜志林二位教授的积极支持表示衷心的感谢。  相似文献   
97.
Old‐world fruit bats are known to provide significant benefits to tropical agroforestry through seed dispersal services. However, the social pathways through which local people perceive and actually utilize these benefits are not well understood. Through interview surveys with plantation owners and farmers in the Western Ghats of Kerala (India), we documented local perceptions and knowledge about the socio‐ecological importance of fruit bat seed dispersal shadows. Respondents’ perceptions were highly positive, with greater benefits reported from seed dispersal, than costs from fruit damage by bats. Interestingly, seed aggregation of commercial fruit crops (cashew, areca) by fruit bats was perceived to reduce agricultural labour costs in plantations. Our study demonstrates that local perceptions can offer valuable insights toward understanding the contribution of bat‐generated ecosystem services for tropical agroforestry systems, and in turn may facilitate effective fruit bat conservation.  相似文献   
98.
99.
西双版纳不同林茶复合生态系统碳储量   总被引:2,自引:0,他引:2  
为了探明上层遮荫树种对茶园碳储量的影响,根据所建立的茶园上层树种及茶树的生物量模型估算了不同林茶复合生态系统的生物量,结合植物、土壤样品碳含量的实测值,对西双版纳州勐海县4种茶园组合模式及纯茶园的碳储量进行了分析。结果表明:樟树+茶、樟-杉+茶2种组合模式的碳储量分别比纯茶园碳储量(223.442t·hm-2)高22.701、3.871t·hm-2,而4种遮荫树种+茶、6种遮荫树种+茶2种组合模式的碳储量则分别比纯茶园低10.828、5.717t·hm-2。各茶园总碳储量以土壤的碳储量所占比例最大,达91.8%~96.0%,随上层树种数量的增加而降低,并在4种遮荫树种+茶组合模式达到最低;而植物体的碳储量仅占总碳储量的4.0%~8.2%,呈现随上层树种数量增加而先增加后降低的趋势。表明西双版纳的人工茶园复合态系统具有很强的碳储存能力。  相似文献   
100.
Aim To improve knowledge of the distribution of species and modern pollen dispersal in the puna vegetation belt (central Andes) for palaeoenvironmental analysis and reconstructions. Location Puna belt, Nevado Coropuna, Western Cordillera, Peru. Methods The vegetation facies and belts of the area were mapped by remote sensing using a March 1998 SPOT4 image. This was complemented by the interpretation of aerial photographs, by field sampling, and by the identification of plants. Data from 1940 to 1994 from the Peruvian meteorological station network were modelled to characterize the relationship between climate and vegetation. Twenty‐four soil‐surface samples were collected in the various vegetation facies identified on the map, and standard palynological techniques were applied to analyse these samples. A principal components analysis was performed on the pollen data set. Results The map shows three bioclimatic belts and seven facies in the puna sensu lato, and identifies the main plants that are characteristic of each bioclimatic area. The pollen results fit the vegetation facies and belts, including the plant species of the distinct facies that are well represented in the pollen assemblages. The mesotropical belt is characterized by the predominance of Asteraceae‐type Ambrosia; the supratropical belt shows significant frequencies of Asteraceae‐type Senecio; the orotropical belt is characterized by high frequencies of Apiaceae and includes Polylepis woodland and peat bogs; and the cryorotropical belt shows significant frequencies of Asteraceae‐type Senecio and Apiaceae. Main conclusions The pollen grains of the plants that grow on the puna sensu lato are generally entomophilous and are therefore not transported far from their plant source. The distinct bioclimatic facies and belts identified by the cartography can thus be well distinguished by their pollen production and deposition. We were therefore able to characterize the relationship between pollen, vegetation and climate that can be used for palaeoenvironmental reconstructions. An altitudinal pollen gradient on the western slopes of the central Andes was revealed by the pollen study, with the succession of Asteraceae‐type Ambrosia (1800–2200 m), Malvaceae (2700–3300 m), Asteraceae‐type Senecio (3500–4100 m) and Apiaceae (above 4600 m).  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