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   16732篇
  免费   19篇
  国内免费   1篇
林业   3656篇
农学   1342篇
基础科学   146篇
  2818篇
综合类   727篇
农作物   2114篇
水产渔业   1788篇
畜牧兽医   1149篇
园艺   1126篇
植物保护   1886篇
  2022年   3篇
  2021年   12篇
  2020年   10篇
  2019年   14篇
  2018年   2752篇
  2017年   2709篇
  2016年   1195篇
  2015年   76篇
  2014年   27篇
  2013年   39篇
  2012年   808篇
  2011年   2136篇
  2010年   2115篇
  2009年   1260篇
  2008年   1320篇
  2007年   1586篇
  2006年   38篇
  2005年   108篇
  2004年   107篇
  2003年   159篇
  2002年   62篇
  2001年   13篇
  2000年   45篇
  1999年   6篇
  1998年   4篇
  1996年   4篇
  1995年   2篇
  1993年   15篇
  1992年   12篇
  1991年   4篇
  1990年   14篇
  1989年   11篇
  1988年   16篇
  1987年   3篇
  1986年   3篇
  1985年   2篇
  1984年   4篇
  1983年   5篇
  1982年   2篇
  1980年   3篇
  1979年   7篇
  1978年   2篇
  1977年   6篇
  1976年   5篇
  1974年   5篇
  1972年   2篇
  1970年   3篇
  1969年   4篇
  1968年   4篇
  1966年   5篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The mechanical behavior of steamed spruce wood changes dramatically with compression along the grain, the change being much more moderate perpendicular to the grain. The stiffness decrement due to increased temperature is greatest in the tangential material direction. The stiffness decrement due to compression is greatest along the grain. Compression to 80% compressive strain at 131°C inverts the order of the material directions regarding stiffness, the stiffness being the least along the grain. Plastic strain due to compression is greater at higher temperatures. The compression-induced decrement of stiffness along the grain is greater at higher temperatures, but the off-axis decrement of stiffness is less at higher temperatures.  相似文献   
992.
This paper describes the use by family forest landowners of educational programs provided by Washington State University Cooperative Extension (WSUCE), and the associated use of technical assistance programs provided by state and federal agencies and the private sector. Approximately 100,000 family forest owners controlled 19% or over 1.2 M ha of Washington’s forestland and accounted for 29% of the timber harvested in the state on a volume basis in 1998. A variety of public and private assistance and education programs are available to encourage and help family forest owners manage their forests. In 1999 a mail survey was conducted to evaluate use and effectiveness of Washington’s family forest assistance and education programs. Over half of the 872 responding family forest landowners had contact with an extension educator, program or educational material, and about three quarters of these respondents gave an overall rating of the usefulness of extension programs and materials as good or excellent. Respondents attending WSUCE forestry educational programs have larger median land ownership size, are older, have owned their forests longer, have a higher rate of absentee ownership, and are better educated than non-users. They are more likely to actively manage their forests for timber production and exhibit a clearer understanding of the multiple-use capabilities of their forests.  相似文献   
993.
A study was conducted with the objective of obtaining a profile of farmers who have participated in the Farm Partnership Scheme in Ireland. This scheme is a joint venture between Coillte (The Irish Forestry Board) and farmers. Under the terms of the scheme, Coillte is responsible for the establishment, management and harvesting of the forest plantation on the farmer’s land. The farmer retains ownership of the land, receives premium payments and shares in the harvesting profits. Fifty farmers who joined the Farm Partnership Scheme between 1993–1997 were interviewed in 1999 and a detailed questionnaire was completed. Survey results indicate that the typical farm partner was male, married and aged 50 years or older. Over half of those surveyed had an off-farm job. The most popular reason for farmers opting to participate in the Farm Partnership Scheme was that they did not have sufficient time to undertake the establishment and management of a forest plantation themselves. The average size of forest established under the scheme was 23.7 ha, with Sitka spruce the most popular species planted. The average annual payment under the scheme was €8077. For 46% of those surveyed, the returns from the Farm Partnership Scheme comprised all of their farm income.  相似文献   
994.
This paper assesses the potential impact of planting of eucalypt trees as a strategy to reduce poverty in a less-favoured area of the highlands of Ethiopia. Results from simulations with a bio-economic model for a less-favoured case study area in the highlands are combined with survey data at community, household and plot level to assess how general the results of the bio-economic model are. Application of the bio-economic model shows clearly that land degradation, population growth, stagnant technology and drought threaten food security. Household welfare and land quality are deteriorating rapidly in the area and interventions are urgently needed to avoid human disaster. Planting of eucalypts on land unsuitable for crop production may substantially increase household incomes if market outlets for trees can be found. Tree planting will not have severe negative effects on food production or land conservation. A policy combining promotion of tree planting and conservation of cropland may achieve win-win benefits in terms of increased household incomes as well as more sustainable land-use. Analysis of survey data from the Amhara Region of Ethiopia reveals that there is a large area of land that is unsuitable for crop production located close to all-weather roads in the less-favoured areas of the region. Few trees have been planted on this type of land up to now. The past policy seems to have discouraged tree planting except on homestead plots that are more suitable for food crops. This paper reports part of the IFPRI/ILRI project ‘Policies for Sustainable Land Management in the East African Highlands’. These two agencies have provided funds and logistical support for the work. The Norwegian Ministry of Foreign Affairs has provided funds for this research in the Amhara Region in Ethiopia. The authors also draw on earlier work funded by Research Council of Norway. Any correspondence should be directed to the first author.  相似文献   
995.
FLORES, the Forest Land Oriented Resource Envisioning System, is a framework to facilitate quantitative modelling of ecological, economic and social issues at the landscape scale. This issue ofSmall-scale Forest Economics, Management and Policy describes the evolution of FLORES from a concept to a series of models calibrated for diverse locations, and documents the lessons learned. The idea to construct and use landscape-scale models of the forest frontier, based on simulating household decisions and land use at a spatial scale close to the field level, arose from a desire to add rigour to land-use policy research at CIFOR, the Center for International Forestry Research (Vanclay 1995). This simulation modelling approach to addressing interdisciplinary issues, where people are strongly interacting with forest resources, became known as FLORES, the Forest Land Oriented Resource Envisioning System (Vanclay 1998). Muetzelfeldtet al. (1998) constructed a simple prototype of a FLORES model to illustrate the concept and demonstrate the ability of a system-dynamics modelling environment to animate such a model (Muetzelfeldt and Taylor 1997, 2001, Muetzelfeldt and Massheder 2003). In 1999, FLORES became a reality, when 50 scientists from diverse disciplines met in Bukittinggi, Indonesia to construct the first FLORES model styled on this prototype (CIFOR 1999, Vanclayet al. 2003). The research reported in this special issue was made possible by generous financial support from the Department for International Development (UK), the European Community, the Asian Development Bank (under RETA 5812) and the Center for International Forestry Research. The views expressed herein are those of the authors alone. We would like to thank the following people who have assisted in the production of this special issue by acting as anonymous referees for these and other papers: Andy Warner, Brendan Moran, Bruno Verbist, Chris Dake, Chris Legg, Doug Sheil, Euan Mason, Francois Bousquet, Frank Vanclay, Geoff Slaughter, James Gambiza, John Herbohn, John Poulsen, Jungho Suh, Mike Spilsbury, Paul Phillips, Paul van Gardingen, Phil Norman, Philip Nyhus, Roger Wheate, Ross Sigley, Steve Harrison and Tom Evans. Thanks also to the editorial staff, especially Steve Harrison, John Herbohn and Jungho Suh, for making this special issue possible.  相似文献   
996.
This paper explores the challenge of defining the scope of a systems model, emphasising three aspects: boundary, granularity and conceptual scope. The significance of these is illustrated by reference to a model of land-use decisions made in villages bordering on the Mafungautsi forest in Zimbabwe. The purpose of this model was to help policy players (Forestry Commission staff, non-governmental organisations, researchers and local people) to understand the impact of policy interventions on local people’s livelihoods. Scoping decisions that were made in building the Mafungautsi model were deliberately liberal, to encompass the interests of all participants in the modelling process. These decisions now present a range of serious challenges: the difficulty of model calibration, the computational expense of running simulations, and the difficulty for new users to understand the model. Facilitators of modelling teams need to consider the serious implications of giving everyone what they want and including all participants’ ideas in a model. In the long run, it may be better to be tough and reject many suggestions at the outset. The former approach is unlikely to lead to a tractable model, while the latter may ultimately offer greater satisfaction for all. We are grateful to the UK’s Department for International Development (DFID) and the European Community for financial support of this project. Thanks also to all of the ACM team who have contributed ideas on the process and content of the modelling work and who make it fun to undertake collaborative research. Thanks also to the participants in the Zimbabwe FLAC workshops for their input into the specification of the Mafungautsi model. Finally thanks to Robert Muetzelfeldt, Jasper Taylor, Jonathan Massheder, Fergus Sinclair and Jerry Vanclay for helping us to ride on the FLORES bandwagon.  相似文献   
997.
Genus Eurydoxa Filipjev in China is reviewed and noted for the first time. Based on the present study, two species are recognized, including rhodopa Diakonoff and advena Filipjev. All available information for the species is reviewed and provided.  相似文献   
998.
1-Hydroxyethyl-2-alkyl-2-imidazoline (HEAI) and its derivative sulfonate amphoteric surfactant were synthesized using tall oil fatty acid as the raw material. Suitable experimental conditions for synthesizing the intermediate and final products were probed. Their chemical structures were also investigated by means of infrared spectroscopy and hydrogen nuclear magnetic resonance spectroscopy. Based on the results of these analyses, it has been found that during the reaction process of HEAI with sodium 3-chloro-2-hydroxypropane sulfonate under alkaline condition the imidazoline ring was hydrolyzed to amides, and the obtained amphoteric surfactant was actually a complex mixture of amides. An etherification reaction in the hydroxyethyl group was also observed when the HEAI/sodium 3-chloro-2-hydroxypropane sulfonate molar ratio was more than 1:1.  相似文献   
999.
 Two kinds of bamboo vinegar from madake bamboo (Phyllostachys bambusoides) and moso bamboo (Phyllostachys pubescens) were prepared to analyze their components by gas chromatography (GC). The original vinegar, distilled vinegar, ether-extracted vinegar, and three fractions including acidic, neutral, and phenolic fractions separated from ether-extracted vinegar were diluted with distilled water 102 to 107 times. These diluted vinegar solutions were used to investigate the effect of bamboo vinegar on the germination and radicle growth of seed plants. High concentrations of all kinds of treated bamboo vinegars (e.g., 102 of original vinegar and 103 of ether-extracted vinegar) showed strong inhibition against germination of the seeds. However, an appropriate dilution of bamboo vinegar showed an obvious promotional effect on germination and radicle growth for the four kinds of tested seeds (lettuce, watercress, honewort, chrysanthemum). Received: December, 12, 2001 / Accepted: July 29, 2002 RID="*" ID="*" Part of this paper was presented at the 51st Annual Meeting of the Japan Wood Research Society, Tokyo, April 2001  相似文献   
1000.
Poor compatibility was found between exploded wood fiber strand (WFS) and cement due to the excessive presence of water-soluble degraded polysaccharides in extractives of exploded WFS obtained from weathered wood waste treated by the water-vapor explosion process (WVEP). This study presents some comparative results from a continuing investigation on the compressive strengths of exploded WFS–cement mixtures. Based on results previously obtained with the hydration test, the relation between hydration behavior and compressive strength of the mixture was explored. In addition, the effect of the curing age on compressive strength development of the mixture with selected additive chemicals was examined. The results supported the results of early studies with hydration tests indicating that adding MgCl2 to the mixtures of exploded WFS mixed with quick-curing cement or ordinary Portland cement and a composite of MgCl2 + CaO added to the mixture of exploded WFS and furnace-slag cement effectively improved the hydration behaviors; it greatly enhanced the compressive strengths of mixtures as well. Compressive strengths were strongly correlated to maximum hydration temperatures (Tmax) of wood–cement mixtures influenced by the cement type, wood wastes (treated or not with WVEP), additive chemicals, and their content levels. The results also indicated that adding selected chemicals had no significant effect on compressive strength among the mixtures of exploded WFS mixed, respectively, with three types of cement at a curing age of 180 days. X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy were used to identify the hydration products and to probe the element distribution of the mixture in the wood–cement interface zone from a fractured surface.Part of this report was presented at the 52nd Annual Meeting of the Japan Wood Research Society, Gifu, April 2002  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