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   29768篇
  免费   705篇
  国内免费   1611篇
林业   5126篇
农学   3242篇
基础科学   1443篇
  5249篇
综合类   4770篇
农作物   2830篇
水产渔业   2286篇
畜牧兽医   2825篇
园艺   1497篇
植物保护   2816篇
  2024年   60篇
  2023年   185篇
  2022年   429篇
  2021年   584篇
  2020年   519篇
  2019年   549篇
  2018年   3040篇
  2017年   3192篇
  2016年   1606篇
  2015年   722篇
  2014年   611篇
  2013年   735篇
  2012年   1664篇
  2011年   2994篇
  2010年   2936篇
  2009年   2049篇
  2008年   2048篇
  2007年   2378篇
  2006年   787篇
  2005年   786篇
  2004年   430篇
  2003年   378篇
  2002年   289篇
  2001年   241篇
  2000年   298篇
  1999年   365篇
  1998年   296篇
  1997年   271篇
  1996年   232篇
  1995年   251篇
  1994年   176篇
  1993年   214篇
  1992年   179篇
  1991年   134篇
  1990年   124篇
  1989年   89篇
  1988年   79篇
  1987年   57篇
  1986年   34篇
  1985年   9篇
  1984年   7篇
  1983年   11篇
  1982年   12篇
  1981年   5篇
  1980年   6篇
  1977年   4篇
  1968年   4篇
  1964年   2篇
  1963年   3篇
  1934年   2篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The effect of uncertainty on the cost-effectiveness of alternative chains of stand establishment in northern Finland was examined. The data were from a reforestation study of Scots pine (Pinus sylvestris L.) consisting of 288 sample plots, which were measured with respect to regeneration success. The study design included four site-preparation methods (patch scarification, ploughing, prescribed burning and disk trenching) combined with three reforestation methods (sowing, planting with containerized seedlings and planting with bare-rooted transplants). Initial reforestation density was 2,500 spots or seedlings per hectare, and the regeneration success was modeled as probability with two thresholds, namely 500 and 1,100 saplings. On formerly spruce-dominated as well as pine-dominated sites the most cost-effective chain was ploughing and planting with containerized seedlings, when threshold was 1,100 saplings per hectare. However, with threshold of 500 saplings the best performer was ploughing and direct sowing on both sites.  相似文献   
992.
The UN’s Millennium Development Goals (MDGs) and other regional and national policy commitments have motivated an upsurge of interest in concepts and practical methods for monitoring forest conditions and trends at very wide geographic scales. Two approaches to sustainability assessment at a global level are reviewed here. One consists of monitoring change in forest conditions over time—the so-called Criteria and Indicators (C&I) approach. Another approach compares nations at a given point in time. An example is the Yale Environmental Performance Index (EPI). Both approaches yield insights. It is widely recognized, though, that severe data weaknesses afflict forest information over much of the world. These weaknesses include weak or absent information on wood consumption in many regions, poor area estimates, and weak or absent information on key ecological conditions in forests. The purpose of this essay is to introduce these efforts at global assessment, and to argue that an entirely new discipline is needed to supply the information needed. The focus of this new discipline would be to design an ecologically based set of definitions for forest and related ecosystems, and then to build and implement the optimum combination of satellite measurements, air photo interpretation, and field plot measurements needed to measure world forest resource conditions and trends. Examples of this new approach are already appearing. This argument is addressed to members of the global forest policy community concerned with assessment, and to scientists, technologists, and managers in the many technical fields already engaged on one or another aspect of measuring and monitoring forest conditions at a national and regional scale.  相似文献   
993.
994.
995.
996.
Tree rings from the Alpine area have been widely used to reconstruct variations in summer temperature. In contrast, estimates of changes in the hydrological cycle are rather scarce. In order to detect altitudinal and species-specific patterns of growth responses to anomalous dry and wet conditions, a large network of 53 tree-ring width site chronologies along the Rhone valley (Valais, Switzerland) covering the 1751–2005 period was compiled and analyzed. A total of 1,605 measurement series from four conifer species—pine, larch, spruce, fir—were detrended to allow inter-annual to multi-decadal scale variability to be preserved. Site chronologies were combined to four altitudinal (colline: <800 m asl, sub-montane: 800–1,000 m asl, montane: 1,000–1,450 m asl, sub-alpine: >1,450 m asl) and species-specific mean time-series. These records were compared with temperature, precipitation, and drought (scPDSI) data. Among the altitudinal records, the colline chronologies showed highest correlation with June precipitation and scPDSI (0.5 and 0.7). Altitudinal effects, via control on climatic conditions, were superimposed upon species-specific characteristics in affecting tree growth and response to moisture variations. In particular, species-specific differences affected the significance level of drought response, with decreasing drought sensitivity towards higher elevations. Growth conditions were found to be optimal at ~1,300 m asl, with precipitation/drought limiting tree growth below and temperature above. Common years of extreme drought and low growth for the colline sites occurred in 1921, 1944, 1976, 1992, and 1998. Our results demonstrate the potential of lower elevation conifers for reconstructing long-term changes in Alpine hydro-climate.  相似文献   
997.
Various approaches have been developed to achieve sustainability in forestry, under changing social needs and, consequently, changing definitions of sustainability. This has led to the confusing situation in which various groups have different understandings of the meaning of ‘sustainability’. Likewise, the concepts utilized to achieve sustainability, often with a poorly defined objective, are sometimes not clear and/or inconsistent as a systematic overview regarding definitions and concepts is lacking. Based on a literature review, this paper discusses related terms such as sustainability, sustained yield management, sustainable forestry, sustainable forest management and sustainable development: their history, concepts and relationships, from a European perspective. Finally, flexibility is proposed as a solution to overcome the identified shortcomings at all scales, while focussing on the enterprise level. The origin of the sustainability concept in forestry was first driven by forest experts, while participatory elements have been considered since the more recent idea of sustainable development. Since then, much effort has been made to achieve intragenerational fairness by creating an improved participatory process. Concurrently, the original idea of sustainable forestry as long-term and future-oriented management, considering future generations’ needs fell behind. An increasing standing timber volume in Europe and the discussion on climate change brought new interest in how to cope with risks in the context of pervasive future uncertainties within the scope of promoting sustainable development. Although the consideration of risk has been concentrated on as a topic in forest science in recent years, studies have mainly focused on the enhancement of forest resistance against disturbances. However, precaution and risk avoidance alone are probably insufficient to achieve an improved sustainable development that focuses on intergenerational fairness, as these more defensive approaches may disregard important management opportunities involved with an uncertain future. A perhaps more promising approach, the idea of future options and the ability to respond to changing social and biophysical circumstances (i.e. flexibility) as criteria for sustainable development have only shown a shadowy existence up to now. To further develop the consistency of sustainability concepts, a shift of sustainability approaches from continuity towards flexibility options is proposed.  相似文献   
998.
中国与“一带一路”国家木质家具贸易现状及趋势   总被引:1,自引:0,他引:1  
鉴于地理距离和区域消费习惯对家具贸易影响程度较大,文中按照地理位置差异将“一带一路”地区分为6大板块,利用世界贸易中心(ITC)数据库2013—2017年中国与“一带一路”国家木质家具贸易数据进行分析发现:中国与“一带一路”国家木质家具贸易总额不断增长,贸易顺差较大,进出口市场分布不均,出口产品种类相对集中,进口产品较分散;为提高我国木质家具在“一带一路”国家的竞争力提出如下建议:制定战略规划以支撑国家“一带一路”倡议,优化产品设计以满足差异化需求,依托大型基础建设项目推进木质家具生产与销售,以高科技赋能家具产业和产品,重视木质家具产品的设计保护,加强政府政策引导。  相似文献   
999.
在酸性土壤中,铝对林木的毒害是限制林木生长、降低林木生产力、进而导致森林退化的主要因素之一。目前,国内外在林木铝毒害及耐铝机制方面已有较多的研究,但关于林木耐铝的生理和分子机制的综述性报道很少。文中综述了近年来林木对铝的富集、铝对林木生理和分子水平上的毒害等方面的研究进展,总结了林木耐铝的生理机制和分子机制,简要介绍了外源添加物对林木耐铝毒的调控机制,提出了今后有关林木铝毒需进一步研究的重点,以期为林木铝毒的深入研究提供参考以及为缓解我国酸性土壤林木铝毒害及森林的健康持续经营提供参考。  相似文献   
1000.
对淬火前的热挤压棒用不同的冷拉拔量进行拉拔,结果表明;冷拉拔量对棒材的组织和性能有不同的影响.当变形量达到10%时,经淬火后的棒材其组织是均匀的细晶组织.  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