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   436篇
  免费   32篇
  国内免费   44篇
林业   55篇
农学   15篇
基础科学   17篇
  75篇
综合类   193篇
农作物   12篇
水产渔业   32篇
畜牧兽医   37篇
园艺   12篇
植物保护   64篇
  2023年   10篇
  2022年   23篇
  2021年   14篇
  2020年   19篇
  2019年   19篇
  2018年   12篇
  2017年   22篇
  2016年   23篇
  2015年   12篇
  2014年   28篇
  2013年   19篇
  2012年   32篇
  2011年   41篇
  2010年   41篇
  2009年   32篇
  2008年   33篇
  2007年   33篇
  2006年   20篇
  2005年   17篇
  2004年   16篇
  2003年   9篇
  2002年   7篇
  2001年   11篇
  2000年   5篇
  1999年   2篇
  1997年   3篇
  1995年   1篇
  1994年   1篇
  1993年   2篇
  1992年   3篇
  1989年   1篇
  1988年   1篇
排序方式: 共有512条查询结果,搜索用时 31 毫秒
11.
There is growing interest in research to develop potential agroforestry models for temperate climates. In Washington State, recent studies and anecdotal information suggest that agroforestry is already employed by land managers, and if so, this experience should inform future research efforts. Because this population is not well defined, a mail survey was designed to: 1) Assess Washington land manager awareness of agroforestry, 2) assess perceptions of agroforestry as a land management tool, 3) assess the perceived potential opportunities or obstacles for land managers to practice agroforestry, and 4) identify landowner groups believed to be practicing agroforestry in Washington State.Three groups of land managers were surveyed: employees of the Soil Conservation Service (SCS), Washington State University Cooperative Extension Service (WSUCE) and OTHER, consisting of university faculty, private land managers, State and Federal land managers and owners of small natural resource businesses.Agroforestry was not a new concept for most (94%) respondents, further 55% of those familiar with agroforestry were practicing agroforestry or providing advice to landowners who were practicing agroforestry. Use in (government mandated) soil conservation plans on farmland (100% of all respondents) was the most frequently cited potential application for agroforestry in the state followed by range and pasture land and managing non-commercial forest land (both 84%), use on commercial forest plantation (83%) and fruit and nut orchards (61%).Diversifies land use (25%), enhanced productivity (18%), aesthetics (13%) and income diversity (13%) were the four most frequently cited potential advantages to practicing agroforestry. Lack of information (28%), lack of technical assistance (18%), establishment costs (14%) and not an established practice (14%) were the most frequently identified potential obstacles to practicing agroforestry. Respondents suggested there is great potential for application of agroforestry throughout the state, and non-industrial private forest land owners were selected for future study of this potential.  相似文献   
12.
Nontimber forest product harvesting in the Pacific Northwest is neither a new activity nor a disappearing relic of the pre-industrial era. Though the emphasis may have shifted from subsistence to commercial and recreational pursuits, harvesting and harvesters of wild species are still widespread throughout the region. Hundreds of businesses and thousands of harvesters earn part or all of their income from the harvests. Every year thousands of pounds and hundreds of nontimber forest products valued in the hundreds of millions of dollars are harvested from regional public and private forests. This harvesting of a large diversity of species represents a considerable subset of the overall terrestrial biodiversity in Pacific Northwest forests. Despite widespread extraction, little investment in research, planning, or basic inventorying and monitoring has been done on nontimber forest products. Without better information, land managers will be increasingly unable to make informed decisions on how to manage nontimber forest product biodiversity sustainably as demand for products increases.  相似文献   
13.
The potential for traditional and local ecological knowledge to contribute to biodiversity conservation has been widely recognized, but the actual application of this knowledge to biodiversity conservation is not easy. This paper synthesizes literature about traditional and local ecological knowledge and forest management in the Pacific Northwest to evaluate what is needed to accomplish this goal. We address three topics: (1) views and values people have relating to biodiversity; (2) the resource use and management practices of local forest users, and their effects on biodiversity; (3) models for integrating traditional and local ecological knowledge into biodiversity conservation on public and private lands. We focus on the ecological knowledge of forest users belonging to three groups who inhabit the region: American Indians, family forest owners, and commercial nontimber forest product harvesters.  相似文献   
14.
本文通过对西北地区生态环境问题形成的历史原因及现状分析,得出该区域主要的生态环境问题包括森林和草原生态破坏、水资源和水生态破坏、水土流失和土地荒漠化等方面。同时,依据该区域生态环境脆弱特性及植被在生态环境建设中的恢复机理,提出较为具体的植被恢复措施,试图为西北地区生态环境的恢复重建提供参考。  相似文献   
15.
对湖南石门壶瓶山景关门、桑植龙潭坪茅垭村样地的长果安息香(Changiostyrax raxdolichocarpa)种群数量和分布格局进行调查和分析。结果表明:长果安息香种群的年龄结构呈衰退型,个体数量主要集中在小树(IV龄段),幼龄级株数和老龄级株数匮乏;长果安息香种群在进入生理死亡年龄前有2个死亡高峰,第1个峰值产生于幼树(II、III龄段),主要原因是种内和种间竞争(主要是光照),第2个峰值产生于中树长成大树阶段(V、VI龄段),除种内、种间对环境的竞争外,生长地土层浅薄、大树倒伏及人为砍伐亦加剧了该峰值的产生;种群整体呈聚集分布类型,小树向中树、大树变化过程表现出聚集分布向随机分布转变,导致长果安息香濒危的主要原因是种子萌发率极低、种群竞争能力较差以及人为干扰等。  相似文献   
16.
养猪产业是畜牧经济中的一项具有发展优势和潜力的产业。由于我国发达地区劳动力成本的不断增加,使得养殖业出现了向中西部转移的趋势,这就为西北地区发展养猪业提供了良好的机遇。分析了西北地区养猪业的发展现状,并对西北地区猪产业化发展规划及经营模式进行了初步探讨。  相似文献   
17.
[目的]探讨土地整治规划的重要内容——土地整治分区,为确定土地整治重点区域和土地整治规划提供参考依据。[方法]以西北部生态脆弱区典型区域甘肃省兰州市为研究区域,基于生态保护视角,在充分考虑区域自然地理特征、土地利用特点及区域主要生态问题的基础上,从地形地貌、降雨、土壤类型、植被类型、水土流失、土地利用等5方面选取指标因子初步构建分区指标体系,并从水资源供给、水系分布及行政区划等方面进一步完善。同时运用ArcGIS空间叠加分析功能,使用综合分析法进行土地综合整治分区。[结果]形成了"方位+地貌+土地利用类型"命名的土地整治两级分区体系,二级分区较为详细。兰州市划分为7个一级区和15个二级区。[结论]中国西北部生态脆弱区域在土地综合整理过程中应该通过选择合理的指标体系分区进行整治。各分区应该针对其土地资源、水资源、生态环境状况等具体特征,基于生态保护视角采取有效措施进行土地综合整治。  相似文献   
18.
Conventional tillage winter wheat (Triticum aestivum) (WW)–summer fallow reduces soil productivity and increases soil erosion. Conservation tillage management, together with intensive cropping may have the potential to reverse these sustainability concerns. The objective of this study was to determine the effects of conventional tillage (CT) and no-tillage (NT) systems on grain yield of long-term annual cropping of monoculture WW, spring wheat (SW), and spring barley (Hordeum vulgare) (SB) grown with or without fertilizer, in the Pacific Northwest region of the USA. In unfertilized crops, grain yield of WW, SW, and SB was 15%, 25%, and 50% higher, respectively, in CT than in NT plots, an indication of the involvement of yield limiting factors under the NT cropping system. When fertilized, there were no significant differences in grain yield of WW. Yields of SW and SB, however, remained 21% and 15% higher, respectively, in CT than in NT, an indication that factors other than fertility were involved. These results suggest that in order for NT management to be widely adopted by area growers, the yield-limiting factors need to be addressed.  相似文献   
19.
分析法律院校图书馆为地方法制建设服务的有利时机和法律院校图书馆服务优势,提出法律院校图书馆拓展其信息服务功能的几种方式。  相似文献   
20.
基于西北地区农业灌溉特点,以新疆乌苏市为例,分析当前的节水灌溉措施和效益,指出了其中的不足之处,并通过对该地农业节水途径的分析,定量计算了未来10年该地和新疆以及西北地区的节水潜力,同时提出几点有针对性的建议和改进措施,以求为广大西北地区农业可持续提供发展思路.  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