全文获取类型
收费全文 | 3845篇 |
免费 | 284篇 |
国内免费 | 1054篇 |
专业分类
林业 | 573篇 |
农学 | 191篇 |
基础科学 | 536篇 |
1679篇 | |
综合类 | 1446篇 |
农作物 | 118篇 |
水产渔业 | 109篇 |
畜牧兽医 | 205篇 |
园艺 | 90篇 |
植物保护 | 236篇 |
出版年
2024年 | 28篇 |
2023年 | 139篇 |
2022年 | 203篇 |
2021年 | 208篇 |
2020年 | 218篇 |
2019年 | 253篇 |
2018年 | 170篇 |
2017年 | 241篇 |
2016年 | 276篇 |
2015年 | 249篇 |
2014年 | 242篇 |
2013年 | 254篇 |
2012年 | 347篇 |
2011年 | 346篇 |
2010年 | 303篇 |
2009年 | 262篇 |
2008年 | 227篇 |
2007年 | 254篇 |
2006年 | 209篇 |
2005年 | 168篇 |
2004年 | 106篇 |
2003年 | 94篇 |
2002年 | 66篇 |
2001年 | 54篇 |
2000年 | 33篇 |
1999年 | 37篇 |
1998年 | 29篇 |
1997年 | 20篇 |
1996年 | 20篇 |
1995年 | 20篇 |
1994年 | 29篇 |
1993年 | 13篇 |
1992年 | 18篇 |
1991年 | 10篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1988年 | 10篇 |
1987年 | 2篇 |
1985年 | 1篇 |
1981年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有5183条查询结果,搜索用时 15 毫秒
11.
本文利用 RS、GIS的高新科技手段 ,对乌拉盖综合开发区土地资源质量等级划分与评价进行了认真研究 ,编制出乌拉盖综合开发区土地资源质量等级分布图 ,对各不同土地资源质量等级进行了评价 ,为该地区确定最佳土地利用方向、提高土地资源的生产效益、经济效益和生态效益提供科学依据 相似文献
12.
Existing spatial patterns of a forest are in part a product of its disturbance history. Using laser altimetry and field measures
of canopy top height to represent pre- and post-hurricane canopy topography, respectively, we measured changes in spatial
patterns of stand structure of a United States southern mixed coniferous-deciduous for est. Autocorrelative and fractal properties
were measured in this opportunistic study to quantify changes in canopy architecture along twelve, 190-250 m transects that
were subjected to moderate to high levels of wind disturbance. Prior to the hurricane, canopy heights were autocorrelated
at scales <40 m with an average fractal dimension of 1.71. After the disturbance, autocorrelation disappeared; the average
fractal dimension rose to 1.94. This shift towards spatial randomness illustrates part of the cyclical nature of ecosystem
development. It shows how a catastrophic collapse of biomass accumulation corresponds to a decrease in ecosystem organization
across a landscape.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
Potato diseases caused by soft rot erwinias: an overview of pathogenesis 总被引:17,自引:0,他引:17
M. C. M. Pérombelon† 《Plant pathology》2002,51(1):1-12
Three soft rot erwinias, Erwinia carotovora ssp. carotovora , E. carotovora ssp. atroseptica and E. chrysanthemi are associated with potatoes causing tuber soft rot and blackleg (stem rot). Latent infection of tubers and stems is widespread. As opportunistic pathogens, the bacteria tend to cause disease when potato resistance is impaired. Pathogenesis or disease development in potato tubers and stems is discussed in terms of the interaction between pathogen, host and environment, microbial competition and recent findings on the molecular basis of pathogenicity. Emphasis is placed on the role of free water and anaerobiosis in weakening tuber resistance and in providing nutrient for erwinias to multiply. Blackleg symptoms are expressed when erwinias predominate in rotting mother tubers, invade the stems and multiply in xylem vessels under favourable weather conditions. Soft rot erwinias tend to out-compete other bacteria in tuber rots because of their ability to produce larger quantities of a wider range of cell wall-degrading enzymes. However, despite extensive studies on their induction, regulation and secretion, little is known about the precise role of the different enzymes in pathogenesis. The putative role of quorum-sensing regulation of these enzymes in disease development is evaluated. The role certain pathogenicity-related characters, including motility, adhesion, siderophores, detoxifying systems and the hrp gene complex, common to most bacteria including symbionts and saprophytes, could play in latent and active infections is also discussed. 相似文献
14.
我国西部干旱区生态环境脆弱 ,自然灾害频繁。塔里木盆地是我国最大的内陆盆地 ,干旱、大风、沙尘暴、洪水、泥石流以及地震、雪灾等自然灾害 ,严重地影响到盆地的生态安全。在RS、GIS及 GPS等技术支持下 ,研究各自然灾害的孕灾机理及过程 ,建立自然灾害的监测评价及预警系统 ,对于塔里木盆地生态环境建设具有重要意义。 相似文献
15.
“三北”防护林甘青宁类型区土地沙漠化遥感调查研究 总被引:1,自引:1,他引:1
本文是“七五”期间“三北”防护林甘青宁类型区土地沙漠化遥感调查成果总结。该区沙漠化土地面积为25878.37km2,其中轻度、中度、重度和极度沙漠化土地分别为3020.30km2,7277.44km2、7085.59km2t和8495.04km2.文中对沙漠化土地的分布及影响因素进行了分析,最后筛出合理的农林牧用地结构。 相似文献
16.
Landscape metrics with ecotones: pattern under uncertainty 总被引:1,自引:0,他引:1
17.
通辽市主要有五大水系,湿地面积广阔。近年来由于人口的增加和社会经济的发 展,对湿地破坏严重,使得湿地面积锐减,湿地功能下降,湿地生态环境恶化。文章运用了遥 感、GIS技术对通辽市湿地进行分析研究,简略提出其保护措施。 相似文献
18.
19.
20.
Latin-American cities can be characterized by dynamic processes of urbanization that encroach upon the natural and semi-natural surrounding landscapes. Our study presents the effects of landscape development, transformed from semi-natural conditions into a mostly disperse suburban settlement. We explore the impact that this transformation has had on this context by three ecosystem services that regulate rainwater runoff, enhance microclimate conditions and help to improve air quality by monitoring vegetation cover. We have designed a spatio-temporal hierarchical analysis which employs remote sensing techniques to capture the structural changes of this landscape over long, medium and short term scales on two spatial levels. This methodological approach was tested in the Metropolitan Area of Santiago (MAS) as case study area. Despite of the increase in impervious surfaces due to urban processes, there has also been an increase in vegetation cover, which has led to an improvement in the provision of the above-mentioned ecosystem services. Hence, if diverse urbanization processes continue and they are coupled with an increase in vegetation cover, the provision of ecosystem services could also expand. This phenomenon can be observed in some areas, where public and private green spaces are created and maintained. Our data analyses give evidence that certain types of suburban areas which increase the share of vegetation cover can provide daily ecological benefits for urban neighborhoods, and beyond, for adjacent areas. Moreover, suburban development can successfully provide ecological benefits to citizens. Such processes can only be ecologically sustainable if the composition of vegetation is well-adapted to the regional climatic conditions. 相似文献