全文获取类型
收费全文 | 8390篇 |
免费 | 486篇 |
国内免费 | 781篇 |
专业分类
林业 | 1177篇 |
农学 | 503篇 |
基础科学 | 332篇 |
1933篇 | |
综合类 | 3695篇 |
农作物 | 366篇 |
水产渔业 | 213篇 |
畜牧兽医 | 608篇 |
园艺 | 343篇 |
植物保护 | 487篇 |
出版年
2024年 | 54篇 |
2023年 | 169篇 |
2022年 | 280篇 |
2021年 | 275篇 |
2020年 | 249篇 |
2019年 | 280篇 |
2018年 | 230篇 |
2017年 | 342篇 |
2016年 | 430篇 |
2015年 | 385篇 |
2014年 | 513篇 |
2013年 | 437篇 |
2012年 | 656篇 |
2011年 | 666篇 |
2010年 | 566篇 |
2009年 | 575篇 |
2008年 | 516篇 |
2007年 | 554篇 |
2006年 | 404篇 |
2005年 | 353篇 |
2004年 | 263篇 |
2003年 | 208篇 |
2002年 | 167篇 |
2001年 | 132篇 |
2000年 | 123篇 |
1999年 | 128篇 |
1998年 | 110篇 |
1997年 | 65篇 |
1996年 | 84篇 |
1995年 | 67篇 |
1994年 | 50篇 |
1993年 | 55篇 |
1992年 | 60篇 |
1991年 | 64篇 |
1990年 | 48篇 |
1989年 | 35篇 |
1988年 | 26篇 |
1987年 | 18篇 |
1986年 | 6篇 |
1984年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1978年 | 4篇 |
1976年 | 1篇 |
1962年 | 3篇 |
1956年 | 1篇 |
排序方式: 共有9657条查询结果,搜索用时 15 毫秒
151.
An analysis of structure of tree seedling populations on a Lahar 总被引:2,自引:0,他引:2
The structure of a tree seedling population is dependent on the interaction of several processes including seed dispersal, germination, survival, and competition on a physical landscape. Structural components (composition, size distributions, spatial distributions, age distributions, density, and history) of a tree seedling population on the Muddy River Lahar on the east side of Mount St. Helens were examined over a range of extents (1/10 m to 1000 m). Many of these component have rarely been examined at the larger extents listed here. Composition reflected distances to seed source and seed morphology. Seedling sizes are inversely proportional to depth to a buried soil if one existed. Spatial patterns indicated that seedling are clustered for tree seedlings less than 200 m apart, random for tree seedling from 200 m to 400 m and uniform for seedling greater than 400 m apart. This was confirmed by two measures of multidimensional spatial point pattern. Age distributions did not reflect the size distributions; old seedlings could be almost any size, young seedlings were constrained to be small in size. Densities appear to be typical for forests in the area. History of disturbance events (the lahar establishment, and successive ash, pumice, and erosion) has strongly influenced this tree seedling community. 相似文献
152.
A model of arctic tundra vegetation derived from topographic gradients 总被引:10,自引:0,他引:10
We present a topographically-derived vegetation model (TVM) that predicts the landscape patterns of arctic vegetation types in the foothills of the Brooks Range in northern Alaska. In the Arctic there is a strong relationship between water and plant structure and function and TVM is based on the relationships between vegetation types and slope (tan ) and discharge (), two independent variables that can be easily derived from digital terrain data. Both slope and discharge relate to hydrological similarity within a landscape: slope determines the gravitational hydrological gradient and hence influences flow velocity, whereas discharge patterns are computed based on upslope area and quantify lateral flow amount. TVM was developed and parameterized based on vegetation data from a small 2.2 km2 watershed and its application was tested in a larger 22km2 region. For the watershed, TVM performed quite well, having a high spatial resolution and a goodness-of-fit ranging from 71–78%, depending on the functions used. For the larger region, the strength of the vegetation types predictions drops somewhat to between 56–59%. We discuss the various sources of error and limitations of the model for purposes of extrapolation. 相似文献
153.
城市生态环境问题已上升到生态安全的层面.以人类生存安全和理想人居环境为目标,基于空间数据和统计资料,利用景观安全格局原理和地理信息系统空间分析方法,构建了包含基本保障格局-缓冲格局-最优格局3个级别的水安全格局、地质灾害安全格局、大气安全格局、生物保护安全格局和农田安全格局,叠加得到高明区综合生态安全格局,在此基础上提出了高明区建设用地扩展最优方案和生态功能分区.结果显示,并非生态安全格局水平越高,城乡发展模式越优,缓冲生态安全格局下的高明区建设用地扩展模式,最可能实现生态安全保护与城镇扩展的和谐发展.综合生态安全格局明确了区域的生态环境敏感区和生态服务功能重要区,为区域生态功能分区提供了新的思路. 相似文献
154.
Weed and nutrient management in cropping systems of semi-arid areas is a major constraint to cereal yield. Where the use of herbicides is banned or discouraged, the competitive ability of a crop is crucial to reduce weed growth and diffusion. Genotypic differences in the competitive abilities of crops are an important trait to reduce weeds, especially for plant height. However, there is contrasting information about the interactions of other management practices and genotypic traits on wheat yield and competitive ability against weeds and weed growth. The present study investigated yield and quality of durum wheat (Triticum durum Desf.) and weed growth and composition for two wheat cultivars with contrasting competitive abilities against weeds. Wheat was grown under three spatial arrangements (5-cm, 15-cm, 25-cm inter-row distance) and three sowing densities, and broadleaf weeds were either removed or not. The sowing rate did not affect the yield of these wheat cultivars or the weed growth. Reduced inter-row distance dramatically reduced weed biomass for both wheat cultivars, and increased wheat yield and nitrogen uptake in the low-competitive, high-yielding, semi-dwarf cv. ‘PR22D89’, when both weed free and with weeds. These results have direct implications for weed and nutrient management in low-input and organic cropping systems. 相似文献
156.
陈萍 《四川畜牧兽医学院学报》2008,(5):154-157
文学总是以话语蕴藉的形态而存在,无论是作者还是读者,都只能通过这种话语蕴藉而参与到文学活动中。符号学理论认为,文学语言是具有审美功能的语言,美学功能不以传达了什么为中心,而以如何传达为中心。笔者拟从传达美学的角度分析文学符号的蕴藉。 相似文献
157.
对吉林市某苗圃柳树中柳九星叶甲若虫的空间分布型进行了研究。结果表明柳九星叶甲若虫种群分布型为聚集分布,分布的基本成分是个体群,而且此虫的聚集程度不高。 相似文献
158.
159.
图书馆信息服务的优化 总被引:1,自引:0,他引:1
徐进 《农业图书情报学刊》2008,20(7)
图书馆信息服务的优化是维护并促进图书馆事业可持续发展的关键,文献的多元性、文化的多样性以及生存环境的丰富性为其提供了拓展的空间.围绕这一主题进行阐述,分析了全新的信息环境下,拓展和延伸图书馆信息服务的必要性和优化的方式,希望借此对图书馆信息服务的发展有所助益. 相似文献
160.