全文获取类型
收费全文 | 32056篇 |
免费 | 1895篇 |
国内免费 | 1632篇 |
专业分类
林业 | 5528篇 |
农学 | 1721篇 |
基础科学 | 1714篇 |
3969篇 | |
综合类 | 13874篇 |
农作物 | 1479篇 |
水产渔业 | 1423篇 |
畜牧兽医 | 2653篇 |
园艺 | 796篇 |
植物保护 | 2426篇 |
出版年
2024年 | 88篇 |
2023年 | 538篇 |
2022年 | 712篇 |
2021年 | 947篇 |
2020年 | 980篇 |
2019年 | 1114篇 |
2018年 | 723篇 |
2017年 | 998篇 |
2016年 | 1305篇 |
2015年 | 1108篇 |
2014年 | 1830篇 |
2013年 | 1900篇 |
2012年 | 2624篇 |
2011年 | 2620篇 |
2010年 | 2045篇 |
2009年 | 2152篇 |
2008年 | 1855篇 |
2007年 | 1975篇 |
2006年 | 1695篇 |
2005年 | 1477篇 |
2004年 | 1163篇 |
2003年 | 968篇 |
2002年 | 790篇 |
2001年 | 689篇 |
2000年 | 567篇 |
1999年 | 462篇 |
1998年 | 348篇 |
1997年 | 305篇 |
1996年 | 285篇 |
1995年 | 247篇 |
1994年 | 211篇 |
1993年 | 170篇 |
1992年 | 163篇 |
1991年 | 148篇 |
1990年 | 118篇 |
1989年 | 102篇 |
1988年 | 81篇 |
1987年 | 39篇 |
1986年 | 18篇 |
1985年 | 6篇 |
1984年 | 2篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1962年 | 1篇 |
1956年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 421 毫秒
961.
Logging pattern and landscape changes over the last century at the boreal and deciduous forest transition in Eastern Canada 总被引:4,自引:0,他引:4
Forestry practices associated with the industrial era (since ~1900) have altered the natural disturbance regimes and greatly
impacted the world’s forests. We quantified twentieth century logging patterns and regional scale consequences in three sub-boreal
forest landscapes of Eastern Canada (117,000, 49,400 and 92,300 ha), comparing forestry maps depicting age and forest cover
types for early industrial (1930) and present-day (2000) conditions. Results were similar for the three landscapes, indicating
large-scale forest change during the twentieth century. In 1930, previous logging activities had been concentrated in the
lowlands and along the main hydrographical network, as compared to a more even distribution over the landscapes in 2000, reflecting
a decreasing influence of the environmental constraints on forest harvesting. In 1930, old-aged forests (>100 years) accounted
for more than 75% of the unlogged areas of the three landscapes, as compared to less than 15% for the present-day conditions.
Logging practices have thus inverted the stand age distribution of the landscapes that are currently dominated by young and
regenerating stands. The 1930 forest cover types showed a clear relationship with elevation, with conifers located in the
lowlands and mixed and deciduous stands restricted to the upper slopes. Between 1930 and 2000, 58–64% of the conifer areas
transformed to mixed and deciduous forests, such that no clear altitudinal relationships remained in 2000. We conclude that
twentieth century logging practices have strongly altered the preindustrial vegetation patterns in our study area, to the
point that ecosystem-based management strategies should be developed to restore conifer dominance, altitudinal gradients,
as well as the irregular structure inspired from old forest stands. 相似文献
962.
Heike Kappes Kurt Jordaens Frederik Hendrickx Jean-Pierre Maelfait Luc Lens Thierry Backeljau 《Landscape Ecology》2009,24(5):685-697
Habitat fragmentation is a major cause for species loss, but its effect on invertebrates with low active dispersal power,
like terrestrial gastropods, has rarely been studied. Such species can not cross a hostile habitat matrix, for which the predictions
of island theory, such as positive relations between species richness and patch size, should apply. In order to test this
prediction, we studied gastropod species diversity by assessing gastropod assemblage characteristics from 35 sites in 19 fragments
of deciduous old-growth forests in the Lower Rhine Embayment, Germany. Assemblages differed between larger (≥700 ha) and smaller
forests (<400 ha), those of large forests held a higher percentage of forest species. Although α-diversity was similar between
the two forest size classes, small forests often comprised matrix species, resulting in a higher β-diversity. Edge effects
on the species richness of matrix species were noticeable up to 250 m into the forest. Hierarchical partitioning revealed
that distance to disturbances (external edge, internal edges like roads) explained most assemblage variables, whereas forest
size and woodland cover within a 1 km radius from the sites explained only a few assemblage variables. Densities of two forest-associated
species, Discus rotundatus and Arion fuscus, decreased with forest size. Yet, forest size was positively correlated with richness of typical forest species and densities
of Limax cinereoniger. The latter species seems to need forests of >1,000 ha, i.e., well above the size of most fragments. In conclusion, the prediction
is valid only for forest species. The response to fragmentation is species specific and seems to depend on habitat specialization
and macroclimatic conditions.
Jean-Pierre Maelfait: Deceased. 相似文献
963.
周永军 《农业图书情报学刊》2004,16(8):31-33
从探讨电子公文的概念形成入手 ,分析了目前电子公文实际管理中存在的一些问题 ,并有针对性的提出了解决问题的对策 相似文献
964.
加强企业内部控制,是企业的最基础性工作,也是企业生存和发展的保证。然而,目前我国许多企业的内部控制还相当脆弱。内部控制已经成为管理企业的有效工具,其健全与否是企业生产成败的关键。依法完善切实可行的内部控制,建立健全的内部控制监督机构,改善内部控制环境,设立良好的控制程序,建立有效的会计控制系统是解决企业内部控制失效的有效措施。 相似文献
965.
渔业公共资源管理既是一个国际性难题,也是我国渔业现代化和市场经济进程中,必须面对和妥善解决的渔业公共资源问题.本文重点分析了当前我国渔业公共资源存在的冲突矛盾,揭示其内在共性特征,总结了福建省渔业公共资源管理的经验和管理模式,结合渔业公共资源管理现状,提出了进一步完善我国渔业公共资源管理的制度设计思路和体制机制设计的原则,研究了适应我国社会主义市场经济条件下的渔业公共资源管理思路. 相似文献
966.
967.
968.
969.
970.
研究木霉菌T23、Ta22菌株对甜瓜枯萎病菌的拮抗作用和T23、Ta22二者复合处理对甜瓜枯萎病的室内及田间防治效果,结果表明:T23与Ta22对枯萎病菌拮抗系数都很高,二者复合处理对甜瓜枯萎病的田间相对防效可达79.00%。 相似文献