全文获取类型
收费全文 | 16403篇 |
免费 | 1011篇 |
国内免费 | 1134篇 |
专业分类
林业 | 3404篇 |
农学 | 1264篇 |
基础科学 | 106篇 |
2189篇 | |
综合类 | 6500篇 |
农作物 | 704篇 |
水产渔业 | 909篇 |
畜牧兽医 | 1383篇 |
园艺 | 1141篇 |
植物保护 | 948篇 |
出版年
2024年 | 103篇 |
2023年 | 310篇 |
2022年 | 542篇 |
2021年 | 557篇 |
2020年 | 624篇 |
2019年 | 699篇 |
2018年 | 415篇 |
2017年 | 648篇 |
2016年 | 805篇 |
2015年 | 722篇 |
2014年 | 967篇 |
2013年 | 1067篇 |
2012年 | 1475篇 |
2011年 | 1517篇 |
2010年 | 1230篇 |
2009年 | 1084篇 |
2008年 | 1041篇 |
2007年 | 1062篇 |
2006年 | 864篇 |
2005年 | 612篇 |
2004年 | 429篇 |
2003年 | 390篇 |
2002年 | 268篇 |
2001年 | 215篇 |
2000年 | 177篇 |
1999年 | 162篇 |
1998年 | 118篇 |
1997年 | 82篇 |
1996年 | 86篇 |
1995年 | 43篇 |
1994年 | 27篇 |
1993年 | 44篇 |
1992年 | 37篇 |
1991年 | 29篇 |
1990年 | 16篇 |
1989年 | 31篇 |
1988年 | 22篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1962年 | 3篇 |
1956年 | 4篇 |
1955年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
982.
983.
以黄土高原坡地刺槐林—草地复合系统景观边界为研究对象,按一定的生态梯度调查分析了边界植物种的分布特点。结果表明,草地、林地斑块内都有自身的植物种类分布;草地斑块内有较高的植物多样性。在草地斑块内,距林缘8~16m区域为边界植物多样性边缘效应的显著区;林地斑块内β多样性变化剧烈,林缘处的群落相异系数(CD)、Cody指数都最大,共有度指数(CP)最小,CD、Cody与CP成反比。 相似文献
984.
985.
以福建省永安市为研究对象,基于3S技术手段,运用景观分析软件Fragstats4.1,计算不同水平上不同景观要素的特征值,对永安市现有城市景观和城市森林景观在类型水平和景观水平上的景观格局进行研究。结果表明:研究区各景观类型分布较为均匀,不存在明显的优势主体;城市森林整体分布不够均衡,研究区还未形成生态功能较为完善的城市森林结构。研究结果可为地区森林城市建设的可持续发展提供决策支持。 相似文献
986.
Rodney L. Robichaud Jeffrey C. Glaubitz Olin E. Rhodes Jr. Keith Woeste 《New Forests》2006,32(2):179-196
We describe the development of a robust and powerful suite of 12 microsatellite marker loci for use in genetic investigations
of black walnut and related species. These 12 loci were chosen from a set of 17 candidate loci used to genotype 222 trees
sampled from a 38-year-old black walnut progeny test. The 222 genotypes represent a sampling from the broad geographic distribution
of the species. Analysis of the samples using the 12 loci revealed an average expected heterozygosity of 0.83, a combined
probability of identity of 3×10−19, and a combined probability of exclusion for paternity analysis of >0.999. The 222 genotyped trees from the progeny test
comprised 39 open-pollinated families, 29 of which (having at least five sampled progeny) were used to estimate the outcrossing
rate for the progeny trial. The same 29 families were used to construct a Neighbor-Joining dendrogram based upon allele sharing
between individuals. The multilocus estimate of the outcrossing rate was 100% (standard error of zero), higher than the 90%
level found in previous studies at the embryo stage, suggesting that both artificial and natural selection against selfs may
have occurred over the 38-year lifespan of the progeny trial. In the Neighbor-Joining dendrogram, the majority of the putative
siblings grouped together in 21 out of the 29 families, showing that the microsatellites were able to discern most of the
family structure in the dataset. Our results indicate that errors were sometimes committed during the establishment of the
progeny test. This set of microsatellite loci clearly provides a powerful tool for future applications in black walnut. 相似文献
987.
988.
猬实植物群落外貌和物种多样性的研究 总被引:1,自引:0,他引:1
通过对国家三级保护植物猬实进行群落外貌和物种多样性研究,结果表明:猬实对生态环境要求不严,群落组成简单。3个分布区灌木层和草本层的物种丰富度、多样性指数和均匀度指数都表现为:山西永济雪花山>山西夏县泗交林场>陕西华阴华山。总结了影响猬实种群拓展的4个因素。 相似文献
989.
A field study was conducted in the Western Ghats of peninsular India to test the following two hypotheses: (1) lower floristic diversity for the cardamom hill reserves (CHR —a traditional agroforestry system of the tropics) compared to undisturbed evergreen forests and (2) a truncated vegetation structure for shade trees in the cardamom areas. The experimental sites involved three CHRs and an evergreen forest site.The CHRs, regardless of their locations, were characterized by lower floristic diversity and density than the evergreen forest site. The undisturbed forest site at Ayyappancoil registered the highest floristic richness and diversity (Simpson's floristic diversity index, D=0.93), followed by the well-managed CHR site, suggesting that managerial interventions may have a strong bearing on the floristic diversity of CHRs. The current suite of species in the CHRs included both heliophilic as well as shade-tolerant components. However, dominant tree species, their density and relative abundance exhibited marked variations among the CHR sites, albeit about one-third of the species were common at all sites.Stand physiognomy was characterized by the dominance of a single layer of trees in the CHRs, while the wet evergreen forest exhibited a multilayered canopy structure. Some of the lower height classes were poorly represented in the cardamom areas, whereas the evergreen forests depicted an inverse J shaped height distribution pattern. Implicit in the truncated stand structure of the CHR is the poor regeneration status, due to systematic removal of the lower size classes. 相似文献
990.
Cynthia G. Locasio B. G. Lockaby Jon P. Caulfield M. Boyd Edwards M. Keith Causey 《New Forests》1990,4(4):261-269
Diversity of understory vegetation was compared among four intensities of site preparation and an adjacent 50-year-old pine-hardwood forest. The study site was a six-year-old loblolly pine (Pines taeda) plantation in the lower Piedmont of Georgia. Species richness and Shannon diversity indices were used to evaluate vine and woody (trees and shrubs) species diversity. Biomass distribution was compared among four plant categories: vines, forbs, grasses, and woody. Moderate intensity treatments (chainsaw and shear and chop) consistently ranked highest overall in diversity, with the mature pine-hardwood forest ranking lowest. Distribution of the four plant categories was not significantly affected by intensity of site preparation.Alabama Agricultural Experiment Station Series No. 9-892325P. 相似文献