全文获取类型
收费全文 | 23353篇 |
免费 | 1375篇 |
国内免费 | 872篇 |
专业分类
林业 | 11716篇 |
农学 | 575篇 |
基础科学 | 460篇 |
2742篇 | |
综合类 | 7956篇 |
农作物 | 311篇 |
水产渔业 | 208篇 |
畜牧兽医 | 654篇 |
园艺 | 331篇 |
植物保护 | 647篇 |
出版年
2024年 | 58篇 |
2023年 | 252篇 |
2022年 | 382篇 |
2021年 | 533篇 |
2020年 | 533篇 |
2019年 | 799篇 |
2018年 | 366篇 |
2017年 | 599篇 |
2016年 | 792篇 |
2015年 | 745篇 |
2014年 | 1322篇 |
2013年 | 1243篇 |
2012年 | 1849篇 |
2011年 | 1781篇 |
2010年 | 1413篇 |
2009年 | 1476篇 |
2008年 | 1399篇 |
2007年 | 1489篇 |
2006年 | 1335篇 |
2005年 | 1174篇 |
2004年 | 933篇 |
2003年 | 740篇 |
2002年 | 629篇 |
2001年 | 640篇 |
2000年 | 483篇 |
1999年 | 365篇 |
1998年 | 350篇 |
1997年 | 273篇 |
1996年 | 269篇 |
1995年 | 263篇 |
1994年 | 235篇 |
1993年 | 171篇 |
1992年 | 182篇 |
1991年 | 149篇 |
1990年 | 117篇 |
1989年 | 132篇 |
1988年 | 87篇 |
1987年 | 30篇 |
1986年 | 9篇 |
1985年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Research needs for restoring seasonal tropical forests in Thailand: accelerated natural regeneration
Accelerated natural regeneration (ANR) is a relatively cheap method of reforestation, which encourages natural establishment of indigenous trees and shrubs. It requires a low input of labour, but a high input of ecological information. In this paper, the knowledge required to predict and manipulate the natural regeneration of seasonal tropical forest is reviewed and areas in need of further research are identified. Regeneration will be influenced by five groups of potentially limiting factors; site resources (soil and microclimate); competition with weeds; site disturbance; occurrence of established woody plants or their propagules; seed dispersal by wild animals and birds. This paper considers each of these, their interactions with seasons, and each other. Collation of existing information on these topics, combined with the suggested further research, should facilitate the creation of tools that will enable practitioners to judge the regeneration potential of sites and to select the most appropriate ANR techniques. 相似文献
992.
使用25%森泰水剂和5%森泰颗粒剂防除香椿幼林地的杂草研究试验结果表明:25%森泰水剂和5%森泰颗粒剂均能够有效防除香椿幼林地的五节芒、苦竹、刚竹、芒萁骨等多年生恶性杂草,防除效果均可达93%以上。使用25%森泰水剂除草比人工除草可减少用工56 1%~92 2%,降低除草成本36 1%~61%。同时,可根据幼林地杂草的种类、高度、盖度和水源等因素选择合适的森泰除草剂的剂型和剂量组合,以达到理想的防除杂草效果。 相似文献
993.
994.
995.
996.
森林有害生物生态控制 总被引:13,自引:1,他引:13
从森林生物灾害与森林生态系统功能的关系、森林病虫害防治历史、社会对森林病虫害防治策略以及森林经营目标的需求出发,系统化森林有害生物控制策略———生态控制;定义森林有害生物生态控制;分析森林有害生物生态控制的特点、理论基础以及技术基石;认为自然控制论和自组织理论是森林有害生物生态控制的理论基础,森林有害生物生态控制的技术基石是森林生态系统管理;对森林有害生物生态控制遵循的原则进行讨论,包括生态系统原则、协调性原则、生态平衡原则、生态位原则、环境保护原则、协同进化原则、林副产品安全原则、生态系统稳定性原则、生态系统的高功能原则(生产力、经济效益、生态效益、社会效益)、森林生态系统生物灾害的可持续控制原则;对森林有害生物生态控制及其适用条件进行评价。森林有害生物生态控制与森林有害生物综合管理相比具有多项优点,是未来森林有害生物控制的主要方向,具有广阔的前景。 相似文献
997.
BudiadiEmail author Yoichi?Kanazawa Hiroaki?T.?Ishii M.?Sambas?Sabarnurdin Priyono?Suryanto 《Agroforestry Systems》2005,64(2):143-155
Kayu putih oil, extracted from the kayu putih tree (Melaleuca leucadendron), is an important non-timber forest product (NTFP)
in Indonesia. We compared productivity of kayu putih tree plantations among three different sites in East, Central, and West
Java, Indonesia. Leaf-twig, and branch production per tree differed significantly among the three sites. Tree biomass production
was highest at the site in West Java practicing tumpangsari (a form of shifting cultivation) of kayu putih with rice (Oryza
sativa) on alluvial soils. Tree biomass production was lowest at the site in East Java, practicing tumpangsari of kayu putih
with maize (Zea mays) and cassava (Mani- hot esculenta). At this site, tree biomass production increased with increasing stand
age and low production was compensated by greater biomass yield of companion crops. Intermediate tree biomass production was
observed at the site in Central Java where intercropping is only practiced for the first two years after plantation establishment.
Production of tree biomass and kayu putih oil did not increase with increasing stand density, indicating that at a given site,
high tree stocking did not lead to high production. Stand-level productivity of the plantations increased with increasing
stand age and maximum productivity was attained at 15 to 35 years, after which productivity declined. This suggested unsustainable
stand productivity as plantations increase in age. 相似文献
998.
999.
Deanna H. Olson Paul D. Anderson Christopher A. Frissell Hartwell H. Welsh Jr. David F. Bradford 《Forest Ecology and Management》2007
Stream–riparian areas represent a nexus of biodiversity, with disproportionate numbers of species tied to and interacting within this key habitat. New research in Pacific Northwest headwater forests, especially the characterization of microclimates and amphibian distributions, is expanding our perspective of riparian zones, and suggests the need for alternative designs to manage stream–riparian zones and their adjacent uplands. High biodiversity in riparian areas can be attributed to cool moist conditions, high productivity and complex habitat. All 47 northwestern amphibian species have stream–riparian associations, with a third being obligate forms to general stream–riparian areas, and a quarter with life histories reliant on headwater landscapes in particular. Recent recognition that stream-breeding amphibians can disperse hundreds of meters into uplands implies that connectivity among neighboring drainages may be important to their population structures and dynamics. Microclimate studies substantiate a “stream effect” of cool moist conditions permeating upslope into warmer, drier forests. We review forest management approaches relative to headwater riparian areas in the U.S. Pacific Northwest, and we propose scenarios designed to retain all habitats used by amphibians with complex life histories. These include a mix of riparian and upslope management approaches to address the breeding, foraging, overwintering, and dispersal functions of these animals. We speculate that the stream microclimate effect can partly counterbalance edge effects imposed by upslope forest disturbances, hence appropriately sized and managed riparian buffers can protect suitable microclimates at streams and within riparian forests. We propose one approach that focuses habitat conservation in headwater areas – where present management allows extensive logging – on sensitive target species, such as tailed frogs and torrent salamanders that often occur patchily. Assuming both high patchiness and some concordance among the distribution of sensitive species, protecting areas with higher abundances of these animals could justify less protection of currently unoccupied or low-density habitats, where more intensive forest management for timber production could occur. Also, we outline an approach that protects juxtaposed headwater patches, retaining connectivity among sub-drainages using a 6th-field watershed spatial scale for assuring well-distributed protected areas across forested landscapes. However, research is needed to test this approach and to determine whether it is sufficient to buffer downstream water quality and habitat from impacts of headwater management. Offering too-sparse protection everywhere is likely insufficient to conserve headwater habitats and biodiversity, while our alternative targeted protection of selected headwaters does not bind the entire forest landscape into a biodiversity reserve. 相似文献
1000.
通过对100Dg圆公式含义的分析,对影响林分断面积精度的2个主要因子:样地株数密度与林分平均胸径估测值进行了误差分析与探讨,提出了样地株数密度估计林分株数密度采用典型选样,林分平均胸径估测值采用目测法等相应解决办法。其中,当目测值有偏差时采用修正系数可使100Dg圆的可靠性进行了理论上的探讨。 相似文献