全文获取类型
收费全文 | 23911篇 |
免费 | 377篇 |
国内免费 | 946篇 |
专业分类
林业 | 4588篇 |
农学 | 2494篇 |
基础科学 | 725篇 |
4187篇 | |
综合类 | 2725篇 |
农作物 | 2562篇 |
水产渔业 | 2078篇 |
畜牧兽医 | 2013篇 |
园艺 | 1395篇 |
植物保护 | 2467篇 |
出版年
2024年 | 10篇 |
2023年 | 75篇 |
2022年 | 204篇 |
2021年 | 309篇 |
2020年 | 251篇 |
2019年 | 258篇 |
2018年 | 2907篇 |
2017年 | 2914篇 |
2016年 | 1408篇 |
2015年 | 386篇 |
2014年 | 351篇 |
2013年 | 393篇 |
2012年 | 1222篇 |
2011年 | 2605篇 |
2010年 | 2612篇 |
2009年 | 1736篇 |
2008年 | 1725篇 |
2007年 | 1946篇 |
2006年 | 467篇 |
2005年 | 518篇 |
2004年 | 295篇 |
2003年 | 314篇 |
2002年 | 189篇 |
2001年 | 142篇 |
2000年 | 216篇 |
1999年 | 264篇 |
1998年 | 207篇 |
1997年 | 194篇 |
1996年 | 166篇 |
1995年 | 131篇 |
1994年 | 123篇 |
1993年 | 146篇 |
1992年 | 123篇 |
1991年 | 108篇 |
1990年 | 81篇 |
1989年 | 69篇 |
1988年 | 57篇 |
1987年 | 26篇 |
1986年 | 27篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1979年 | 7篇 |
1977年 | 4篇 |
1972年 | 1篇 |
1969年 | 1篇 |
1968年 | 4篇 |
1967年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
991.
A NMR study of water distribution in hardwoods at several equilibrium moisture contents 总被引:1,自引:0,他引:1
The water state of one tropical (Robinia coccinea) and two temperate (Acer saccharum and Fagus grandifolia) hardwoods was determined at different equilibrium moisture contents (EMC) during desorption at 25°C. NMR technique was used
to separate different components of water in wood. The species studied presented different structures, which were apparent
on the spin–spin relaxation T2 values. Three different water components were separated: slow T2 (liquid water in vessel elements), medium T2 (liquid water in fiber and parenchyma elements) and fast T2 (bound or cell wall water). The NMR results showed that even at equilibrated conditions a region exists where loss of liquid
water and bound water takes place simultaneously. This region will vary according to the wood structure. Finally, liquid water
was present at EMC lower than the fiber saturation point, which contradicts the concept of this point when considered as a
bulk property of wood. 相似文献
992.
Shigeru Yamauchi Yoichi Sakai Yasuo Watanabe Michael Kenya Kubo Hideaki Matsue 《Journal of Wood Science》2007,53(4):324-331
The distributions of boron in Japanese cedar (Cryptomeria japonica D. Don) sapwood blocks treated with aqueous or methanolic boric acid [B(OH)3] solutions were explored through Raman spectroscopy and prompt gamma-ray analysis (PGA). B(OH)3 was the sole boron species observed in Raman spectra of the wood blocks treated with either solution. Plots of weight gain
of the treated wood blocks versus boron concentration in treatment solutions were found to be linear. The results indicated
that the methanolic solution makes it possible to impregnate wood with much larger amounts of boron than the aqueous solution.
PGA confirmed that B(OH)3 was highly enriched near the end grains of the treated wood blocks. Raman measurements suggested that boron content in the
bulk of the wood block is not as large as expected from the weight gain of the treated wood blocks when an ordinary air-drying
method is used. It was concluded that the aqueous solution impregnates the cell walls of wood with boron more easily than
the methanolic solution.
Part of this report was presented at the 55th (Kyoto, March 2005) and 56th (Akita, August 2006) Annual Meetings of the Japan
Wood Research Society. This article follows the previous rapid communication “Analysis of boron in wood treated with boric
acid solutions using Doppler broadening method of prompt gamma-rays.” J Wood Sci (2006) 52:279–281 相似文献
993.
The effects of thermo-mechanical refining conditions on the properties of medium density fiberboard (MDF) made from black
spruce (Picea mariana) bark were evaluated. The bark chips were refined in the MDF pilot plant of Forintek Canada Corporation under nine different
refining conditions in which preheating retention time was adjusted from 3 to 5 to 7 min and steam pressure was set at either
0.6, 0.9 or 1.2 MPa. The resulting bark fibers were blended with 12% UF resin (based on oven-dry fiber weight) using a mechanical
blender. The resinated fibers were manually formed into fiber mats and hot-pressed into MDF panels using consistent parameters.
Two panels for each refining condition were produced, resulting in a total of 18 panels. Analysis of variance (ANOVA) was
used to analyze the significance of factors. Regression coefficients and 3D contour plots were used to quantify the relationship
between panel properties and the two test factors. The results from this study indicated that the preheating retention time
was a significant factor for both modulus of rupture (MOR) and modulus of elasticity (MOE), the steam pressure was a significant
factor for internal bond strength (IB), MOR and MOE, whereas both factors were insignificant for thickness swelling, water
absorption and linear expansion. The properties of MDF panels were quadratic functions of retention time and steam pressure.
Compared to the ANSI standard for 120-grade MDF, most panels with a nominal density of 950 kg/m3 had very high IB (>1 MPa) and acceptable MOR, MOE and dimension stabilities. These results suggest that black spruce bark
residues can be considered as a potentially suitable raw material for manufacturing MDF products. 相似文献
994.
Weston F. Mwase Åsmund Bjørnstad James M. Bokosi Moses B. Kwapata Brita Stedje 《New Forests》2007,33(3):297-307
Biodiversity is declining throughout southern African miombo woodlands due to poor land use practices that are detrimental
to soil, vegetation and habitat. We aimed at examining tree and shrub species diversity and distribution in Uapaca kirkiana-dominated woodlands under three land tenure types; forest reserves, customary and leasehold land. The study was conducted
at different forest sites within similar agroecological zone in southern Malawi. Study sites were located at a range of altitudes
between 900 and 1,200 m a.s.l. Three circular plots each with 32.60 m radius were established at each study site to sample
tree and shrub species composition, structure and distribution under three land tenure types. We found that forest reserves
had higher species diversity with an average of 16 tree families, 27 genera and 34 species as compared to only 10 tree families,
6 genera and 6 species on customary forest lands. Comparisons of diameter at breast height size class distributions showed
that customary land had significantly low numbers of small (5.0–10.0 cm) and very large (≥30 cm) diameter classes suggesting
lower levels of regeneration and high rate of anthropogenic activities. The high species diversity and richness in forest
reserves and leasehold land indicate high potential for protected lands to restore tree species diversity. It is concluded
that levels of human activities as influenced by land tenure type reduce tree species diversity, composition and distribution
at the different sites, and this confirms the hypothesis that open access lands are not compatible with conservation of tree
and shrub species diversity because of high anthropogenic activities. 相似文献
995.
996.
997.
998.
对20年生马尾松人工林生物量的研究表明,全林生物量38.42吨/公顷,间伐后年生产量6.49吨/公顷林木根量5.73吨/公顷,相对生长率19.0%,净同化率0.253公斤/米~2·年,生产力水平偏低。林分抚育间伐后,林木的生物生产力得到提高。建立了林木各器官干重与直径树高的8个相关方程,相关系数均在0.92以上。通过叶面积及其生长函数分析,林分密度过高,生长速率降低,建议及时合理地调节林分密度。 相似文献
999.
不同生长条件下复叶槭生态型叶片显微形态特征的变化(英文) 总被引:1,自引:0,他引:1
复叶槭 (AcernegundoLinn .)自 10 0年前从北美引种到中国 ,在内蒙古东部的生态交错带上 ,为了适应严酷的生存环境 ,形成了很多特殊的形态特征 .在不同的植被带生长的两个复叶槭生态型 ,其叶片并没有肉眼可见的差异 ,但它们的叶片显微形态特征却完全不同 .来自更加干旱温暖的种源的叶片具有更多的表皮毛、更多而小的气孔、更大的副卫细胞、更厚的栅栏组织 ,且其叶表角质层更加平滑 .来自干热地区种源的叶片显微特征有利于植物降低叶温、保持水分 .植物长期适应不同生存环境会形成具有不同叶部显微形态特征的生态型 ,当叶部显微形态特征用作植物分类的依据时应予以考虑 . 相似文献
1000.
Lili Zhou Addo-Danso Daniel Shalom Pengfei Wu Zongming He Chunhua Liu Xiangqing Ma 《林业研究》2016,27(2):357-368
Biomass production and nutrient (N, P, K, Ca and Mg) accumulation, distribution and cycling were quantified in young, mature and over-mature (10-, 22-, and 34-year old) Chinese fir [Cunninghamia lanceolate (Lamb.) Hook] plantations in southern China. Total stand biomass of young, mature and over-mature stands was 38, 104 and 138 t ha?1 respectively. Biomass production increased significantly with age. Stem wood represented the highest percentage of stand biomass, accounting for 41, 55 and 63 % in the young, mature and over-mature plantations respectively. Nutrients concentration was highest in live needles and branches, and lowest in stem wood. The plantations accumulated more N, followed by K, Ca, Mg, and P. Nutrient return amount, nutrient utilization efficiency, nutrient turnover time, the ratio of nutrient return and uptake increased with stand age, which implies that young Chinese fir deplete soil nutrients to maintain growth, and efficiently utilize nutrients to decrease dependence on soil nutrients as they age. Harvesting young Chinese fir plantations would therefore lead to high nutrient loss, but prolonging the rotation length could improve soil recovery, and help sustain productivity in the long-term. Improved nutrient return through litterfall as stands get older may also be beneficial to nutrient pool recovery. 相似文献