全文获取类型
收费全文 | 18512篇 |
免费 | 1030篇 |
国内免费 | 20篇 |
专业分类
林业 | 922篇 |
农学 | 598篇 |
基础科学 | 129篇 |
2700篇 | |
综合类 | 2913篇 |
农作物 | 958篇 |
水产渔业 | 1097篇 |
畜牧兽医 | 8991篇 |
园艺 | 242篇 |
植物保护 | 1012篇 |
出版年
2021年 | 182篇 |
2020年 | 212篇 |
2019年 | 230篇 |
2018年 | 354篇 |
2017年 | 405篇 |
2016年 | 302篇 |
2015年 | 233篇 |
2014年 | 361篇 |
2013年 | 734篇 |
2012年 | 597篇 |
2011年 | 751篇 |
2010年 | 497篇 |
2009年 | 472篇 |
2008年 | 720篇 |
2007年 | 638篇 |
2006年 | 594篇 |
2005年 | 536篇 |
2004年 | 495篇 |
2003年 | 492篇 |
2002年 | 435篇 |
2001年 | 502篇 |
2000年 | 496篇 |
1999年 | 394篇 |
1998年 | 188篇 |
1997年 | 207篇 |
1996年 | 164篇 |
1995年 | 199篇 |
1993年 | 172篇 |
1992年 | 283篇 |
1991年 | 322篇 |
1990年 | 318篇 |
1989年 | 324篇 |
1988年 | 295篇 |
1987年 | 256篇 |
1986年 | 298篇 |
1985年 | 263篇 |
1984年 | 218篇 |
1983年 | 213篇 |
1979年 | 251篇 |
1978年 | 174篇 |
1977年 | 163篇 |
1975年 | 176篇 |
1974年 | 241篇 |
1973年 | 217篇 |
1972年 | 254篇 |
1971年 | 237篇 |
1970年 | 209篇 |
1969年 | 200篇 |
1967年 | 163篇 |
1966年 | 165篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Wood Science and Technology - 相似文献
992.
Exotic species possess abilities to harm the ecosystems they invade. This study assesses the density, frequency and cover of exotic plants in roadside right-of-ways, logged areas and wildfire sites within mixedwood sections of the southern boreal forest of Saskatchewan. A total of 23 exotic species were observed including nine species of Gramineae, seven species of Leguminosae and five species of Compositae. Average density of exotic species in areas recently disturbed by timber harvesting or wildfire was 0.2 stems m−2 with a frequency of 72%. Exotic species adapted for wind dispersal were best represented including common dandelion (Taraxacum officinale), perennial sow thistle (Sonchus arvensis) and annual hawksbeard (Crepis tectorum). Only two exotic species, T. officinale and Canada bluegrass (Poa compressa), were observed in mature forest; both occurred with a frequency of 13% and an average density of 0.002 stems m−2. A total of 22 exotic species was found in the right-of-ways quadrats with an average density of 117 stems m−2 and a frequency of 94%. The most frequently observed exotic species in the roadside right-of-way areas were T. officinale, alsike clover (Trifolium hybridum), S. arvensis, creeping red fescue (Festuca rubra) and smooth brome grass (Bromus inermis). These species are either common agricultural weeds or were part of the original seed mixture used to establish a plant cover in the roadside right-of-ways. 相似文献
993.
Suvi P. Pietarinen Stefan M. Willför Markku O. Ahotupa Jarl E. Hemming Bjarne R. Holmbom 《Journal of Wood Science》2006,52(5):436-444
The antioxidant properties of hydrophilic extracts of knotwood of several industrially important tree species were evaluated
by lipid-peroxidation inhibition and peroxyl-trapping capacity tests. The results were compared with the antioxidant properties
of hydrophilic extracts of bark, and pure lignans and flavonoids isolated from knotwood extracts. The knot extracts from several
tree species were stronger antioxidants than the bark extracts, which can, however, also be classified as strong antioxidants.
In addition, the antioxidant properties of most of the knotwood extracts are stronger than the pure compounds. It is concluded
that knotwood is a rich source of natural antioxidants. 相似文献
994.
Intra-annual variations in climate influence growth and wood density of Norway spruce 总被引:2,自引:0,他引:2
Intra-annual radial growth variations of two Norway spruce trees (Picea abies (L.) Karst.) were monitored over 4 years, at four heights up the stem, by means of point-dendrometers. The trees were then felled and radial wood samples were cut from the radii that had been monitored by the dendrometers and analyzed for density. From the radial growth measurements recorded by the dendrometers, we related positions within the rings to dates, thus making possible investigation of the relationships between changes within the rings in wood density and fluctuations in climate or growth rate. Radial growth started in early April and ended, with large intra-annual differences, in August or September. Short-term variations in growth rate were related to fluctuations in climate parameters and soil water reserves. The sensitivity of radial growth to climate decreased with stem height. Wood density responded strongly to drought events, and a dry period in June 1996 induced false-ring formation. Wood density was relatively independent of growth rate and climatic conditions during the first part of the growing season, but increased with decreasing radial growth rate later in the growing season. 相似文献
995.
An experiment was conducted in southern Chile to study the effect of tree cover (0 or 200 stems of 13-year-old Pinus radiate/ha), grazing severity (8 or 12 cm height at commencement of grazing by sheep) and fertilizer application rate (low or high)
on the establishment and growth of Lotus uliginosus and Trifolium subterraneum, when sown in conjunction with Festuca arundinacea. The establishment of both legumes was greater in the agroforestry than the open field, and the percentage of viable seeds
that were established was greater for Trifolium than Lotus. Festuca established better when sown in conjunction with Lotus in the open field. During the two years after stand establishment there was a greater reduction in the ground cover and dry
matter (DM) yield of both legumes, but in particular Trifolium, in the agroforestry than in the open field. In the second year of grazing Trifolium contributed less than 9% of total DM yield in the agroforestry, compared with 22% for Lotus. The ground cover of Lotus was increased by a tall grazing height in the agroforestry, whereas Trifolium increased its cover when the grazing height was short. Additional fertilizer at establishment had a greater effect in increasing
herbage yield in the open field than in agroforestry. It is concluded that both Lotus and Trifolium establishments are greater in agroforestry than in an open field, but that in agroforestry there is a reduction in legume
yield over time, particularly of Trifolium.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
996.
O. Auersch 《Journal of pest science》1975,48(3):37-38
Caterpillars of the buff-tip moth, Phalera bucephala L. (Lepid., Notodontidae) attacking cherry trees
Observations were made on caterpillars of P. bucephala attacking cherry trees in 1972 near Schkopau (DDR). Informations are given about the symptoms of damage as well as about the food quantity needed by the pest larvae. 相似文献
997.
O. Auersch 《Journal of pest science》1961,34(1):7-12
Ohne ZusammenfassungFür die Förderung der Arbeit danke ich Herrn Prof. Dr. H. v. Lengerken. 相似文献
998.
Karen A. Kainer Lúcia H.O. Wadt Christina L. Staudhammer 《Forest Ecology and Management》2007,250(3):244-255
Brazil nut is widely recognized as the cornerstone of the Amazonian extractive economy. Tight linkages between Brazil nut production, regional income, and intact mature forests have thrust this species into focus as a key component of Amazonian conservation and income generation strategies. Nonetheless, a comprehensive synthesis of factors explaining Brazil nut fruit production variation is lacking. We aimed to address this knowledge gap, asking: (1) What are the rates and annual variation of Bertholletia excelsa fruit production at individual and population levels? (2) What factors explain B. excelsa production variation, focusing on spatial and temporal variables, diameter at breast height (dbh), crown attributes, liana loads, and soil attributes? and (3) Does liana cutting affect fruit production? 相似文献
999.
With aid of fluorescein isothiocyanate (FITC) and gold-labelled lectins, together with antibodies against chitinase and β-1,3-glucanases, the spatial distribution of chitin and glucans in hyphal walls of Pythium dimorphum, a causative agent of root die-back disease of conifer seedlings was studied. Contrary to previous reports, this study revealed that chitin, a polymer of N-acetyl-d -glucosamine, is a component of the cell wall of this oomycetous fungus. However, compared with the other amino sugar, N-acetyl-galactosamine, it is not present in significant amounts. Ultrastructural and labelling studies demonstrated that the chitin region is embedded in or masked by glucan residues. 相似文献
1000.
Field studies carried out in a forest transition site on a tropical Alfisol in southwestern Nigeria, show that shading by
gliricidia (Gliricidia sepium (Jacq.) Steud) and leucaena (Leucaena leucocephala (Lam.) de Wit) hedgerow species reduced density of speargrass (Imperata cylindrica (L.) Raeuschel) by 67% and 51%, respectively. Shoot biomass of speargrass decreased by 81% and 78% in gliricidia and leucaena
hedgerows, respectively. Reduction in speargrass rhizome biomass in gliricidia plots was 96% while rhizome reduction in the
leucaena plots was nearly 90%. Rhizome mortality was significantly higher in gliricidia plots than in leucaena and control
plots. The bulk of rhizomes was found between 10 cm and 20 cm of soil depth but rhizome did not penetrate further than 30
cm depth in the Alfisol in which this study was carried out. Gliricidia was better than leucaena hedgerow species in suppressing
speargrass. 相似文献