全文获取类型
收费全文 | 2442篇 |
免费 | 32篇 |
国内免费 | 79篇 |
专业分类
林业 | 1185篇 |
农学 | 99篇 |
基础科学 | 56篇 |
124篇 | |
综合类 | 795篇 |
农作物 | 97篇 |
水产渔业 | 32篇 |
畜牧兽医 | 95篇 |
园艺 | 26篇 |
植物保护 | 44篇 |
出版年
2024年 | 5篇 |
2023年 | 27篇 |
2022年 | 33篇 |
2021年 | 44篇 |
2020年 | 82篇 |
2019年 | 81篇 |
2018年 | 42篇 |
2017年 | 43篇 |
2016年 | 91篇 |
2015年 | 78篇 |
2014年 | 104篇 |
2013年 | 79篇 |
2012年 | 134篇 |
2011年 | 122篇 |
2010年 | 100篇 |
2009年 | 107篇 |
2008年 | 114篇 |
2007年 | 108篇 |
2006年 | 95篇 |
2005年 | 109篇 |
2004年 | 109篇 |
2003年 | 94篇 |
2002年 | 103篇 |
2001年 | 101篇 |
2000年 | 99篇 |
1999年 | 77篇 |
1998年 | 63篇 |
1997年 | 40篇 |
1996年 | 41篇 |
1995年 | 45篇 |
1994年 | 38篇 |
1993年 | 38篇 |
1992年 | 23篇 |
1991年 | 27篇 |
1990年 | 25篇 |
1989年 | 18篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有2553条查询结果,搜索用时 15 毫秒
941.
Physical and mechanical properties of medium density fiberboards (MDF) made from various mixtures of wood fibers and stone pine (Pinus pinea L.) cones were evaluated using European standards. MDF panels were manufactured using standardized procedures that simulated industrial production at the laboratory. Six panel types were made from mixtures of wood fiber/cone flour, 100/0, 90/10, 80/20, 70/30, 60/40, and 50/50 percents, respectively. Addition of the cone flour into the MDF significantly reduced formaldehyde emission from the panel. In addition, the addition of 10% cone flour also improved water resistance of the MDF panels made using urea–formaldehyde (UF) resin. However, further addition of the cone flour into the panel negatively influenced their water resistance. Flexural properties and internal bond strength decreased with the increase of cone flour content in the panel. The UF resin is the main source of formaldehyde emission from the UF-bonded wood-based panels. Depending on addition of the cone flour in the panels, the formaldehyde emission values ranged from 2.6% to 55.3% lower than the panels made from 100% wood fiber. Based on the findings obtained from this study, pine cone can be used as a renewable biological formaldehyde catcher as an alternative to the traditional formaldehyde catchers for E1 Class MDF manufacture. 相似文献
942.
Roots, stems, branches and needles of 160 Norway spruce trees younger than 10 years were sampled in seven forest stands in central Slovakia in order to establish their biomass functions (BFs) and biomass expansion factors (BEFs). We tested three models for each biomass pool based on the stem base diameter, tree height and the two parameters combined. BEF values decreased for all spruce components with increasing height and diameter, which was most evident in very young trees under 1 m in height. In older trees, the values of BEFs did tend to stabilise at the height of 3–4 m. We subsequently used the BEFs to calculate dry biomass of the stands based on average stem base diameter and tree height. Total stand biomass grew with increasing age of the stands from about 1.0 Mg ha−1 at 1.5 years to 44.3 Mg ha−1 at 9.5 years. The proportion of stem and branch biomass was found to increase with age, while that of needles was fairly constant and the proportion of root biomass did decrease as the stands grew older. 相似文献
943.
944.
The present study examined branch and stem characteristics of trees growing around gaps in Norway spruce plantations. Trees located at the edge of gaps with a radius of either 5 m or 7 m either unplanted or with supplementary planting were compared to trees in the original closed plantation. The experiment was carried out in two locations in the south of Sweden and the design included four blocks at each location and one replicate of each treatment in each block. The measurements were carried out on standing and felled trees in 2005 at the time of the first thinning. The results indicate a significant effect of gaps and supplementary planting on the diameter of the largest branch, the number of living branches close to breast height, branch basal area, height to the first living branch and taper. The effect of supplementary planting on branch and stem characteristics was greater in large gaps than in small ones. This study indicated that supplementary planting may be useful in affecting stem-form and branch parameters, although other studies have shown that the trees that result from supplementary planting contribute little to total production. 相似文献
945.
946.
Spatial variation in respiration from coarse woody debris in a temperate secondary broad-leaved forest in Japan 总被引:1,自引:0,他引:1
We measured the rates of respiration from snags and logs (“coarse woody debris”, CWD) of Japanese red pine (Pinus densiflora Sieb. et Zucc.) to examine the rate of decomposition and CO2 efflux from these materials in a temperate secondary broad-leaved forest in Japan. At this site, a high quantity of CWD of P. densiflora had accumulated as a result of pine wilt disease during the 1970s. Respiration rates were measured using a dynamic closed chamber method combined with an infrared gas analyzer. We measured the respiration rate of 7 samples of snags and 10 samples of logs from August 2003 to January 2004. The responses of the respiration rates of snags (Rsnag) and logs (Rlog) to changing temperature were both exponential and the responses to water content were quadratic, and the same function could be used to estimate annual values of both Rsnag and Rlog. Intensive measurements of water contents of snags and logs showed a marked difference in water content. The mean water content of snags was 20% of log water content. This difference was likely responsible for the observed difference in annual Rsnag and Rlog. The decay rate constants estimated from the respiration rates measurement of snags and logs were 0.019 and 0.081 year−1, respectively. Despite being lower than Rlog, Rsnag was a significant compartment of the CWD carbon budget at this site. 相似文献
947.
Calycophyllum spruceanum (Benth.) Hook. f. ex K. Shum. is an important timber-tree species in the Peruvian Amazon Basin. Markets currently prefer
wood with a uniform, light yellow color, but these preferences may change in the future. As farmers and industry commonly
use wood from young trees, it is important to investigate genetic and environmental variation in juvenile-wood properties
to assess whether tree improvement programs could make adjustments to changing preferences. A provenance/progeny test was
established to evaluate genetic variation in growth and wood properties of young trees, the strength of their genetic control
as well as their interrelationships both at the genetic and the phenotypic level in different planting zones. This paper presents
analyses of variation in wood color at 39 months, and their correlations with tree growth and wood basic density. CIELab means
for lightness ranging from black to white (L), green to red hues (a*) and blue to yellow hues (b*) were 67.63, 5.34 and 22.12,
respectively; means for chroma (C) and hue angle (h)* were 22.76 and 76.43, respectively (C and h* were estimated from a*
and b*). Significant variation due to provenances and especially due to families within provenances was found in some wood
color characteristics, and some color characteristics also differed significantly among planting zones. Genetic correlations
indicate that, in general, selection of faster growing trees and/or trees with denser wood would have little effect on wood
color and its uniformity. In general, wood color had relatively low heritability (h
i
2): among all trees, h
i
2 = 0.31 for L; and variance due to families was not significant for a* and b*. Genetic control of color was highest in the
planting zone where trees grew most rapidly: h
i
2 = 0.48 and 0.52 for a* and b*, respectively; but variance due to families was not significant for L. Results suggest that
selection based on wood color would be more effective in zones with more fertile soils and higher rainfall.
相似文献
948.
Yoshitaka Kubojima Seiichi Kanetani Takeshi Fujiwara Youki Suzuki Mario Tonosaki Hiroshi Yoshimaru Hiroharu Ikegame 《Journal of Wood Science》2008,54(6):443-450
A dead tree of Pinus armandii Franch. var. amamiana (Koidz.) Hatusima (abbreviated to PAAm) was obtained from a natural habitat on Tanega-shima Island and various properties
of its wood were investigated. Grain angle was measured and soft X-ray analysis was undertaken to obtain the density in each
annual ring. Unit shrinkage and dynamic properties were measured by shrinkage, bending, and compression tests. Variations
of wood properties in the radial direction, relationships of wood properties to density, and annual ring width were examined.
Roughly speaking, variations in the radial direction of the grain angle, twist angle by drying, Young’s modulus and strength
in static bending, absorbed energy in impact bending, compressive Young’s modulus, compressive strength, and compressive proportional
limit corresponded to the variation of annual ring width. As a result, it was determined that if PAAm is afforested artificially
for the purposes of lumber production and conservation, the annual rings of logs should not be too widely spaced. Wood properties
of PAAm were similar to those of Japanese black pine (Pinus thunbergii Parl.), which is another representative pine on Tanegashima Island.
This study was presented in part at the 56th Annual Meeting of the Japan Wood Research Society, Hiroshima, August 2007 相似文献
949.
Typical symptoms of rubberwood canker found in East Kalimantan, Indonesia, differ from those of conventional rubberwood peach
canker. Rubberwood canker showed a conspicuous longitudinal canker with exposed xylem that was discolored and decayed. Bark
wounds, resulting from regular tapping, were enclosed within the xylem, and the tissues had become integrated. The characteristics
of abnormal and decayed xylem in this rubberwood canker were analyzed by light microscopy and scanning electron microscopy.
Xylem in the vicinity of the enclosed bark contained fewer vessels of smaller diameter and shorter length, and significantly
wider rays compared with normal xylem. Around the wide growth zones of the canker, axial cells were disoriented and warped
toward the canker zones. In view of the separation among cells, and the concentric degradation of the cell walls starting
from the lumen surfaces, decayed xylem appeared to be caused mainly by white-rot fungal attack. 相似文献
950.
Yuan Zhou Fu-Quan Chen Si Chen Qiong Xiao Hui-Fen Weng Qiu-Ming Yang An-Feng Xiao 《Marine drugs》2021,19(9)
In this work, the physicochemical properties of maleic anhydride (MAH)-modified κ-carrageenan (κCar) (MC) were characterized and compared with those of native κ-carrageenan (NC). The Fourier transform infrared spectrum of MC exhibited that κCar was successfully modified. Thermogravimetric analysis indicated that the thermal stability of MC was decreased. When the degree of substitution was 0.032, MC exhibited a low gel strength (759 g/cm2), gelling temperature (33.3 °C), and dehydration rate (60.3%). Given the excellent film-forming ability of κCar, MC films were then prepared and were found to have better mechanical and barrier properties (UV and water) than NC films. With regard to optical properties, MC films could completely absorb UV light in the range of 200–236 nm. The water contact angle of MC films was higher than that of NC films. Moreover, the elongation at break increased from 26.9% to 163%. These physicochemical property changes imply that MC can be employed in polysaccharide-based films. 相似文献