全文获取类型
收费全文 | 16570篇 |
免费 | 6篇 |
国内免费 | 1篇 |
专业分类
林业 | 3637篇 |
农学 | 1298篇 |
基础科学 | 138篇 |
2790篇 | |
综合类 | 713篇 |
农作物 | 2101篇 |
水产渔业 | 1821篇 |
畜牧兽医 | 1092篇 |
园艺 | 1114篇 |
植物保护 | 1873篇 |
出版年
2022年 | 1篇 |
2021年 | 4篇 |
2020年 | 4篇 |
2018年 | 2746篇 |
2017年 | 2704篇 |
2016年 | 1181篇 |
2015年 | 67篇 |
2014年 | 31篇 |
2013年 | 34篇 |
2012年 | 795篇 |
2011年 | 2135篇 |
2010年 | 2115篇 |
2009年 | 1259篇 |
2008年 | 1322篇 |
2007年 | 1581篇 |
2006年 | 37篇 |
2005年 | 103篇 |
2004年 | 106篇 |
2003年 | 166篇 |
2002年 | 62篇 |
2001年 | 10篇 |
2000年 | 45篇 |
1999年 | 1篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1993年 | 12篇 |
1992年 | 7篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 6篇 |
1988年 | 11篇 |
1987年 | 3篇 |
1985年 | 1篇 |
1977年 | 6篇 |
1974年 | 1篇 |
1972年 | 3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 15 毫秒
991.
Proanthocyanidins (PAs) in the hot water extract prepared from Pinus radiata bark were isolated by a Sephadex LH-20 chromatography. The PAs were characterized using UV/Visible, FT-IR and 13C nuclear magnetic resonance (NMR) spectroscopic techniques, matrix-assisted laser desorption/ionization time of flight (MALDI-TOF)
mass spectrometry (MS), and gel permeation chromatography (GPC) analysis. Bate-Smith acidic alcoholysis and FT-IR spectrum
of the PAs indicated the predominant presence of the procyanidin (PC) structure. The 13C NMR spectrum revealed that the PAs comprise PC (94%) together with a small amount of prodelphinidin (PD, 6%), and also that
they contain the higher trans configuration rather than the cis configuration of C2–C3. The MALDI-TOF MS results obviously proved the presence of PD units, but the maximum degree of polymerization
(DP) was limited to octamer. The average molar mass (Mw, Mn), polydispersity (Mw/Mn), and DP of the pine bark PAs were 3,800
(Mw), 1,200 (Mn), 3.2, and 13, respectively. 相似文献
992.
Current silvicultural treatments in beech forests are aimed at achieving thick logs without discoloured hardwood. Therefore
intensive thinning is applied already in younger stands with the objective of large-sized trunks at an age of 100 years. However,
this approach bears the risk that dead wood structures and broken trees are completely removed from the forest. The impact
of three different silvicultural management intensity levels on wood-inhabiting fungi over decades was investigated in a large
beech forest (>10,000 ha) in southern Germany in 69 sampling plots: A Intensive Thinning and Logging with high-value trees,
B Conservation-Oriented Logging with integration of special structures such as dead wood and broken trees and C Strict Forest
Reserves with no logging for 30 years. The analysis of community showed marked differences in the fungus species composition
of the three treatments, independent of stand age. The relative frequencies of species between treatments were statistically
different. Indicator species for naturalness were more abundant at sites with low silvicultural management intensity. Fomes fomentarius, the most common fungus in virgin forests and strict forest reserves, is almost missing in forests with high-management intensity.
The species richness seemed to be lower where intensive thinning was applied (P = 0.051). Species characteristic for coarse woody debris were associated to low management intensity, whereas species with
a significant preference for stumps became more frequent with increasing management intensity. A total amount of dead wood
higher than 60 m3/ha was found to enable significantly higher numbers of species indicators of naturalness (P = 0.013). In conclusion, when applying intensive silvicultural treatment, the role of dead wood needs to be actively considered
in order to maintain the natural biocoenosis of beech forests. 相似文献
993.
Agroforestry trees are now well known to play a central role in the build up of nutrients pools and their transformations
similar to that of forest ecosystem, however, information on the potential of homegarden trees accumulating and releasing
nitrogen (mineralization) is lacking. The present study reports seasonal variations in pool sizes of mineral N (NH4+-N and NO3−-N), and net N-mineralization rate in relation to rainfall and temperature under coconut (Cocos nucifera L.), clove (Eugenia caryophyllata Thunb) and nutmeg (Myristica fragrans Houtt. Nees) trees in a coconut-spice trees plantation for two annual cycles in the equatorial humid climate of South Andaman
Island of India. Concentration of NH4+-N was the highest during wet season (May–October) and the lowest during post-wet season (November–January) under all the
tree species. On the contrary, concentration of NO3−-N was the lowest in the wet season and the highest during the post-wet season. However, concentrations of the mineral N were
the highest under the nutmeg and the lowest under the coconut trees. Like the pool sizes, mean annual mineralization was the
highest under the nutmeg (561 mg kg−1 yr−1) and the lowest under the coconut trees (393 mg kg−1 yr−1). Rate of mineralization was the highest during the post-wet season and the lowest during the dry season (February–April)
under all the tree species. High rainfall during the wet season, however, reduced the rate of nitrification under all the
tree species. The mean annual mineralization was logarithmically related with rainfall amount and mean monthly temperature. 相似文献
994.
To study the bark beetle attacks on Turkish fir stands and to determine the principle site and stand factors influencing beetle
attacks, trap log method was used in the Western Black Sea Region of Turkey. The study was carried out in ten different locations
prior to flying time in February 2003. On each experimental plot, three healthy fir trees with 30–50 cm diameter and 200 m
apart were cut down with a chainsaw. From each sampling tree, needles were sub-sampled for determination of specific leaf-area
and macro-nutrient concentrations. In addition, on each experimental plot, crown closure, basal-area, and stand density were
measured. To determine the bulk density and nutrient concentrations of the soil, samples were taken around trees on each plot.
The relationship between measured stand variables, the number of beetle species and beetle populations were evaluated using
correlation and stepwise multiple regression procedure. Eight different species belonging to four different families from
Coleoptera were identified in examining the trap logs. Six of these eight species (Pissodes piceae Illig., Pissodes notatus Fabr., Cryphalus piceae Ratz., Pityophthorus pityographus Ratz., Xyloterus lineatus Oliv., Pityokteines curvidens Germ.) were identified as harmful beetles (HB). The other two (Thanasimus formicarius L. and Rhizophagus dispar Payk.) were identified as predator species. The analysis showed that the total number of HB increased as the site’s slope
and needle specific leaf area increased. However, as needle N concentrations increased, HB population decreased. Increased
Ca concentration in fir needle reduced total insect attack. 相似文献
995.
Martin Häglund 《Wood Science and Technology》2007,41(6):477-490
Knowledge of effects of moisture exposure on timber structures is important for serviceability and load bearing capacity of
timber elements and systems. Varying climate affects the moisture content profiles in timber as the relative humidity changes
in the ambient air. This paper presents methods for calculation of diffusion based moisture transport and corresponding results
on how timber responds to varying moisture with regard to moisture penetration and distribution. The calculations are based
on measured outdoor relative humidity and temperature at different climatic locations in Sweden. No remarkably large deviations
between the different climatic locations were found in general, which is positive in a design code perspective. 相似文献
996.
Twigs of 2–3-year-old Platanus occidentalis L. were used as experimental material to find the causes for the contamination and browning in the initial stages of tissue cultures. To compare the degree of browning of explants picked off from different growing seasons, the experimental material was excised from trees on each of the first ten days in January, March, May and July, 2006. The results indicated that the contamination and browning rates of the material cut off in January (14.2% and 30.6%, respectively) and March were somewhat lower than those in July. The pretreatment of soaking the explants in different anti-oxidants and absorbents at the same time could diminish some side effects. The pretreatment of using 10 g·L–1 vitamin C reduced the contamination and brown-ing rate effectively. An orthogonal experiment showed that the optimal factor and level arrangement is 0.5 mg·L–1 BA, 2.0 g·L–1 ac-tive carbon and 1.5 g·L–1 PVP which resulted in a browning rate of only 16.5%. In general, sampling period, physical properties and pretreatment of explants are the main factors responsible for the contamination and browning of material in the initial stages of P. occidentalis tissue cultures. 相似文献
997.
Policies Affecting Forestry Entrepreneurship 总被引:1,自引:0,他引:1
Anssi Niskanen Anders Lunnan Ikuo Ota Keith Blatner John Herbohn Lyndall Bull Ian Ferguson Gordon M. Hickey 《Small-Scale Forestry》2007,6(3):233-255
Many demand and supply-side policies impede or foster forestry entrepreneurship. A study was conducted to consolidate existing knowledge on policies influencing forestry entrepreneurship
in Finland, Norway, Japan, Australia, the Philippines and the USA, and to draw conclusions on these impeding and fostering
factors. From the country studies it was difficult to find common structures on policies affecting forestry entrepreneurship.
This is understandable because most policies in forestry are aimed at supporting sustainable forest management, wood production
and ecological services of the forests rather than entrepreneurship as such. Despite the high variety of policies applied
in the study countries, it can be concluded that strict public control on forests’ use and management potentially impedes
forestry entrepreneurship. While these policies assist to correct market failure and to promote sustainability of forest management,
they may also result into unnecessary and ineffective regulations that limit the opportunities for forestry entrepreneurship.
A common feature promoting the demand for forestry entrepreneurship in some of the countries studied is the strong emphasis
on forestry cooperatives, which were important institutions to support small-scale forestry entrepreneurship. In many study
countries, different ad hoc programs are implemented to find new economic and entrepreneurial opportunities aside from the
current use of wood and forests. Subsidies and tax incentives are commonly applied to reduce risks from making forestry investments
or otherwise increase the economic return from timber production. 相似文献
998.
To determine the effect of shade on morphology, growth and biomass allocation in Picea sitchensis, Larix × eurolepis and Thuja plicata, seedlings were grown in the open or under shadehouses providing 25%, 50% and 75% reductions of full-light for two growing
seasons. For most of the characteristics assessed there was no significant interaction between species and shade indicating
that the morphological responses to changing shade treatments were not species-dependent. After two growing seasons the mean
height increment for the three species was significantly greater in 25% (76.1 cm) and 50% shade (74.9 cm) than in the open
(69.5 cm). Root collar diameter increment, shoot, root and total biomass declined significantly with increasing shade while
the opposite was true for the height:diameter ratio. In both western red cedar and hybrid larch the shoot:root ratio was significantly
greater in the shade while in Sitka spruce this characteristic was not influenced by shade. While all species had significantly
greater specific shoot areas in 75% shade than in 0% shade, this trend was particularly pronounced in hybrid larch. In hybrid
larch and western red cedar, the normalised specific projected shoot area increased significantly with increasing shade. The
opposite trend was observed for Sitka spruce. We conclude that in the main the species studied demonstrated similar shade
acclimation responses despite their reported differences in shade tolerance. 相似文献
999.
Interactions between scarification treatments and seedling types of Norway spruce were examined at two different locations in southern Sweden. The scarification treatments were not scarified control, mounding and soil inversion and the seedling types were a 10 week-old containerized seedling (mini seedling), a 2 year-old containerized seedling and a 2 year-old hybrid seedling, grown as a containerized seedling the first year and as a bare-root seedling the second year. Site preparation intensity had a greater effect on the mini seedlings. The results indicated that mini seedlings established faster in the soil inversion treatment compared to the larger seedling types. At the same seedling age, the mini seedlings had faster or similar growth rate as the containerized and the hybrid seedlings regardless of scarification treatment. Although mini seedlings grew as well as or even better than larger seedlings if they were successfully established, they were more sensitive to their planting environment and proper handling was critical. Problems with frost heaving and competing vegetation have to be taken into consideration when choosing site preparation method and seedling type. 相似文献
1000.
This research examined the first year growth characteristics of cold stored and transplanted nursery-produced aspen (Populus tremuloides) seedlings (container and bareroot (BR)) and compared it to the growth of seedlings that had not been transplanted (established
from germinants in the field) and therefore had an unrestricted root system (UR). Prior to planting, nursery-produced seedlings
were placed in cold storage (−3°C) and root growth potential (RGP) and total non-structural carbohydrate (TNC) root reserves
were tested at 0, 10, 75 and after 150 (container) and 190 days (BR) of storage. Both container and BR stock had much lower
root to shoot ratios (RSRs) and root carbohydrate reserves compared to UR seedlings after 170 days. During storage, root reserves
in container stock declined faster than in the BR and UR seedlings. RGP in all nursery stock was the highest after 75 days
of storage, while longer storage resulted in shoot dieback and reduced root growth. After the first growing season, UR seedlings
were one tenth the size of the nursery stock; however, in the second growing season they had no stem dieback and grew twice
the height and stem diameter. The higher RSRs and root reserves in the UR seedlings was likely caused by early bud set in
its first year of growth. This suggests that inducing bud set earlier in the growing regime might allow seedlings to increase
root mass and carbohydrate reserves. 相似文献