全文获取类型
收费全文 | 5521篇 |
免费 | 355篇 |
国内免费 | 2篇 |
专业分类
林业 | 398篇 |
农学 | 136篇 |
基础科学 | 43篇 |
937篇 | |
综合类 | 1020篇 |
农作物 | 217篇 |
水产渔业 | 275篇 |
畜牧兽医 | 2335篇 |
园艺 | 132篇 |
植物保护 | 385篇 |
出版年
2023年 | 35篇 |
2022年 | 40篇 |
2021年 | 77篇 |
2020年 | 97篇 |
2019年 | 87篇 |
2018年 | 109篇 |
2017年 | 95篇 |
2016年 | 120篇 |
2015年 | 89篇 |
2014年 | 137篇 |
2013年 | 242篇 |
2012年 | 256篇 |
2011年 | 336篇 |
2010年 | 210篇 |
2009年 | 172篇 |
2008年 | 308篇 |
2007年 | 272篇 |
2006年 | 294篇 |
2005年 | 262篇 |
2004年 | 222篇 |
2003年 | 268篇 |
2002年 | 213篇 |
2001年 | 93篇 |
2000年 | 102篇 |
1999年 | 111篇 |
1998年 | 56篇 |
1997年 | 49篇 |
1996年 | 46篇 |
1995年 | 39篇 |
1994年 | 35篇 |
1993年 | 36篇 |
1992年 | 63篇 |
1991年 | 75篇 |
1990年 | 68篇 |
1989年 | 72篇 |
1988年 | 57篇 |
1987年 | 65篇 |
1986年 | 59篇 |
1985年 | 77篇 |
1984年 | 53篇 |
1983年 | 57篇 |
1982年 | 34篇 |
1981年 | 33篇 |
1979年 | 46篇 |
1978年 | 33篇 |
1976年 | 32篇 |
1974年 | 32篇 |
1973年 | 33篇 |
1970年 | 37篇 |
1969年 | 35篇 |
排序方式: 共有5878条查询结果,搜索用时 15 毫秒
201.
Characteristic site disturbance due to harvesting and extraction machinery traffic on sensitive forest sites with peat soils 总被引:2,自引:0,他引:2
Ciaran Nugent Christopher Kanali Philip M. O. Owende Martin Nieuwenhuis Shane Ward 《Forest Ecology and Management》2003,180(1-3):85-98
The effects of wood harvesting and extraction machinery traffic on sensitive forest sites with peat soils were characterised with the objective of quantifying the threshold levels beyond which significant site impacts (compaction and rutting) would occur. The treatments involved running the machines in selected extraction racks (i.e., 3 m wide machine routes) while conducting normal wood thinning and extraction operations comprising one and two passes by the harvester and the forwarder with full payload, respectively. Soil disturbance thresholds were established by testing the level of significance of the difference in induced soil damage and compaction before and after machine traffic treatments. For volumetric soil water content lying between 10.0 and 14.9%, threshold cone penetration resistance levels for two 600/55–30.5 tyres were found to range from 594 to 640 kPa for deep-raised peat soil with initial strength lying between 524 and 581 kPa. In general, the proportion of the total rut depth data in each rack that exceeded the threshold level of 21.5 cm was about 5%. The threshold value corresponds to sinkage equivalent to 15% of the overall wheel diameter of the harvester, above which machine mobility would be hampered considerably. In addition, after harvester traffic the mean rut depth per unit rack length was 10.2×10−2 cm/m, and it ranged from 0.7 to 24.7×10−2 cm/m. 相似文献
202.
203.
Takuya?NishimuraEmail author Jaymeen?Amin Martin?P.?Ansell 《Wood Science and Technology》2004,38(4):297-309
The technique of image analysis has been used to assess the quality of model oriented strand board panels by investigating the relationships between shape and size of strands, the distribution of strands and bending properties. A batch of commercial strands was analysed by image analysis and the distribution of the shape and size of strands was quantified. The strands were categorised into five strand types as a function of size and aspect ratio. In general, strand shapes were observed to be mostly rectangular and there was also a wide variation in strand dimensions in commercial material. Bigger area strands had low aspect ratios and small strands had high aspect ratios. Half of the commercial strands were longer than 100 mm.Model OSB panels were manufactured in the laboratory by hot pressing strand mats formed from each of the five strand types. Strands were laid up by hand into the forming mat and following pressing the orientation and shape of strands was evaluated by image analysis and the panels were tested in a three point bending. Large area (type 3) strands with high aspect ratios produced model panels with optimum strand orientation and mechanical properties.Type 3 panels were also fabricated from strands dropped through a slotted forming device in order to simulate the delivery of strands to the forming line under factory conditions. As the height of strand delivery increased from 0 to 100 to 200 mm the disorientation of strands in the pressed panels progressively increased and as a result mechanical properties in bending were reduced.Image analysis is therefore a powerful tool for evaluating the distribution of commercial strand shapes and the relationship between strand geometry, strand orientation and the mechanical properties of oriented strand board. 相似文献
204.
Somatic embryos of oak (Quercus robur L.) were matured on P24 media differing in gel strength (0.8, 0.9 and 1.0% (w/v) agar). Viscosity and osmotic potential (Psipi,medium) of the media were determined. Developing cotyledonary embryos were analyzed at maturity Stages I-III for water content, osmotic potential (Psipi,embryo) and concentrations of abscisic acid (ABA) and proline. Proliferation of embryogenic tissue, germination rates and the number of embryos formed were also determined in order to relate embryo quality to physiological parameters. Viscosity increased with agar concentration, a phenomenon apparently related to water availability. Many Stage III embryos with high germination potentials were obtained on P24 medium containing 1.0% agar. Embryo water content decreased progressively from 94 to 80% during embryo maturation. Stage I and II embryos that matured on media containing 0.8 or 0.9% agar had similar values of Psipi,embryo, whereas Psipi,embryo of Stage III embryos that matured on medium containing 1.0% agar was significantly lower, although Psipi,medium was unaffected by gel strength. Stage III embryos showed a nearly 16-fold increase in proline concentration and a 50% decrease in ABA concentration compared with Stage I embryos. We conclude that tissue water status and a complex relationship between ABA and proline concentrations, modulated by medium gel strength, are important factors in the maturation process and the quality of oak somatic embryos. 相似文献
205.
Bruno Vincent Fotini Koutroumpa Valérie Altemayer Géraldine Roux-Morabito Jeremy Gevar Carine Martin François Lieutier 《Annals of Forest Science》2008,65(1):111-111
As a consequence of the recent introduction of the pine wood nematode Bursaphelenchus xylophilus in Portugal, nematodes of the genus Bursaphelenchus were looked for in various French pine forests, in trees attacked by Monochamus galloprovincialis, the vector insect of B. xylophilus, and in the insects themselves. Trap trees were felled in 12 localities distributed all over the country. Nematodes were extracted from transversal stem discs; insects emerging from the trap trees were studied. B. hellenicus, B. leoni, B. mucronatus and B. sexdentati were isolated, but not B. xylophilus. The presence of B. mucronatus and the absence of B. xylophilus were confirmed by molecular markers. B. mucronatus was isolated from several regions with an average prevalence of 19%. The infestation of M. galloprovincialis by B. mucronatus reached 26.7%. The wide distribution of B. mucronatus in France could have an effect on the extension of B. xylophilus in a case of an introduction. 相似文献
206.
Martin Kohler Julia Sohn Gregor Nägele Jürgen Bauhus 《European Journal of Forest Research》2010,129(6):1109-1118
To investigate whether drought tolerance of individual trees can be increased through the provision of more growing space,
trees from a thinning experiment were analysed for reductions in radial growth during drought years and their subsequent recovery.
Tree-ring widths were quantified on increment cores as well as stem discs of 32 trees from stands of a thinning trial established
in 1974 in 27-year-old spruce stands in the Alpine Foreland of Southwest Germany. Three different thinning regimes of the
trial were selected for this study: unthinned control (8 trees), moderate (13 trees) and heavy thinning (11 trees) intensities.
All trees sampled were of a co-dominant to dominant canopy status. The standardisation of growth data was carried out using
the software program ARSTAN (University of Arizona). For the year 1976—a widespread and severe drought year in Germany—we
found year ring widths were not reduced compared to those of the pre-drought years for all treatments. However, we observed
the formation of false year rings and resin ducts for this year in all trees investigated. The drought events in 1992 and
2003 led to severe growth depressions in the year of the drought event in all trees, regardless of previous thinning regimes.
However, the resilience—the recovery of basal area growth in subsequent years—was significantly more rapid in trees from heavily
thinned stands, even if the drought event occurred more than 10 years after the last thinning intervention. This indicates
a shorter stress period for trees with more growing space, which may reduce the susceptibility to secondary pathogens or pest
species such as bark beetles. 相似文献
207.
In this paper, a literature overview is presented on the use of laser rangefinder techniques for the retrieval of forest inventory
parameters and structural characteristics. The existing techniques are ordered with respect to their scale of application
(i.e. spaceborne, airborne, and terrestrial laser scanning) and a discussion is provided on the efficiency, precision, and
accuracy with which the retrieval of structural parameters at the respective scales has been attained. The paper further elaborates
on the potential of LiDAR (Light Detection and Ranging) data to be fused with other types of remote sensing data and it concludes
with recommendations for future research and potential gains in the application of LiDAR for the characterization of forests. 相似文献
208.
Allometric models for estimation of aboveground carbon stocks in improved fallows in eastern Zambia 总被引:1,自引:1,他引:1
This paper presents allometric functions for estimation of C stocks in aboveground tree biomass in 2-year-old improved fallows
in eastern Zambia. A total of 222 individual trees representing 12 tree species were destructively harvested for C analysis
by LECO CHN-1000 analyzer. Allometric models relating collar diameter (D10) and total tree height (H) to stem and total aboveground C stocks were developed using data from tree fallows. Logarithmically
transformed power functions displayed a good ability to stabilize variance of aboveground C stocks and showed a good fit (84 < R
2 < 99) with a bias of 0.7–3.6%. D10 alone and in combination with H explained most of the variability in total aboveground C stocks. Validation of the species-specific
and generalized models with field data indicated that they accurately predicted aboveground tree C stocks. Generalized C estimation
functions were also validated and described 73–97% of variability in aboveground C stocks with an average unsigned deviation
of 1.5–4.9%. The C functions will serve as a vital tool for predicting and monitoring C pool sizes in long-term studies and
agroforestry projects, especially where destructive sampling is not possible. 相似文献
209.
Allometric equations based on a fractal branching model for estimating aboveground biomass of four native tree species in the Philippines 总被引:1,自引:0,他引:1
F. Santos Martin R. M. Navarro-Cerrillo R. Mulia M. van Noordwijk 《Agroforestry Systems》2010,78(3):193-202
Fractal branching models can provide a non-destructive and generic tool for estimating tree shoot and root length and biomass,
but field validation is rarely described in the literature. We compared estimates of above ground tree biomass for four indigenous
tree used on farm in the Philippines based on the WanFBA model tree architecture with data from destructive sampling. Allometric
equations for the four species varied in the constant (biomass at virtual stem diameter 1) and power of the scaling rule (b in Y = aD
b
), deviating from the value of 8/3 that is claimed to be universal. Allometric equations for aboveground biomass were 0.035
D
2.87 for Shorea contorta, 0.133 D
2.36 for Vitex parviflora, 0.063 D
2.54 for Pterocarpus indicus and 0.065 D
2.28 for Artocarpus heterophyllus, respectively. Allometric equations for branch biomass had a higher b factor than those for total biomass (except in Artocarpus); allometric equations for the leave + twig fraction a lower b. The performance of the WanFBA model was significantly improved by introduction of a tapering factor “τ“ for decrease of
branch diameter within a single link. All statistical tests performed on measured biomass versus biomass predicted from the
WanFBA results confirm the viability of the WanFBA model as a non-destructive tool for predicting above-ground biomass equations
for total biomass, branch biomass and the leaf + twig fraction. 相似文献
210.
Oisín F. McD. Sweeney Rebecca D. Martin Sandra Irwin Thomas C. Kelly John O’Halloran Mark W. Wilson Peter M. McEvoy 《Forest Ecology and Management》2010
Dead wood is an important component of forest ecosystems and volumes vary depending on forest age, management intensity and productivity. This is the first large-scale study to quantify dead wood in Irish forests and to compare them to forests in other locations. We measured the volume and size distribution of logs, the density and size distribution of snags and the volume of dead wood contained in stumps in Oak (Quercus spp.) and Ash (Fraxinus excelsior) forests and in Sitka spruce (Picea sitchensis) plantations throughout Ireland. We also assigned each log, snag and stump to one of three decay classes (intact, part-rotted and well-rotted). We found no significant difference in log volume between any of the forest types. The majority (>90%) of logs were less than 20 cm in diameter, and large logs (>40 cm diameter) were scarce. We found a relatively high density of snags in all forest types but, as in the case of logs, over 90% of snags were <20 cm DBH and large snags (>40 cm DBH) were rare. The volume of dead wood contained in stumps was significantly higher in plantations than in Oak or Ash forests as a result of thinning and harvesting. Most logs and snags were moderately decayed but, in plantations, most stumps were intact. Log volume and the size of logs and snags were considerably lower than in old-growth forests in other regions. These patterns may reflect historical use of Irish forests for coppice and timber production. Management for biodiversity should aim to accelerate dead wood accumulation to increase the frequency of large-diameter logs and snags. Although management seeking to replicate the dead wood volumes of old-growth forests is ideal, it may be unrealistic in the short term. 相似文献