全文获取类型
收费全文 | 21454篇 |
免费 | 263篇 |
国内免费 | 545篇 |
专业分类
林业 | 4287篇 |
农学 | 2065篇 |
基础科学 | 485篇 |
3471篇 | |
综合类 | 2199篇 |
农作物 | 2423篇 |
水产渔业 | 1944篇 |
畜牧兽医 | 1936篇 |
园艺 | 1264篇 |
植物保护 | 2188篇 |
出版年
2024年 | 10篇 |
2023年 | 56篇 |
2022年 | 146篇 |
2021年 | 210篇 |
2020年 | 169篇 |
2019年 | 175篇 |
2018年 | 2852篇 |
2017年 | 2851篇 |
2016年 | 1360篇 |
2015年 | 287篇 |
2014年 | 263篇 |
2013年 | 304篇 |
2012年 | 1119篇 |
2011年 | 2450篇 |
2010年 | 2430篇 |
2009年 | 1537篇 |
2008年 | 1548篇 |
2007年 | 1860篇 |
2006年 | 310篇 |
2005年 | 313篇 |
2004年 | 234篇 |
2003年 | 266篇 |
2002年 | 218篇 |
2001年 | 139篇 |
2000年 | 169篇 |
1999年 | 123篇 |
1998年 | 106篇 |
1997年 | 94篇 |
1996年 | 93篇 |
1995年 | 83篇 |
1994年 | 78篇 |
1993年 | 85篇 |
1992年 | 68篇 |
1991年 | 75篇 |
1990年 | 39篇 |
1989年 | 37篇 |
1988年 | 38篇 |
1987年 | 15篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1977年 | 4篇 |
1972年 | 2篇 |
1968年 | 4篇 |
1963年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 421 毫秒
871.
This paper presents a probabilistic method of evaluating the final moisture content (MC) of lumber obtained at the end of the kiln-drying process. The final MC data of three different drying tests conducted in past studies were analyzed using the bootstrap method. Target MC was tentatively set below 20 % in the analysis. Two characteristic parameters representing the final MC were estimated with bootstrap confidence intervals. These parameters were the standard deviation (SD) and the percentage of the population that met the MC requirement of less than 20 % (P 20). The histograms of the final MC and the subsequent goodness-of-fit tests revealed that the final MC data of two drying tests did not follow any classical probability distributions, including Normal, Log-Normal, Weibull, and Gamma distributions, thus indicating the need for nonparametric statistics. The uncertainty of the final MC could be evaluated with the estimated SD and P 20. After deriving the relationships between P 20 and the corresponding probability that P 20 is not achieved, we demonstrated how such relationships could provide a kiln operator with information to facilitate better decision-making in optimizing a drying schedule. 相似文献
872.
Key message
When areas of interest experience little change, remote sensing-based maps whose dates deviate from ground data can still substantially enhance precision. However, when change is substantial, deviations in dates reduce the utility of such maps for this purpose.Context
Remote sensing-based maps are well-established as means of increasing the precision of estimates of forest inventory parameters. The general practice is to use maps whose dates correspond closely to the dates of ground data. However, as national forest inventories move to continuous inventories, deviations between map and ground data dates increase.Aims
The aim was to assess the degree to which remote sensing-based maps can be used to increase the precision of estimates despite differences between map and ground data dates.Methods
For study areas in the USA and Norway, maps were constructed for each of two dates, and model-assisted regression estimators were used to estimate inventory parameters using ground data whose dates differed by as much as 11 years from the map dates.Results
For the Minnesota study area that had little change, 7-year differences in dates had little effect on the precision of estimates of proportion forest area. For the Norwegian study area that experienced considerable change, 11-year differences in dates had a detrimental effect on the precision of estimates of mean biomass per unit area.Conclusions
The effects of differences in map and ground data dates were less important than temporal change in the study area.873.
József Garay Zoltán Sebestyén Zoltán Varga Manuel Gámez Alejandro Torres José E. Belda Tomás Cabello 《Journal of pest science》2015,88(2):343-358
Over the last few decades, important advances have been made in understanding of host–parasitoid relations and their applications to biological pest control. Not only has the number of agent species increased, but new manipulation techniques for natural enemies have also been empirically introduced, particularly in greenhouse crops. This makes biocontrol more complex, requiring a new mathematical modeling approach appropriate for the optimization of the release of agents. The present paper aimed at filling this gap by the development of a temperature- and stage-dependent dynamic mathematical model of the host–parasitoid system with an improved functional response. The model is appropriate not only for simulation analysis of the efficiency of biocontrol agents, but also for the application of optimal control methodology for the optimal timing of agent releases, and for the consideration of economic implications. Based on both laboratory and greenhouse trials, the model was validated and fitted to the data of Chelonus oculator (F.) (Hym.: Braconidae) as a biological control agent against the beet armyworm, Spodoptera exigua Hübner (Lep.: Noctuidae). We emphasize that this model can be easily adapted to other interacting species involved in biological or integrated pest control with either parasitoid or predator agents. 相似文献
874.
This paper presents results of survey research into the role of the forestry nursery sector in the development of smallholder
forestry in Leyte, Philippines. Seedling production in the province is basically carried out by three categories of nurseries:
individual, communal and the government. Seedlings in the government nurseries are primarily raised for free distribution
to landholders but the extension scheme fails to reach the majority of the smallholder farmers. Seedling demand of landholders
is mostly catered for by the more numerous and accessible individual and communal nurseries. However, problems of capital
outlay, sources and supply of germplasm, technical knowledge in nursery management for both timber and fruit trees and low
sales are among the major impediments in the sustainability of small-scale nurseries. Most of the communal nurseries fail
to sustain the operation after the withdrawal of supporting agencies. Knowledge on site-species matching is needed not only
by small-scale nursery operators but also by the managers of government nurseries. It is concluded that the seedling industry
in the province is not well established and there is a need to organise better the overall nursery system so as to improve
the availability of seedlings and promote appropriate site and species combinations. 相似文献
875.
Florence Z. Tarun-Acay 《Small-Scale Forestry》2005,4(1):101-116
An assessment was made of the attributes of People’s Organisations (POs) and of the institutional viability of Community-Based
Forest Management (CBFM) programs in three project sites in the Philippines. In CBFM, the capacity of People’s Organisations,
as de facto managers of forest resources, is being developed by non-government organisations under the guidance of local government units
(LGUs) and the Department of Environment and Natural Resources (DENR). A survey was conducted of community members (case informants)
and members of the various agencies (key informants) to ascertain the characteristics of successful POs and the relationship
between these characteristics and the viability of CBFM. Although commendable contributions have been made to the forest management
capability of POs, their standard of living and environmental conditions, there are also many instances in which institutional
conflict among stakeholders is apparent. The great challenge for the various stakeholders is to orchestrate their differing
interests so as to address successfully the critical issues of the low socioeconomic status of PO members, corruption, lack
of transparency and accountability of the organisations involved, loss of interest of PO members in project activities, lack
of technical assistance, loss of credibility of the DENR, and lack of equitable benefit sharing. 相似文献
876.
Nancy Alejandre-Melena Roberto Lindig-Cisneros Cuauhtémoc Sáenz-Romero 《New Forests》2007,34(1):25-30
Major disturbances that remove vegetation cover create conditions that differ greatly from those in adjacent forests. In volcanic
areas, tephra deposition can be a major disturbance that eliminates vegetation cover and creates a barrier for plant establishment.
The eruption of the Paricutín volcano, in Michoacán, México, from 1943 to 1952, created large tephra deposits adjacent to
forest stands. Many of these deposits still lack native vegetation and understanding of the limiting factors for establishment
of native tree species is necessary to restore these areas. Pinus pseudostrobus early growth in response to the amount of fertile growing medium on top of a variable-depth tephra layer was evaluated in
a greenhouse experiment. Increasing amounts of fertile growing medium increased height of 28-week-old plants (no growing medium = 5.5 cm
tall ±1.8 cm, 3 cm of growing medium = 21.2 ± 4.6 cm and 6 cm of growing medium 24.8 ± 4.4 cm). Other variables showed the
same trend (diameter, number of fascicles, belowground biomass and aboveground biomass). Our results suggest that P. pseudostrobus establishment in tephra covered areas will depend on the accumulation of a fertile layer of more than 3 cm in thickness. 相似文献
877.
A provenance trial of Michelia chapensis Dandy in the south of China was carried out in three sites to assess yield and stability analyses. The trials were conducted as randomized, complete block designs with four replications at each site. Additive main effects and multiplicative interaction (AMMI) and genotype main effect and genotype × environment interaction (GGE) were employed in the evaluation of provenances; AMMI analyses showed that significant genotype × environment interaction effects (P < 0.05) existed between provenances, capturing 15.11 % of the total sum of squares. The AMMI stability value revealed that provenance 2 was stable, but had low yield. GGE-biplot models showed that provenances 7, 8, 10, and 11 were more stable and high-yielding. The GGE results also confirmed that among the three sites, site JiuQuShui was the most ideal environment for representation and discrimination. 相似文献
878.
Emila Akroume Bernd Zeller Marc Buée Philippe Santenoise Laurent Saint-André 《Annals of Forest Science》2016,73(4):1005-1013
Key message
Near- and mid-infrared spectroscopy allows for the detection of local patterns of forest soil properties. In combination with dendrometric data, it may be used as a prospective tool for determining soil heterogeneity before setting up long-term forest monitoring experiments.Context
Forest soils and stands generally exhibit higher spatial heterogeneity than other terrestrial ecosystems. This variability needs be taken into account before setting up long-term forest monitoring experiments to avoid multiple interactions between local heterogeneity and the factors tested in the experiment.Aims
We hypothesized that raw near- and mid-infrared spectra can be used as an integrated proxy of a large set of soil properties. The use of this method, in combination with dendrometric data, should provide a quick and cost-effective tool for optimizing the design of experimental forest sites.Methods
We assessed the local soil heterogeneity at 11 experimental sites in oak and beech stands, which belong to a new forest long-term ecological research (LTER) network. We used near- and mid-infrared spectroscopy in soil and litter samples. The spectra were subjected to principal components analyses (PCA) to determine the intra-site variability of the soil and litter layers.Results
Based on mapped PCA coordinates and basic dendrometric data, it was possible to design the experiment and minimize the interactions between the treatment layout and the tested variables. The method was validated with chemical analyses of the soil. No interaction was detected at the set-up of the experiment between the treatment layout and chemical soil properties (C, N, C/N ratio, pH, CEC, Al, Mg, P2O5, Fe, Mn, Na, and K).Conclusion
Near-infrared (NIR) and mid-infrared (MIR) spectroscopy is a useful tool for characterizing the overall heterogeneity of soil chemical properties. It can be used without any preliminary calibration. In combination with dendrometric data, it provides a reliable method for optimizing LTER plots in different types of ecosystems.879.
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
880.