全文获取类型
收费全文 | 2707篇 |
免费 | 194篇 |
国内免费 | 1806篇 |
专业分类
林业 | 371篇 |
农学 | 188篇 |
基础科学 | 173篇 |
2097篇 | |
综合类 | 881篇 |
农作物 | 133篇 |
水产渔业 | 258篇 |
畜牧兽医 | 301篇 |
园艺 | 81篇 |
植物保护 | 224篇 |
出版年
2024年 | 50篇 |
2023年 | 90篇 |
2022年 | 103篇 |
2021年 | 155篇 |
2020年 | 164篇 |
2019年 | 215篇 |
2018年 | 207篇 |
2017年 | 231篇 |
2016年 | 298篇 |
2015年 | 231篇 |
2014年 | 182篇 |
2013年 | 292篇 |
2012年 | 323篇 |
2011年 | 296篇 |
2010年 | 257篇 |
2009年 | 214篇 |
2008年 | 131篇 |
2007年 | 153篇 |
2006年 | 149篇 |
2005年 | 133篇 |
2004年 | 99篇 |
2003年 | 99篇 |
2002年 | 81篇 |
2001年 | 64篇 |
2000年 | 63篇 |
1999年 | 68篇 |
1998年 | 52篇 |
1997年 | 34篇 |
1996年 | 44篇 |
1995年 | 46篇 |
1994年 | 31篇 |
1993年 | 36篇 |
1992年 | 25篇 |
1991年 | 16篇 |
1990年 | 28篇 |
1989年 | 16篇 |
1988年 | 18篇 |
1987年 | 7篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有4707条查询结果,搜索用时 15 毫秒
51.
The standardization of environments (E) encouraged by modern society and by the productivist model of agriculture has resulted
in the standardization of genotypes (G) thereby reducing G × E interaction. New societal values call for the diversification
of agriculture to fit contrasted environments. This process can be depicted by four models defined by two axes, one socio-economic
(individual logics versus collective governance), and the other agro-ecological (reductionist versus systemic approaches).
These models differ in (i) their objectives (from improvement in yield to the empowerment of farmers), (ii) their specific
expectations with respect to genotypes (from inherited genetic resources to varieties that represent genetic, ethical and
social progress), (iii) their specific representations of the environment (E) (from a simple interaction between the bio-physical
environment (B) and the crop management (C), to a complex interaction including the competences of the actors (A), outlets
(O), regulations (R), society (S)), (iv) their particular relations between G and E (from G × E to G × B × C × A under evolving
constraints represented by R × O × S). Taking this diversity into account changes the way plant improvement is considered.
Thus, depending on the model, the order, interest and status of the five classic stages of plant improvement (setting objectives,
creating variability, selecting, evaluating and disseminating) may be called into question. Between the existing analytical
model (Model I) and a holistic model (Model IV) which remains to be developed, lies the challenge of ensuring the sustainability,
efficiency and acceptability of plant breeding and resulting innovations. From a simple “statistical parameter” that we, as
plant breeders, attempt to reduce, the G × E interaction is becoming an “objective” that we try to predict and valorize. Structuring
the different components of E, G and G × E, enables us to extend the basic concept of representivity to both the cultivation
conditions and the relational socio-economic positions of the actors involved.
相似文献
D. DesclauxEmail: |
52.
棉花植株水分含量的高光谱监测模型研究 总被引:3,自引:2,他引:3
精确灌溉对无损、快速的水分监测技术有迫切需求。研究棉花冠层高光谱参数与水分的定量关系并建立水分估测模型,以实现棉花水分及时、准确监测。通过2年试验,测定棉花冠层高光谱及植株水分,根据光谱参数与植株含水量的相关关系,建立了植株含水量监测模型。结果表明:棉株含水量与叶片含水量在一定范围内随灌溉量增减而增减,并能区分棉花干旱程度;棉株及叶片含水量与冠层460~514 nm、605~698 nm、1451~1576 nm和1960~2457 nm反射率极显著负相关,与727~1345 nm反射率极显著正相关,且棉株的相关性好于叶片含水量。所选作物水分指数、归一化差值水分指数1、归一化差值水分指数2、水分胁迫指数1、水分胁迫指数2、水分波段指数、水分指数与归一化差值植被指数之比均与棉株及叶片含水量极显著相关;构建了棉株含水量和叶片含水量的最佳监测模型;所建模型精度能满足大田生产对棉花水分监测的要求。 相似文献
53.
在分析宁德市国有生态公益型林场分类经营的现状、存在问题的基础上,就其分类经营、可持续发展提出了5点对策和5条管理措施。 相似文献
54.
尾叶桉人工林二元材积表的研制 总被引:3,自引:0,他引:3
通过收集福建省永安市不同林龄、立地、密度的尾叶桉人工林林分的样木资料,采用多模型优选法和逐步回归法研制尾叶桉人工林二元立木材积表,并用相关指数、剩余标准差、平均系统误差、平均相对误差绝对值、预估精度作为评价指标,应用灰色关联分析法建立综合评价指数,评价模型优劣。经检验,所编的材积表完全符合生产上的精度要求,且理论基础扎实,编表结果可靠,可在森林资源调查中推广应用。 相似文献
55.
The aim of this study was to develop models on occluded branch characteristics for Fagus sylvatica (beech) based on 41 sample trees. A total of 717 beech branches were sampled; this information was then used to predict (1) the time for a complete occlusion, (2) the total radius of the occluded branch inside the trunk, (3) the branch insertion angle at the year of its death and during branch development, and (4) the dead branch portion of the occluded branch (loose knot). Generalized hierarchical mixed models with nonlinear forms were used in this analysis. The models explained between 6.3 and 52.2% of the total variance (including random effects 23.8–77.1%). The diameter of the occluded branch and the stem radial increment played dominant roles as predictors. Larger branches showed a significantly longer occlusion time, a larger occluded branch radius, a steeper insertion angle, and a higher loose knot portion. Simulations showed a biologically reasonable overall behavior of the models. The residual variation was tolerable for integrating the models into a growth simulation system. 相似文献
56.
This study quantifies biomass, aboveground and belowground net productivity, along with additional environmental factors over a 2-3 year period in Barnawapara Sanctuary of Chhattisgarh, India through satellite remotesensing and GIS techniques. Ten sampling quadrates20×20, 5×5 and 1×1 m were randomly laid for overstorey (OS), understorey (US) and ground vegetation(GS), respectively. Girth of trees was measured at breast height and collar diameters of shrubs and herbs at 0.1 m height. Biomass was estimated using allometric regression equations and herb biomass by harvesting. Net primary productivity (NPP) was determined by Ssumming biomass increment and litter crop values. Aspect and slope influenced the vegetation types, biomass and NPP in different forests. Standing biomass and NPP varied from 18.6 to101.5 Mg ha-1 and 5.3 to 12.7 Mg ha-1 a-1, respectively,in different forest types. The highest biomass was found in dense mixed forest, while net production recoded in Teak forests. Both were lowest in degraded mixed forests of different forest types. OS, US and GS contributed 90.4, 8.7and 0.7%, respectively, for the total mean standing biomass in different forests. This study developed spectral models for the estimation of biomass and NPP using Normalized Difference Vegetation Index and other vegetation indices.The study demonstrated the potential of geospatial tools for estimation of biomass and net productivity of dry tropical forest ecosystem. 相似文献
57.
Stem analysis data of 432 trees were obtained from even-aged, pure natural stands of Calabrian pine in the central Mediterranean Region of Turkey. Eight dynamic site equations derived with the Generalized Algebraic Difference Approach (GADA) were compared, based on autoregressive analysis and a thorough evaluation of the goodness of fit. We used generalized nonlinear least squares methods for model fitting. The adjusted coefficients of determination (0.9825–0.9842), root-mean-square errors (0.8004–0.8435 m), and Akaike’s information criterion differences (0–145) indicated a good fit of the eight site index equations. The Hossfeld equation (M3) provided the best result. The Durbin-Watson test statistic did not reveal an autocorrelation issue while the Hossfeld equation provided a satisfactory solution to the serial correlation problem in stem analysis data as time series using autoregressive modeling. This study presents new site index models for Calabrian pine forests in the central Mediterranean region of Turkey where it is the most important commercial tree species. The site index equation, based on the Hossfeld model is recommended for height growth prediction and site classification of Calabrian pine stands in the central Mediterranean region of Turkey, providing a new basis for growth prediction and yield estimation in these important forest ecosystems. 相似文献
58.
59.
张家口市区环境空气质量现状及其发展对策初探 总被引:2,自引:0,他引:2
空气质量状况如何,不仅影响到人们的生活质量,同时也影响到当地经济的发展。为了更进一步探讨张家口市区环境质量状况及其发展,从市环保局监测站了解到,张家口市区空气质量污染程度严重,主要污染物是降尘、总悬浮颗粒物及燃煤、化工企业及车辆排放的有毒有害气体。因此,要达到城市环境的可持续发展,就必须加强生态建设,搞好城市绿化,从根本上改善空气质量。 相似文献
60.
论述了环境伦理学指导我国园林发展的必要性,以及环境伦理学对我国园林发展的启示,从而对环境伦理学在园林事业发展中的应用进行探讨,提出在城市发展决策和风景园林规划设计,以及在相关法律、法规和行政机构、社会结构中确立环境伦理学价值观的重要意义。 相似文献