首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   21976篇
  免费   375篇
  国内免费   12篇
林业   3888篇
农学   1530篇
基础科学   187篇
  3718篇
综合类   1187篇
农作物   2314篇
水产渔业   2176篇
畜牧兽医   3723篇
园艺   1253篇
植物保护   2387篇
  2022年   40篇
  2021年   65篇
  2020年   113篇
  2019年   111篇
  2018年   2831篇
  2017年   2806篇
  2016年   1300篇
  2015年   177篇
  2014年   137篇
  2013年   277篇
  2012年   1040篇
  2011年   2421篇
  2010年   2327篇
  2009年   1456篇
  2008年   1587篇
  2007年   1814篇
  2006年   286篇
  2005年   329篇
  2004年   314篇
  2003年   373篇
  2002年   273篇
  2001年   154篇
  2000年   171篇
  1999年   120篇
  1998年   61篇
  1997年   44篇
  1996年   45篇
  1995年   46篇
  1994年   52篇
  1993年   70篇
  1992年   93篇
  1991年   83篇
  1990年   88篇
  1989年   85篇
  1988年   58篇
  1987年   65篇
  1986年   48篇
  1985年   62篇
  1984年   68篇
  1983年   58篇
  1982年   57篇
  1981年   39篇
  1979年   42篇
  1978年   47篇
  1977年   50篇
  1976年   47篇
  1975年   38篇
  1974年   43篇
  1972年   35篇
  1969年   43篇
排序方式: 共有10000条查询结果,搜索用时 765 毫秒
821.
Forest commons are regarded as a means to support local development and sustainable forest conditions. To evaluate the development impact of Swedish forest commons, comparative surveys have been undertaken in three regions, and the differences in forest condition and management between categories of commons as well as their relation to other forest ownerships have been assessed. Regional differences between the by-laws, historical development and geographical conditions are apparent. It is concluded that two of three regions have an overly restrictive harvesting policy given the purpose of the forest commons and the official forest policy. The study results underline the importance of evaluation of the performance of forest management in relation to management objectives, to ownership alternatives and to the impact of local variations in preconditions.  相似文献   
822.
In the past, utilization of bamboo resources in China has been traditionally dominated by direct consumption of local farmers as minor forest products with weak linkage with market. In recent years, the over-supply of grains and rapid degradation of agricultural environment call for alternative crops that can be developed through integrating the environmental plantation with the market demands. Closely associated with forestry and agriculture, bamboo is able to deal with the new challenges which China‘s agriculture is facing. Of 534 documented bamboo species in China, 153 species produce edibleshoots and of which 56 species are recommended for agricultural plantation; 139 species provide timbers and of which 58 species recommended; 116 species can be splited as good strips for weaving and of which 22 species recommended; 88 species are considered as garden bamboos and of which 34 species recommended; 45 species are able to produce paper pulp and of which 18 species recommended.  相似文献   
823.
Fast-growing hybrid poplar trees (Populus trichocarpa Torr. & A. Gray x P. deltoides Bartr. ex Marsh.) were compared with slower-growing parental types in both field and laboratory experiments to determine physiological components of leaf growth that could be closely related to biomass production. Stem volume was correlated with individual leaf area (r = 0.81) and leaf growth rate (r = 0.82). Hybrids had a greater total leaf area, not because they produced more leaves, but because they had larger leaves than either parental type. The greater leaf size of the hybrids may be explained by inheritance of larger cell number from P. deltoides and larger cell size from P. trichocarpa. Rates of enlargement of isolated leaf discs in liquid culture were approximately 50% of those observed in intact leaves of field-grown plants.  相似文献   
824.
We evaluated differences between the forest floors and the establishment and growth of coniferous seedlings in fenced (13 years) and unfenced plots on Mt Ohdaigahara where the sika deer (Cervus nippon) population density is high. Large coniferous seedlings (height > 0.05 m) were less abundant in the unfenced plot, as a result of deer browsing. Small coniferous seedlings (height < 0.05 m), however, were more abundant in the unfenced plot, where most seedlings of Abies homolepis were found on bare ground and those of Picea jezoensis var. hondoensis were found on buttresses and fallen logs. The large area of bare ground in the unfenced plot was caused by deer browsing. Deer therefore have an indirect effect on the emergence and growth of small coniferous seedlings by modifying the forest floor.  相似文献   
825.
In order to develop a fireproof wooden material, the synergic effect of fire-retardant chemicals and wood coatings was studied. The fire performance was evaluated by cone calorimeter. Impregnation of fire retardants including polyphosphatic carbamate, and ceramic coatings including alkoxy metal salt improved the fire performance of wooden materials. This treatment made it possible to meet the guidelines for fire performance of noncombustible materials in Japan. In addition to the vacuum-pressure impregnation treatment, hot-and-cold-bath impregnation treatment is an effective way to develop fire-retardant wood by impregnating fire retardant and ceramic coating. The weatherability of the developed material was also investigated. The ceramic coating was resistant to light and moisture. Part of this report was presented at 54th Annual Meeting (Sapporo, August 2004) and the 55th Annual Meeting (Kyoto, March 2005) of the Japan Wood Research Society  相似文献   
826.
This study is part of an effort to improve the quality of dried shiitake mushrooms (Lentinula edodes) in accordance with consumer preference. Results are presented of a trend survey that examined the preferences of consumers using questionnaires combined with sensory evaluation. From the original preference (OP) survey, the distributions of OP divided by age class (AC) were statistically equal to the results obtained in 2000. The medians of OP for teenagers and twenties were “neutral” and “slightly like,” respectively. A firm preference change occurred between teenagers and those in their twenties. It is empirically realized that sensory intensity (SI) scores that were too high or too low led to a low hedonic preference (HP) score. The same tendency was seen for “neutralists” and “likers.” In addition, almost all distributions concerned with HP had no significant difference between 2000 and 2005; for example, that of SI divided by AC. These results showed that the preference for dried shiitake mushroom has been unaffected by the passage of the past 5 years. Dried shiitake mushrooms have been used from ancient times and for many dishes; therefore, the overall preference appears to have remained unchanged.  相似文献   
827.
世界贸易组织(简称WTO),作为处理世界自由贸易机制的国际组织,允许为可持续发展和保护自由贸易和环境选择利用世界自然资源。中国作为世界贸易组织成员国,签定入世议定书,入世后中国在林业的可持续发展方面受到世界贸易组织的条约和原则的制约和挑战。中国的林业资源应当作为基础资源加以保护,政府的行政管理职能在世界贸易组织的原则下加以改革。因此,中国林业的可持续发展的目.标是优先处理环境保护和经济发展的关系。在入世条件下,按照世界惯例,中国林业资源可持续发展问题瞄准中国林业体制改革,以便适用中国天然林业战略,依靠科技进步依法保护中国环境,特别是在中国林业保护和处理环境和自然资源等关系方面。依据世界贸易组织的协议和规则,建立林业的生态化体系,尤其是GATT20条的规定,保护中国林业可持续发展,为中国林业创建国际环境。表1参21。  相似文献   
828.
The optimal formula of Beta-Cypermethrin aqueous capsule suspension was screened out through the experiment,preparation and manufacture of basic formulas, which has no sedimentation, suspended drops of oil, peels and agglomerating and the diameter of capsule is in range of 10-30μm, conformed to the technical requirement. The aftereffect of the 3.3% Beta-Cypermethrin aqueous capsule suspension which was manufactured according to the optimal formula was up more than 25 days. The experiments on controlling the larvae of Dendrolinmus superans Butler and the adults of Xylotrechus rusticus L were carried out with different concentrations of this chemical. The death rate reached 80% when 250 times solution of the chemical was sprayed on stem to control the larvae of D. superans. For control of the adults of X. rusticus, 200, 400 and 600 times solution of the chemical were applied and their control effects (death rate) reached 85.23%, 74.21% and 66.59% respectively. Two kinds of solution (200 times and 300 times) of the chemical were used to control the larvae of D. superans in large area, and the control effect for both concentrations was over 90%.  相似文献   
829.
This review details the Korean species of the Eilema group in the subfamily Arctiinae of Erebidae: 19 species in five genera (Manulea, Collita, Katha, Dolgoma, and Wittia), a new species (Collita hwacheonensis Bayarsaikhan & Bae, sp. nov.), and an unrecorded species [C. vetusta aegrota (Butler 1877)]. All species are diagnosed, and figures of the adults and genitalia and a key to species based on male genital structure of Eilema group in South Korea are included.  相似文献   
830.
Little is known about the extent of work-related accidents occurring during the performance of forestry work by the non-industrial private forest owner or their assistant (hereafter called self-employed NIPF). The absence of an effective accident registration system largely excludes this group from accurate representation in official statistics. In this study, data from 1996 to 2001 were collected from hospital injury records managed by the Umeå Accident Analysis Group at the University Hospital in Umeå. During that period, it was found that 485 persons in the Umeå region were given medical attention due to injuries possibly acquired during self-employed forestry work. Questionnaires about the accidents were sent to each of the 485 injured persons and 80% were returned. Responses revealed that 225 of the respondents were injured during self-employed forestry work. Out of these, 81% performed fewer than 31 days of forestry work annually. Due to the injury, 29% had been on sick leave for some period of time and 42% had persistent symptoms. On average, each injury led to 13 days of sick leave and 24 hours of institutional care. Firewood production was shown to be a key factor behind the large number of accidents for self-employed NIPF. Further studies on the lack of knowledge about how the work should be done and conscious risk taking should be considered as an important link to the development of effective accident prevention strategies.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号