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   5677篇
  免费   402篇
  国内免费   1547篇
  2024年   6篇
  2023年   84篇
  2022年   112篇
  2021年   152篇
  2020年   147篇
  2019年   275篇
  2018年   235篇
  2017年   225篇
  2016年   211篇
  2015年   271篇
  2014年   310篇
  2013年   417篇
  2012年   419篇
  2011年   399篇
  2010年   274篇
  2009年   381篇
  2008年   383篇
  2007年   402篇
  2006年   383篇
  2005年   352篇
  2004年   306篇
  2003年   294篇
  2002年   301篇
  2001年   178篇
  2000年   170篇
  1999年   152篇
  1998年   111篇
  1997年   72篇
  1996年   100篇
  1995年   125篇
  1994年   82篇
  1993年   61篇
  1992年   48篇
  1991年   39篇
  1990年   27篇
  1989年   22篇
  1988年   17篇
  1987年   20篇
  1986年   11篇
  1985年   7篇
  1984年   12篇
  1983年   10篇
  1982年   13篇
  1981年   4篇
  1980年   1篇
  1979年   3篇
  1978年   1篇
  1958年   1篇
排序方式: 共有7626条查询结果,搜索用时 15 毫秒
1.
Understanding the state of forest resource utilization in China and correctly evaluating the role and function of forest resources in national economic development are essential for realizing balanced development of forests, the environment, and the economy. This is especially true given the present situation of increasingly scarce forest resources. Using data from Chinese forest industry statistical yearbooks, forestry development reports, and other documents, this paper examines the current state of forest resource utilization in China from the angle of combining quantities and values based on input–output tables. We show that demand for and input use of forest resources varies greatly across industrial sectors; the paper products and furniture manufacturing industries have the greatest direct consumption coefficient for timber use. When considering direct and indirect demand, it is clear that forest resources restrict different industrial sectors in diverse ways. These results provide an important set of reference values regarding the utilization of forest resources and coordinated industrial development in China.  相似文献   
2.
华北地区针叶林下凋落物层化学性质的研究   总被引:10,自引:0,他引:10  
森林凋落物中贮积了大量的有机、无机养分物质,它是森林土壤自然肥力的重要来源之一。在森林生态系统中,养分物质的内部循环主要是通过凋落物来实现的。在分解、转化过  相似文献   
3.
Middle Permian Pseudoalbaillella from the Xiaodong area in South China were studied using fossil morphology. Short forms of Pseudoalbaillella fusiformis (Holdsworth and Jones), Pseudoalbaillella internata Wang, and Pseudoalbaillella monacantha (Ishiga and Imoto), characterized by a short and skirt-like pseudoabdomen, were confirmed. We conclude that the short forms were derived from their respective species by breaking off during disruptive processes. If this is true, there may be structurally weakened parts in the pseudoabdomen.  相似文献   
4.
5.
Building resource-conserving and environmental-friendly society (referred to as “two-oriented society”, TOS) is an important way proposed by the Chinese government to achieve sustainable development. In this paper, a pilot city of constructing TOS in China-Wuhan is taken as a case to evaluate the performance of TOS from 2005 to 2012. Treating the indicators of TOS as multi-dimensional vectors, this paper proposes a methodological framework by integrating the methods of vector angle and Euclidean distance to measure the angle and distance between the vector of annual status of TOS and the vector of planning target of TOS. Based on this, the paper employs coordination (the angle between the two vectors) and effectiveness (the distance between the two vectors) to describe the performance of TOS and its subsystems, including economic development (ED), social and people's well-being (SW), resource consumption (RC), resource recycling (RR), environmental quality (EQ) and pollution control (PC). Moreover, grey relational analysis approach is used to analyse the core factors influencing TOS construction. Results reveal that the performance of TOS in Wuhan is continually improved in the study period while the performance of its subsystems shows several differences: (1) both coordination and effectiveness of TOS, ED and SW keep on improving, while those of EQ fluctuate; (2) the effectiveness of both RR and PC shows a downward trend while the coordination of them fluctuates; (3) the actual value of RC indicators reaches the planning targets. Based on the analysis of determinants, we suggest that strengthening technological ability and adding investment are extremely important to improve the performance of RC and PC. For the sake of improving RR, it is critical to provide more government public expenditure and encourage financial institutions to provide more loans to stimulate and support the businesses. Moreover, increasing the amount of R&D funding and maintaining steady external economic environment are proved to be effective to improve all subsystems of TOS.  相似文献   
6.
7.
8.
殷嘉迪  董金玮  匡文慧  崔耀平  江东 《生态学报》2020,40(17):5904-5914
改革开放以来中国经济和城市化的快速发展促使生产和生活空间挤占大量生态空间,系统认识和评估我国生态空间演化的宏观格局和过程对于生态文明建设具有重要的理论和现实意义。为揭示全国生态空间变化的时空过程,在对生态空间内涵进行界定的基础上,结合全国尺度时序土地利用数据构建生态空间分类体系,并评估1990-2015年中国生态空间演化特征。结果表明:1990-2015年中国生态用地面积持续减少,主要向半生态用地转变,发生在重要的粮食生产区域及周边;半生态用地面积波动明显,前期主要表现为不断扩张,后期大量转换为弱生态用地,发生在主要城市群地区;弱生态用地持续扩张,与城镇化进程不断加速相关。中国生态空间演变过程表现出一定的区域差异和梯度效应,不同区域生态空间变化发生的拐点时间有所不同,呈现"自东向西、由南到北"的3级梯度特征,区域生态空间状况与经济发展战略及生态空间管控具有较强的相关性。本研究对于国家生态空间管控近远期战略方案制订具有一定启示,建议处于不同梯度的各地区应基于区域生态空间演化所处阶段和不同驱动机制,确定分区域生态空间安全红线目标和生态空间管控方案。  相似文献   
9.
记述我国鞘蛾科一新种:中华金鞘蛾Goniodoma sinica sp. nov.。这是金鞘蛾属在我国的首次报道,对属征和世界名录进行了扼要介绍。  相似文献   
10.
Aim Data and analyses of elevational gradients in diversity have been central to the development and evaluation of a range of general theories of biodiversity. Elevational diversity patterns have, however, been severely understudied for microbes, which often represent decomposer subsystems. Consequently, generalities in the patterns of elevational diversity across different trophic levels remain poorly understood. Our aim was to examine elevational gradients in the diversity of macroinvertebrates, diatoms and bacteria along a stony stream that covered a large elevational gradient. Location Laojun Mountain, Yunnan province, China. Methods The sampling scheme included 26 sites spaced at elevational intervals of 89 m from 1820 to 4050 m elevation along a stony stream. Macroinvertebrate and diatom richness were determined based on the morphology of the specimens. Taxonomic richness for bacteria was quantified using a molecular fingerprinting method. Over 50 environmental variables were measured at each site to quantify environmental variables that could correlate with the patterns of diversity. We used eigenvector‐based spatial filters with multiple regressions to account for spatial autocorrelation. Results The bacterial richness followed an unexpected monotonic increase with elevation. Diatoms decreased monotonically, and macroinvertebrate richness showed a clear unimodal pattern with elevation. The unimodal richness pattern for macroinvertebrates was best explained by the mid‐domain effect (r2 = 0.72). The diatom richness was best explained by the variation in nutrient supply, and the increase in bacterial richness with elevation may be related to an increased carbon supply. Main conclusions We found contrasting patterns in elevational diversity among the three studied multi‐trophic groups comprising unicellular and multicellular aquatic taxa. We also found that there may be fundamental differences in the mechanisms underlying these species diversity patterns.  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