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   2105篇
  免费   66篇
  国内免费   36篇
林业   717篇
农学   60篇
基础科学   12篇
  249篇
综合类   847篇
农作物   134篇
水产渔业   16篇
畜牧兽医   53篇
园艺   99篇
植物保护   20篇
  2024年   4篇
  2023年   33篇
  2022年   29篇
  2021年   24篇
  2020年   31篇
  2019年   40篇
  2018年   18篇
  2017年   44篇
  2016年   55篇
  2015年   65篇
  2014年   97篇
  2013年   111篇
  2012年   123篇
  2011年   217篇
  2010年   183篇
  2009年   220篇
  2008年   175篇
  2007年   173篇
  2006年   155篇
  2005年   92篇
  2004年   58篇
  2003年   44篇
  2002年   42篇
  2001年   27篇
  2000年   21篇
  1999年   14篇
  1998年   19篇
  1997年   18篇
  1996年   13篇
  1995年   13篇
  1994年   8篇
  1993年   12篇
  1992年   6篇
  1991年   9篇
  1990年   3篇
  1989年   2篇
  1988年   2篇
  1987年   3篇
  1986年   1篇
  1984年   1篇
  1983年   1篇
  1980年   1篇
排序方式: 共有2207条查询结果,搜索用时 31 毫秒
101.
湖南野生观赏植物及其园林应用   总被引:4,自引:0,他引:4  
湖南坐落在温带和中亚热带的植物过渡地区,拥有维管束植物5 500余种(含种下单位)。以现有欧亚和北美栽培的观赏植物为基础,湖南有2 000多种野生植物可作为栽培观赏植物,有很高的观赏价值和选育潜力。根据它们的观赏特性、抗寒性、选育潜力与园林应用的不同,可将湖南观赏植物划分为10类。将观赏植物引入园林建设,对创建地带性、个性化的园林景观具有特别重要的意义。  相似文献   
102.
对海南 9种优势树种构筑型进行研究 ,通过对树木分枝方式和发芽方式的细致观察和描述 ,发现有 1种属Massart型 ,1种属Troll型 ,7种属Rauh型。对这 9种树木的构筑型特征与国外的研究进行了比较 ,确定了构筑模式与属及发芽模式与属之间的关系。  相似文献   
103.
选取 2种遮荫条件下 (8%、2 5 %全光照 )西双版纳地区热带雨林上层树种滇南红厚壳和中层树种玉蕊的幼苗 ,测定了雨季末自然降温过程中 2种植物叶片光合特性、抗氧化酶 (SOD、CAT、APX)活性、丙二醛 (MDA)含量的变化。研究结果表明 :低温使 2种植物单位面积叶绿素含量和净光合速率明显下降 ,生长在中等光强下的幼苗比低光强下的幼苗下降的比率大 ,滇南红厚壳比玉蕊下降的比率大。在降温初期 ,即从 10月到 12月初 ,2种光强下 2种植物SOD和APX活性上升 ,CAT活性出现波动 ,黎明光化学效率和MDA含量基本没有变化 ,光合机构没有受到氧化破坏。 12月末 ,中等光强下滇南红厚壳黎明光化学效率降低 ,SOD和APX活性有所下降 ,MDA含量显著上升 ;而低光强下的滇南红厚壳和 2种光强下玉蕊SOD和APX活性仍在上升 ,MDA含量变化较小。这表明 ,2种光强下 2种热带树苗是以不同的方式响应自然降温 ,低温对中等光强下的植物比低光强下的影响大 ,上层树种滇南红厚壳受到的影响比中层树种玉蕊大。同时 ,根据降温过程中抗氧化酶活性和MDA含量的变化 ,大致可将西双版纳热带雨林幼苗对低温的响应分为 2个阶段 ,即第 1阶段为低温适应阶段 ,抗氧化酶活性上升 ,光合机构没有受到明显的氧化破坏 ;第 2阶段 ,中等光强下滇南红厚壳抗氧化酶  相似文献   
104.
孙克君  林鸿辉 《广东园林》2007,29(B09):99-101
介绍园林有机废物变栽培基质的原因、过程,产品应用实例,表明园林有机废物变栽培基质是节约型园林绿化发展的新途径,具有较好的社会效益和生态效益。  相似文献   
105.
西双版纳望天树林的群落组成和结构特征研究   总被引:1,自引:0,他引:1  
利用1992~1993年在西双版纳自然保护区的勐腊县补蚌片区建立的5块望天树林固定样地的观测资料,从物种组成、科属区系特征、水平布局和垂直布局方面对该群落的组成和结构特征进行了研究,采用α多样性指数及联合熵H(X,Y)分析了望天树林群落结构的多样性;采用β多样性指数比较了样地间的多样性。结果表明:西双版纳望天树林物种相当丰富,在5块50 m×50 m的固定样地内计有种子植物68科176属380种。该群落中热带科有57科,占89.1%,热带属有163属,占92.6%,反映出其强烈的热带性质。根据树高的分布情况,将该群落的地上垂直结构分为5层,即计有乔木3层,灌木层和草本层。而对群落内物种的水平分布格局所进行的检测结果显示,所选择的3个主要树种(望天树、木奶果、假海桐)在5块固定样地内均为典型的集群分布。联合熵分析表明,其群落结构的多样性指数为15.800 8~21.005 3。  相似文献   
106.
The coastal forest of Gabon abounds in monospecific secondary Aucoumea klaineana stands derived from natural regeneration after shifting cultivation. This paper aims to describe the changes in the structure and dynamics of these stands with age. It then assesses the impact of selective thinning in the upper storey on both structural and dynamic parameters.

The experiment consisted of 34 Permanent Plots in stands from establishment to more mature stages (ca. 50 years old). Thirteen plots (17–45 years old) were thinned. More than 80% of the removal came from supernumerary dominant A. klaineana.

A. klaineana represented 60% of the total density in stands ca. 15 years old but >90% of the dominant trees in older stands. The changes with age in the floristic composition of the unthinned stands showed three successional stages during which pioneer species associated with A. klaineana (from establishment to ca. 15 years) were progressively replaced by mature forest species.

Basal area increased and density decreased with age before reaching stable values at ca. 40–45 years. Mortality was very high in young stands but decreased in the older ones. Mortality generally affected small diameter individuals in the dominated storey. Diameter and basal area increments showed that the stand growth resulted from the growth of dominant A. klaineana. Diameter increments of A. klaineana were elevated during the first years of colonisation (1.9 cm/year) and were still ca. 0.7 cm/year for 50-year-old dominant trees.

Thinning did not increase the mortality of the dominant population. It favoured the individual growth of A. klaineana. The gain was substantial for dominated trees and small dominant trees (from 60 to 100%) but was lower for large dominant trees (ca. 25–30%). Therefore, stimulation of individual growth did not compensate for the loss of basal area at the stand level.  相似文献   

107.
Coppicing is an alternative reforestation tool for teak in the tropics, substantially reducing regeneration time/costs and associated demands for labor and seed when available. Growth rates of coppiced material are rapid in most situations, but stem/wood quality is a frequent concern. We compared coppiced teak plantations to paired seed-origin plantations, at ages 3, 8, and 13 years, on Forest State Corporation managed land located in Java, Indonesia. Teak trees were evaluated for height, diameter, lower-bole straightness, and presence of disease in both plantation types and at three ages. Mean height and diameter of trees in coppiced plantations were both significantly greater than that in their paired seed-origin plantations at all three ages. Furthermore, heights and diameters in coppiced plantations were higher than expected based on established growth tables for Java. Coppiced plantations were less symptomatic of disease than seed-origin plantations, which promises better wood production and quality. Lower-bole stems in coppiced plantations developed less straight than those in seed-origin plantations, but these deviations faded with time and will likely become insignificant within a 60-year rotation. Based on these results from existing coppiced plantations in Java, coppiced plantations can make a major contribution to teak production in Indonesia.  相似文献   
108.
In July 1992 the Rondônia Agroforestry Pilot Project (RAPP) was launched in two agricultural municipalities (Nova União and Alto Paraiso) in the western Brazilian Amazon State of Rondônia. The purpose of the RAPP was to assess the conditions under which colonist farmers in the western Amazon would integrate agroforestry plantings into their small-scale farming systems and to assess the performance of those plantings over time. An experimental group consisting of 50 small-scale farmers was selected to participate. Plots were designed to accommodate between 3 and 25 different species, each producing one or more commodities with local market potential (hardwood, fruits, nuts, latexes, oils). Farmers planted seedlings typically on a 1-ha plot, located and designed by each farmer with the advice of a professional Brazilian extensionist. During the first phase of the project (1992–1998), the growth performance of the seedlings and changes in household characteristics were monitored on an annual basis. By 2002, 32 (64.0%) of the original 50 agroforest plots were found in place. This paper updates the research findings based on a 2002 follow-up visit to these 32 farms. In addition to growth performance, the authors’ found that 17.95% of the farms in the neighboring control group had planted trees and other agroforest crops between 1992 and 2002, compared to only 5.38% of farms outside the project area, suggesting spontaneous diffusion. The authors also found a potentially synergistic relationship between agroforestry and secondary forest regeneration with the use of satellite image analysis. The experience of the RAPP indicates that colonist farmers in Amazonia can be successful managers of agroforest plots with minimal external inputs over the long-term (10 years).  相似文献   
109.
牡丹观赏园的绿化构思   总被引:3,自引:1,他引:3  
花卉专类园的提出对生态园林的建设,经济可持续发展,生物多样性保护等诸方面具有重要的现实意义。本文介绍了牡丹观赏园规划设计应重点把握的原则,提出了具体的绿化模式。  相似文献   
110.
用可持续发展观对建设生态园林城市工作进行分析,探讨我省创建园林城市和生态园林城市的思路和方法。  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