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   2117篇
  免费   123篇
  国内免费   75篇
林业   863篇
农学   46篇
基础科学   15篇
  139篇
综合类   948篇
农作物   38篇
水产渔业   29篇
畜牧兽医   115篇
园艺   77篇
植物保护   45篇
  2024年   17篇
  2023年   52篇
  2022年   59篇
  2021年   68篇
  2020年   83篇
  2019年   91篇
  2018年   31篇
  2017年   81篇
  2016年   89篇
  2015年   74篇
  2014年   141篇
  2013年   142篇
  2012年   177篇
  2011年   221篇
  2010年   197篇
  2009年   178篇
  2008年   133篇
  2007年   131篇
  2006年   94篇
  2005年   55篇
  2004年   40篇
  2003年   34篇
  2002年   28篇
  2001年   17篇
  2000年   13篇
  1999年   13篇
  1998年   14篇
  1997年   8篇
  1996年   5篇
  1995年   11篇
  1994年   4篇
  1992年   3篇
  1991年   1篇
  1990年   1篇
  1989年   1篇
  1988年   2篇
  1987年   1篇
  1984年   1篇
  1983年   2篇
  1982年   1篇
  1980年   1篇
排序方式: 共有2315条查询结果,搜索用时 0 毫秒
51.
江门市东湖公园北园区绿化景观设计   总被引:1,自引:0,他引:1  
江门市东湖公园北园区是东湖公园的新扩区,在绿化景观设计上,突出自身特点,营造自然、鲜明的绿化景观,并与其它园区相协调和互相衔接,构建了优美的城市景观。  相似文献   
52.
白马雪山国家级自然保护区位于迪庆州德钦县和维西县境内,总面积281640hm^2。详细介绍了保护区对当地社区在生产、生活、旅游、宗教等方面产生的影响,分析了资源保护和利用之间的矛盾,提出了协调不同利益群体的对策。  相似文献   
53.
我省天保工程公益林建设项目作业设计存在问题与对策   总被引:2,自引:0,他引:2  
我省天保工程实施两年来,公益林建设项目作业设计率已达95%以上,然而还存在着一些单位领导对作业设计重要性认识不到位等诸多问题,据此提出了今后若干对策。  相似文献   
54.
Environmental factors, such as global solar radiation, wind speed, air temperature, humidity, and CO2 concentration, were measured above and within the canopy of a tropical rainforest in Lambir Hills National Park, Sarawak, Malaysia. Few data concerning the environment of this forest have been reported. Intensive observations were carried out in 1998, 1999, and 2000 with the following results: (1) The fraction of global solar radiation reaching the upper layer of the canopy varied with global solar radiation above the canopy. Even though the global solar radiation above the canopy fluctuated, the fraction of that reaching the lower canopy and the ground was constantly approximately 5%. (2) The fraction of wind speed reaching each layer of the canopy increased with wind speed above the canopy. Little wind was usually present at the lower canopy. (3) The daytime air temperature at the canopy top was higher than that near the ground. The maximum difference between the air temperature at the canopy top and that at the ground was about 5°C, and the diurnal temperature ranges at the canopy top and those at the ground were about 8°C and about 5°C, respectively. The highest daytime water vapor pressure occurred within the canopy and particularly near the ground. Vertical gradients of water vapor pressure during the day were steep, probably because of high transpiration. (4) In the 1998 observation the minimum and the maximum CO2 concentrations were 360 ppm in the day and 450 ppm at night, while in the 2000 observation the minimum and the maximum CO2 concentrations were 350 ppm in the day and 540 ppm at night. The higher CO2 concentration in the daytime and the lower concentration at night observed during the 1998 observation period were probably due to reduced photosynthesis and soil respiration caused by exceptional dry conditions during the observation period.  相似文献   
55.
图牧吉国家级自然保护区鸟类资源   总被引:1,自引:0,他引:1  
文章阐述了图牧吉国家级自然保护区地理位置、鸟类种类、区系成分特征、国家重点保护种类、生态分布及迁徙时间、途径等。  相似文献   
56.
首次报道了凤头百灵在甘肃民勤连古城国家级自然保护区有分布。该鸟为留鸟型,主要栖息在白刺群落、红砂群落、盐爪爪群落等荒漠植被中,取食荒漠植物种子及荒漠林昆虫,对控制荒漠林害虫的发生、维持荒漠生态系统平衡具有重要作用。该鸟的发现,使该自然保护区鸟类的种类由原来的66种增加到现在的67种。  相似文献   
57.
The management of burnt wood after a fire may affect seed predation by vertebrates due to the change produced in habitat structure. We analyze the effect of burnt wood management on post-dispersal seed predation in the Holm oak. Three plots were established in a burnt forest, with three treatments per plot: (1) non-intervention (NI, all trees left standing), (2) “partial cut plus lopping” (PCL, felling 90% of trees, cutting their main branches, leaving all the biomass in situ), and (3) “salvage logging” (SL, felling the logs for their removal and masticating the woody debris). Acorns were buried to mimic dispersal by jays or rodents two and three years after fire, with two trials per year (7200 monitored acorns), and the predation rate was evaluated until the time of seedling emergence. The spatial patterns of acorn predation were assessed by computing a transformed-Ripley's K function and Moran's I correlograms. There was a large spatial and temporal variability in acorn predation, with differences among trials, plots, and replicates within treatments and plots. Overall, PCL showed the lowest predation values (83.0% versus 87.4 in NI and 88.0 in SL). Predator species (mice versus wild boar) also differed among treatments, wild boar having a negligible effect in PCL, presumably due to the physical barrier of felled logs and branches. The results support that: (1) salvage logging offers no advantage against predators and (2) that post-fire burnt wood management alters the guild of acorn predators and may reshape the pattern of seedling establishment.  相似文献   
58.
The suitability of plantation monocultures for the conservation of forest animals is an issue under continous debate. The adaptability of forest dwellers and the forest management regime seem to play key roles. In this study, I investigated the habitat selection of a generalist bird, the great spotted woodpecker (Dendrocopos major), within a pine (Pine spp.) monoculture, as well as the importance of the native habitat features within the pine matrix for the species’ conservation. I compared 52 plots with woodpecker presence against 121 plots where the species was absent, as well as 68 nest-trees against 90 random ones. Regression analyses were used to investigate the habitat attributes involved in the habitat selection. Although the great spotted woodpecker is considered a generalist forest dweller, it shows a marked habitat selection. Based on presence/absence records, the woodpecker prefers well-forested patches with high levels of tree diversity and with good coverage of a secondary species such as the strawberry tree (Arbutus unedo). To excavate their nests, woodpeckers select large trees in patches where other trees are also larger, rejecting patches with a high number of small trees. The most striking conclusion from this work is the preference shown for native trees, especially Portuguese oaks (Quercus faginea), as nest-trees. This is noteworthy because native trees are smaller and they are surrounded by smaller trees than pines. These findings support that woodpecker conservation benefits from an increase of habitat heterogeneity, particularly by the retention of native woodland patches within the plantation matrix.  相似文献   
59.
Trees are able to respond to their local biotic and abiotic environment with morphological adjustments which improve resource acquisition and, thus, growth. In forests, light is broadly recognised as one of the major factors determining growth, and morphological responses comprise changes in crown architecture and stem stature. On sloping terrain, the interplay of phototropism and gravitropism may further affect morphological growth characteristics. However, different tree species are expected to show species-specific responses. In this study, we analysed three growth characteristics of tree individuals belonging to four species of two functional groups (evergreen: Schima superba, Castanopsis eyrei, deciduous: Quercus serrata var. brevipetiolata, Castanea henryi) in a species-rich Chinese subtropical forest. Crown projection area, relative crown displacement and stem inclination were related to biotic (local species richness, functional richness, competition, stand age) and abiotic (slope aspect and inclination, soil depth) variables in the local neighbourhood of the tree individuals. We hypothesised that (i) there are species-specific differences in the morphological response of crown architecture and stem stature and (ii) that crown size and asymmetry as well as stem inclination are influenced by both, biotic and abiotic factors. In contrast to our expectations we were unable to reveal any species-specific differences in any of the three growth characteristics. The results of mixed effects models showed that crown area was mainly affected by the target tree's dbh and biotic variables related to neighbours (competition, functional diversity), whereas stem inclination was mainly influenced by slope. Relative crown displacement was influenced by both, biotic and abiotic variables. We conclude that growth responses resulting in crown displacement and stem inclination seem to be an important mechanism to ameliorate foraging for light in our study area, but that these responses appear to be species-independent. The interplay of stem inclination and crown displacement allows for a plastic response of tree individuals in biotically and abiotically heterogeneous environments. Our results indicate that forest management in this region should focus on functionally diverse stands which are promoting crown area positively resulting in increased growth rates of individual trees.  相似文献   
60.
采用定点观察、影像调查、追踪观察、瞬时扫描等方法对云南省昆明市翠湖公园内越冬红嘴鸥的日间行为进行调查研究,归纳并建立了红嘴鸥日间行为谱及其PAE编码系统。共记录到红嘴鸥9大类44种行为,类别包括非飞行类运动、迁飞、觅食、通讯、集群、清洁、停歇、竞争对抗、避害。结合人为干扰及斑块化生境中红嘴鸥的行为,在其行为生态及行为功能基础上进行分类。结果表明,人为因素是影响红嘴鸥在翠湖越冬期间行为的主要因素。  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