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   441篇
  免费   141篇
  国内免费   29篇
  2024年   4篇
  2023年   26篇
  2022年   31篇
  2021年   36篇
  2020年   31篇
  2019年   33篇
  2018年   24篇
  2017年   29篇
  2016年   28篇
  2015年   27篇
  2014年   23篇
  2013年   40篇
  2012年   17篇
  2011年   31篇
  2010年   11篇
  2009年   24篇
  2008年   26篇
  2007年   24篇
  2006年   26篇
  2005年   19篇
  2004年   16篇
  2003年   20篇
  2002年   10篇
  2001年   7篇
  2000年   8篇
  1999年   12篇
  1998年   6篇
  1997年   8篇
  1996年   3篇
  1995年   1篇
  1994年   3篇
  1993年   2篇
  1992年   1篇
  1991年   1篇
  1989年   2篇
  1978年   1篇
排序方式: 共有611条查询结果,搜索用时 312 毫秒
31.
Larger and more frequent disturbances are motivating efforts to accelerate recovery of foundational perennial species by focusing efforts into establishing island patches to sustain keystone species and facilitate recovery of the surrounding plant community. Evaluating the variability in abiotic and biotic factors that contribute to differences in survival and establishment can provide useful insight into the relative importance of these factors. In the western United States, severe degradation of the sagebrush steppe has motivated substantial efforts to restore native perennial cover, but success has been mixed. In this study, we evaluated survival of more than 3,000 sagebrush seedlings transplanted on 12 patches totaling 650 ha within a 113,000 ha burn area, and related the survival to organismal and subtaxonomic traits, and to landscape variables. Big sagebrush has high intraspecific diversity attributed to subspecies and cytotypes identifiable through ultraviolet (UV)‐induced fluorescence, length:width of leaves, or genome size (ploidy). Of these organismal traits, survival was related only to UV fluorescence, and then only so when landscape variables were excluded from analyses. The most significant landscape variable affecting survival was soil taxonomic subgroup, with much lower survival where buried restrictive layers reduce deep water infiltration. Survival also decreased with greater slope steepness, exotic annual grass cover, and burn severity. Survival was optimal where perennial bunchgrasses comprised 8–14% of total cover. These soil, topographic, and community condition factors revealed through monitoring of landscape‐level treatments can be used to explain the success of plantings and to strategically plan future restoration projects.  相似文献   
32.
Gravel augmentation is often applied to rivers and streams to rehabilitate salmonid spawning and incubation habitat. However, the effect of gravel size on salmon spawning utilization and embryo survival during incubation is not well understood. We conducted an experiment on a regulated and previously mined Northern California salmonid‐bearing stream in which different sized gravel (small, medium, and large) patches were placed into the stream's degraded spawning reach. We documented Oncorhynchus tshawytscha (Chinook salmon) spawning activity within the three gravel sizes for two seasons. In addition, we deployed Chinook salmon embryos into each gravel size patch and allowed them to incubate until estimated emergence time. Although all experimental gravel sizes were predicted to be within the spawning population's mobilization capabilities, model results indicated the probability of salmon building redds decreased as substrate size increased. Conversely, embryo survival increased as gravel size increased. A possible mechanism of disparate Chinook salmon embryo survival is provided by an observed decrease in embryo survival correlating with greater presence of embryo predators (leeches), which are associated with smaller gravel. Our results indicate a parent‐offspring conflict in optimal spawning gravel size for Chinook salmon, and suggest that an intermediate gravel size would maximize overall reproductive success across both spawning and incubation life stages.  相似文献   
33.
  • In degraded dryland systems, native plant community re‐establishment following disturbance is almost exclusively carried out using seeds, but these efforts commonly fail. Much of this failure can be attributed to the limited understanding of seed dormancy and germination traits.
  • We undertook a systematic classification of seed dormancy of 26 species of annual and perennial forbs and shrubs that represent key, dominant genera used in restoration of the Great Basin ecosystem in the western United States. We examined germination across a wide thermal profile to depict species‐specific characteristics and assessed the potential of gibberellic acid (GA3) and karrikinolide (KAR1) to expand the thermal germination envelope of fresh seeds.
  • Of the tested species, 81% produce seeds that are dormant at maturity. The largest proportion (62%) exhibited physiological (PD), followed by physical (PY, 8%), combinational (PY + PD, 8%) and morphophysiological (MPD, 4%) dormancy classes. The effects of chemical stimulants were temperature‐ and species‐mediated. In general, mean germination across the thermal profile was improved by GA3 and KAR1 for 11 and five species, respectively. We detected a strong germination response to temperature in freshly collected seeds of 20 species. Temperatures below 10 °C limited the germination of all except Agoseris heterophylla, suggesting that in their dormant state, the majority of these species are thermally restricted.
  • Our findings demonstrate the utility of dormancy classification as a foundation for understanding the critical regenerative traits in these ecologically important species and highlight its importance in restoration planning.
  相似文献   
34.
The Yangtze (Changjiang) river-floodplain is one of the most important ecosystems in China and the world, but is seriously threatened by multiple stresses. Thus, it is crucial and urgent to rehabilitate and conserve the river-floodplain. This paper reviews ecological studies conducted on the Yangtze river-floodplain, and presents suggestions for conservation and rehabilitation. First, basic concepts and research advances of riverscape and hydrological connectivity are introduced. Second, the history and current status of the Yangtze River system are summarized. Before 23 Ma, the Yangtze River cut through the Three Gorges, forming the river much like the modern one. Numerous rivers, streams, lakes (the total area 15770 km2 at present) and wetlands are distributed in the mid-lower Yangtze river-flood-plain. Such a river-lake complex ecosystem holds a unique and diverse biota, and is the most important fishery area of China. Third, main threats to the Yangtze river-floodplain ecosystem are identified, i.e., a) habitat loss, including river channelization, sharp shrinkage of lake area (ca. 10000 km2 since the 1950s), degradation of lakeshore zones and sand over-mining; b) alternations of hydrological regimes, including construction of ca. 47000 reservoirs and disconnection of most lakes from the mainstem; c) water pollution, including eutrophication, heavy metals, organic pollutants and microplastic; d) overexploitation of biological resources, including overfishing and intensive pen culture. Fourth, effects of river-lake disconnection on lake ecosystems are summarized. It was found that a) disconnection is one of the main causes of lake eutrophication; b) species diversity, biomass, production of macrophytes and macrobenthos reach maxima at some levels of intermediate river connectivity; c) disconnection greatly reduces fish species richness of each habitat guild, and natural fish larvae is severely depleted; d) disconnection simplifies macroinvertebrate food web structure, and trophic basis is more heavily relied on detritus in disconnected lakes. Last, conservation strategies are proposed. Since the Yangtze river-floodplain is a huge integrated system, the biodiversity conservation must be conducted on the whole basin scale. By establishing species-area models of fishes, the minimum protected area of Yangtze-connected lakes is estimated to be ca. 14400 km2. It means that at least 8900 km2 of disconnected lakes should be reconnected with the Yangtze mainstem, and ecohydrological operation of dams and sluices is the feasible approach. Based upon our studies on environmental flow requirements, the following measures are suggested: a) lower water levels during spring to improve germination of macrophytes, and control rising rates of water levels during spring-summer to ensure development of macrophytes; b) open sluice gates to restore migration routes for juveniles migrating into lakes during April-September, and for adults migrating back to the Yangtze mainstem during November-December. © 2019, Institute of Hydrobiology, Chinese Academy of Sciences. All rights reserved.  相似文献   
35.
为了解决黑果枸杞中花青素稳定性问题,本文采用Box-Behnken设计对黑果枸杞提取物泡腾片配方进行优化,并对其进行质量评价。采用酸碱混合制粒压片法,通过单因素实验,筛选出片剂所需的辅料:崩解剂、填充剂、润滑剂以及甜味剂。采用响应面试验,结合感官评价进行处方优化,从而确定最优配方:柠檬酸32%、黑果枸杞提取物25%、碳酸氢钠24%、乳糖15%、甜蜜素3%、聚乙二醇6000 1%;对最优配方进行质量评价,各项指标均符合规定,其中花青素含量为8.06 mg/g。该泡腾片表面光滑,泡腾效果好,具有黑果枸杞香气,为实际生产提供理论依据。  相似文献   
36.
目的:探讨盐酸多奈哌齐联合综合康复训练对老年痴呆患者认知功能、事件相关电位及血清脑源性神经营养因子(BDNF)、胰岛素样生长因子1(IGF-1)的影响。方法:选取2017年3月~2020年3月期间来我院接受治疗的120例老年痴呆患者,根据随机数字表法分为对照组(n=60,盐酸多奈哌齐治疗)和联合组(n=60,盐酸多奈哌齐联合综合康复训练治疗)。对比两组疗效,治疗前、治疗3个月后的认知功能评分、事件相关电位、血清BDNF和IGF-1水平变化,以及治疗期间不良反应发生情况。结果:联合组的临床总有效率为91.67%,高于对照组的70.00%(P<0.05)。两组治疗3个月后日常生活能力评估量表(ADL)、临床痴呆量表(CDR)评分降低,且联合组低于对照组(P<0.05);简易精神状态量表(MMSE)评分升高,且联合组高于对照组(P<0.05)。两组治疗3个月后N1潜伏期、P2潜伏期均缩短,P3波幅均升高,且联合组治疗3个月后P2潜伏期短于对照组,P3波幅高于对照组(P<0.05);两组治疗3个月后N1潜伏期比较未见统计学差异(P>0.05)。联合组治疗3个月后BDNF水平高于对照组(P<0.05);两组治疗3个月后IGF-1水平比较未见统计学差异(P>0.05)。对照组与联合组不良反应发生率对比无统计学差异(P>0.05)。结论:老年痴呆患者在盐酸多奈哌齐基础上联合综合康复训练,认知功能可得到明显改善,同时还可调节患者事件相关电位及血清BDNF、IGF-1水平。  相似文献   
37.
摘要 目的:观察葛兰心宁软胶囊联合替格瑞洛片治疗冠心病心绞痛的疗效及对心功能和血管内皮功能的影响。方法:选择我院心内科收治的150例冠心病心绞痛患者,按门诊号单双数分为对照组和研究组,各为75例。对照组给予替格瑞洛片治疗,研究组给予葛兰心宁软胶囊联合替格瑞洛片治疗,对比两组疗效、血管内皮功能、心功能、心绞痛发作次数和持续时间以及不良反应发生率。结果:治疗1个月后,研究组的临床总有效率85.33%(64/75)高于对照组的69.33%(52/75),差异有统计学意义(P<0.05)。治疗1个月后,研究组左心室射血分数(LVEF)、心输出量(CO)、每搏输出量(SV)高于对照组(P<0.05)。治疗1个月后,与对照组比较,研究组的心绞痛发作次数更少,持续时间更短(P<0.05)。治疗1个月后,研究组一氧化氮(NO)、血流介导的舒张功能(FMD)高于对照组,内皮素-1(ET-1)、胞间黏附分子-1(ICAM-1)、血管紧张素Ⅱ(Ang Ⅱ)低于对照组(P<0.05)。两组不良反应发生率组间对比无明显差异(P>0.05)。结论:冠心病心绞痛患者经葛兰心宁软胶囊联合替格瑞洛片治疗后,疗效明确,可缓解患者临床症状,促进心功能和血管内皮功能恢复。  相似文献   
38.
摘要 目的:探讨电针阴部神经刺激疗法联合Kegel盆底康复训练对产后压力性尿失禁(SUI)患者盆底肌力、尿流动力学和生活质量的影响。方法:选取2019年6月~2021年11月期间于我院就诊的产后SUI患者109例,按照入院就诊奇偶顺序分为两组,其中对照组54例,接受Kegel盆底康复训练,研究组55例,接受电针阴部神经刺激疗法联合Kegel盆底康复训练。对比两组疗效、漏尿量、尿失禁程度、盆底肌力、尿流动力学和生活质量。结果:研究组的临床总有效率高于对照组(P<0.05)。两组治疗后盆底肌肌力各指标(手测肌力和Ⅰ类肌纤维最大值、Ⅱ类肌纤维平均值)均升高,且研究组高于对照组(P<0.05)。两组治疗后漏尿量、尿失禁程度评分均下降,且研究组低于对照组(P<0.05)。两组治疗后尿流动力学相关指标[腹压漏尿点压(AL-PP)、最大尿流率(Qmax)和最大尿道闭合压力(MUCP)]均升高,且研究组高于对照组(P<0.05)。两组治疗后尿失禁生活质量量表(I-QOL)各维度(限制性行为、心理影响、社交活动受限)评分及总分均升高,且研究组高于对照组(P<0.05)。结论:电针阴部神经刺激疗法联合Kegel盆底康复训练可有效改善产后SUI患者的盆底肌肌力和尿失禁情况,减少漏尿量,同时可促进尿流动力学恢复,进而提高患者的生活质量。  相似文献   
39.
40.
The modification of posture in children using sEMG is a new area that offers much potential to aid disabled children regain function. This article describes a new protocol, the minimax procedure, to aid children regain motor function in sitting. The protocol uses selective reinforcement to encourage recruiting/relaxing target sites (gluteus medius/maximus) in a predetermined way so as to encourage good form in sitting. Children often are unaware how to make changes in posture, and when they do succeed the progress is so slow that sustaining motivation is a challenge. This paper describes a procedure that helps the child recognize when they have achieved the correct posture, immediately rewards them for doing so, and generates data using Boolean operations to numerically substantiate the clinically observed changes in posture.  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