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   3882篇
  免费   724篇
  国内免费   314篇
林业   956篇
农学   122篇
基础科学   170篇
  1624篇
综合类   1029篇
农作物   55篇
水产渔业   49篇
畜牧兽医   409篇
园艺   100篇
植物保护   406篇
  2024年   3篇
  2023年   128篇
  2022年   212篇
  2021年   182篇
  2020年   195篇
  2019年   132篇
  2018年   127篇
  2017年   191篇
  2016年   283篇
  2015年   167篇
  2014年   195篇
  2013年   286篇
  2012年   332篇
  2011年   312篇
  2010年   251篇
  2009年   273篇
  2008年   285篇
  2007年   284篇
  2006年   232篇
  2005年   173篇
  2004年   151篇
  2003年   107篇
  2002年   72篇
  2001年   60篇
  2000年   36篇
  1999年   39篇
  1998年   34篇
  1997年   22篇
  1996年   36篇
  1995年   26篇
  1994年   22篇
  1993年   8篇
  1992年   14篇
  1991年   17篇
  1990年   5篇
  1989年   18篇
  1988年   7篇
  1987年   2篇
  1985年   1篇
排序方式: 共有4920条查询结果,搜索用时 15 毫秒
1.
Snow cover is an important water source for vegetation growth in arid and semi-arid areas,and grassland phenology provides valuable information on the response of terrestrial ecosystems to climate change.The Mongolian Plateau features both abundant snow cover resources and typical grassland ecosystems.In recent years,with the intensification of global climate change,the snow cover on the Mongolian Plateau has changed correspondingly,with resulting effects on vegetation growth.In this study,using MOD10A1 snow cover data and MOD13A1 Normalized Difference Vegetation Index(NDVI)data combined with remote sensing(RS)and geographic information system(GIS)techniques,we analyzed the spatiotemporal changes in snow cover and grassland phenology on the Mongolian Plateau from 2001 to 2018.The correlation analysis and grey relation analysis were used to determine the influence of snow cover parameters(snow cover fraction(SCF),snow cover duration(SCD),snow cover onset date(SCOD),and snow cover end date(SCED))on different types of grassland vegetation.The results showed wide snow cover areas,an early start time,a late end time,and a long duration of snow cover over the northern Mongolian Plateau.Additionally,a late start,an early end,and a short duration were observed for grassland phenology,but the southern area showed the opposite trend.The SCF decreased at an annual rate of 0.33%.The SCD was shortened at an annual rate of 0.57 d.The SCOD and SCED in more than half of the study area advanced at annual rates of 5.33 and 5.74 DOY(day of year),respectively.For grassland phenology,the start of the growing season(SOS)advanced at an annual rate of 0.03 DOY,the end of the growing season(EOS)was delayed at an annual rate of 0.14 DOY,and the length of the growing season(LOS)was prolonged at an annual rate of 0.17 d.The SCF,SCD,and SCED in the snow season were significantly positively correlated with the SOS and negatively correlated with the EOS and LOS.The SCOD was significantly negatively correlated with the SOS and positively correlated with the EOS and LOS.The SCD and SCF can directly affect the SOS of grassland vegetation,while the EOS and LOS were obviously influenced by the SCOD and SCED.This study provides a scientific basis for exploring the response trends of alpine vegetation to global climate change.  相似文献   
2.
Coal mining has led to serious ecological damages in arid desert region of Northwest China. However, effects of climatic factor and mining activity on vegetation dynamics and plant diversity in this region remain unknown. Wuhai City located in the arid desert region of Northwest China is an industrial city and dominated by coal mining. Based on Landsat data and field investigation in Wuhai City, we analyzed the vegetation dynamics and the relationships with climate factors, coal mining activity and ecological restoration projects from 2000 to 2019. Results showed that vegetation in Wuhai City mostly consisted of desert plants, such as Caragana microphylla, Tetraena mongolica and Achnatherum splendens. And the vegetation fractional coverage(VFC) and greenness rate of change(GRC) showed that vegetation was slightly improved during the study period. Normalized difference vegetation index(NDVI) was positively correlated with annual mean precipitation, relative humidity and annual mean temperature, indicating that these climate factors might play important roles in the improved vegetation. Vegetation coverage and plant diversity around the coal mining area were reduced by coal mining, while the implementation of ecological restoration projects improved the vegetation coverage and plant diversity. Our results suggested that vegetation in the arid desert region was mainly affected by climate factors, and the implementation of ecological restoration projects could mitigate the impacts of coal mining on vegetation and ecological environment.  相似文献   
3.
土壤电导率是表征土壤水溶性盐的一个重要指标,可反映土壤盐渍化程度。为了研究高寒草甸退化对土壤电导率的影响,以三江源区未退化高寒草甸和退化高寒草甸为研究对象,系统分析了退化高寒草甸的植被特征和土壤特征与土壤电导率的相互关系。结果表明:高寒草甸退化会对土壤电导率产生显著负影响,且土壤电导率与评价高寒草甸的退化指标植被盖度、地上生物量、土壤有机质、土壤全氮含量表现出一致的变化趋势。因而,认为高寒草甸的退化是会引起土壤电导率的变化,土壤电导率作为土壤盐渍化程度的衡量指标,亦可作为评价草甸退化的客观指标之一。  相似文献   
4.
整理福建省4个有居民海岛和13个无居民海岛的植被现状,从地理环境影响和人为活动影响两方面综述福建海岛植被变化及面临的问题,包括土地风沙化严重,防护林效能衰退,园林植物生长遭受威胁,自然灾害破坏植物生境,以及海岛城市扩张,外来植物入侵威胁等。提出提高生态保护意识,推进海岛植被修复工作,发展生态旅游业,改善海岛防护林体系等福建海岛植被资源保护与开发策略。  相似文献   
5.
  1. Humanity is facing a biodiversity crisis, with freshwater-associated biodiversity in a particularly dire state. Novel ecosystems created through human use of mineral resources, such as gravel pit lakes, can provide substitute habitats for the conservation of freshwater and riparian biodiversity. Many of these artificial ecosystems are subject to a high intensity of recreational use, however, which may limit their biodiversity potential.
  2. The species richness of several taxa (plants, amphibians, dragonflies, damselflies, waterfowl, and songbirds) was assessed and a range of taxonomic biodiversity metrics were compared between gravel pit lakes managed for recreational fisheries (n = 16) and unmanaged reference lakes (n = 10), controlling for non-fishing-related environmental variation.
  3. The average species richness of all the taxa examined was similar among lakes in both lake types and no substantial differences in species composition were found when examining the pooled species inventory. Similarly, there were no differences between lake types in the presence of rare species and in the Simpson diversity index across all of the taxa assessed.
  4. Variation in species richness among lakes was correlated with woody habitat, lake morphology (surface area and steepness), and land use, but was not correlated with the presence of recreational fisheries. Thus, non-fishing-related environmental variables had stronger effects on local species presence than recreational fisheries management or the presence of recreational anglers.
  5. Collectively, no evidence was found that anglers and recreational fisheries management constrain the development of aquatic and riparian biodiversity in gravel pit lakes in the study region; however, the conservation of species diversity in gravel pit lakes could benefit from an increasing reliance on habitat enhancement activities.
  相似文献   
6.
基于无人机多时相植被指数的冬小麦产量估测   总被引:1,自引:0,他引:1  
通过无人机搭载多光谱相机,对不同水分亏缺条件下冬小麦多个生育期进行遥感监测,采用不同种类多光谱植被指数表征冬小麦的生长特征,分析了植被指数与冬小麦产量的相关关系,并利用多时相植被指数构建产量估测数据集,采用偏最小二乘回归、支持向量机回归和随机森林回归3种机器学习算法进行冬小麦产量估测。结果表明,随着冬小麦的生长,多个植被指数与产量的相关性不断增强,灌浆末期相关系数达到0.7,植被指数与产量的线性回归决定系数也达到最大。多时相植被指数反映了冬小麦生长的变化特征,进一步提高了冬小麦产量估测精度,采用开花期和灌浆初期的多时相植被指数进行估产比采用单个生育期的植被指数估测产量的精度高,采用偏最小二乘回归模型的估测精度R2提高约0.021,支持向量机回归模型R2提高约0.015,随机森林回归模型R2提高约0.051。采用灌浆末期的多时相植被指数,3种模型均有较高的估测精度,偏最小二乘回归模型估测精度最高时的R2、RMSE分别为0.459、1 822.746 kg/hm2,支持向量机回归模型估测精度最高时的R2、RMSE分别为0.540、1 676.520 kg/hm2,随机森林回归模型估测精度最高时的R2、RMSE分别为0.560、1 633.896 kg/hm2,本文数据集训练的随机森林回归模型估测精度最高,且稳定性更好。  相似文献   
7.
工程扰动严重破坏原有地形地貌和生态植被,加剧生态环境的恶化与生态系统的退化,为减少水土流失和浅层滑坡,加速植被恢复和生态修复,对工程扰动区植被群落恢复进行研究是比较有意义的课题。研究选择生态治理后处于自然恢复期的石葵河作为研究对象,对工程扰动区不同水位的植被恢复进行季节性监测和实地调查,统计河道边坡植被恢复种类及数量、植被群落类型及其结构特征。经调查统计,常水位以上植被种类为43种,常水位以下只有17种;常水位以上Pielou均匀度指数为0.774,而常水位以下Pielou均匀度指数仅为0.397;常水位以上基本形成乔灌草垂直分布的植被群落结构,常水位以下全部是草本植物。说明水位变化对工程扰动区植被恢复有很大影响,水位变化不仅对植被恢复种类和数量有影响,而且还影响植被的生活型和生长型。  相似文献   
8.
基于不同植被指数的TRMM数据降尺度及误差校正研究   总被引:1,自引:0,他引:1  
[目的] 对不同时间尺度的热带测雨卫星(TRMM)数据进行空间降尺度及误差校正研究,为华中地区洪涝灾害监测等提供科学参考。[方法] 主要借助增强型植被指数(EVI)和归一化植被指数(NDVI)分别运用地理加权回归(GWR)模型实现2001—2019年华中地区TRMM数据的空间降尺度,并结合地理差异分析(GDA)和地理比率分析(GRA)对年、季和月的降尺度结果进行误差校正,通过气象站数据对校正前后的数据进行对比分析。[结果] ① TRMM数据和气象站数据的决定系数(R2)在年(0.630)、季(0.710~0.865)和月(0.637~0.875)尺度都表明了TRMM数据在华中地区具有较好的适用性;②通过GWR模型实现了TRMM数据空间分辨率由0.25°到1 km的降尺度转换,且TRMMEVI数据精度优于TRMMNDVI数据,说明华中地区TRMM数据与EVI的关系比NDVI更为密切;③对优选的TRMMEVI数据分别进行GDA,GRA校正,结果表明GDA校正结果优于GRA校正,且在降雨量越多的月份校正效果越好。[结论] 在华中地区,EVI比NDVI更加适合TRMM数据降尺度研究。降尺度数据采用GDA校正比GRA校正效果更为显著。  相似文献   
9.
祁连山东段高寒植被类型对土壤理化特征的影响   总被引:1,自引:1,他引:0  
[目的]探讨祁连山东段不同高寒植物的土壤理化特征,为区域水资源合理利用提供理论依据。[方法]对祁连山东段6种灌丛植被和高寒草地的土壤基本性状、土壤持水能力和土壤渗透性进行了相关指标的测定。[结果]①祁连山东段高寒植被的土壤容重随着土层深度的增加而增大,土壤含水量则随着土层深度的增加而降低。②祁连山东段土壤总孔隙度随着土层深度的增加而减小,毛管孔隙和非毛管孔隙无明显的垂直变化规律;③祁连山东段土壤最大持水量随着土层深度的增加而递增,土壤毛管持水量表现为先减少后增加的变化趋势;土壤非毛管持水量变化规律则不太明显。④祁连山东段土壤初渗透率(0.58~2.81 mm/min)高于平均渗透率(0.05~1.26 mm/min)和稳渗透率(0.04~1.31 mm/min),6种高寒植被的土壤初渗透率表现为高寒草地>山生柳>硬叶柳>绣线菊>千里香杜鹃>金露梅>头花杜鹃;平均渗透率表现为绣线菊>金露梅>千里香杜鹃>山生柳>硬叶柳>高寒草地>头花杜鹃;稳渗透率表现为绣线菊>金露梅>千里香杜鹃>山生柳>硬叶柳>头花杜鹃灌丛>高寒草地。[结论]研究区域高寒植物类型的不同会对该区域土壤理化特征产生不同影响,即土壤理化特征与植物之间存在相互联系与相互作用的关系。  相似文献   
10.
林分空间结构会显著影响其林下幼树更新。为明确其影响林下幼树更新的关键因子,以湖南省青羊湖国有林场青冈栎林天然次生林为研究对象,测量幼树株数密度、树高、地径和冠幅,计算混交度、大小比数、开敞度指数以及聚集指数代表林分空间结构,评价林下幼树更新情况,并用灰色关联度分析方法探究两者间的相关关系。结果表明,青羊湖国有林场13块青冈栎林样地林下更新幼树有6种,分属于4科5属,分别是青冈栎、苦槠、多脉青冈、南酸枣、黄檀、马尾松,青冈栎在更新幼树中占明显优势,是优势树种,幼树重要值为63.09%。青冈栎林各林分空间结构指数与林下更新指数均大于0.5,关联度较高,综合所有林分空间结构指数对林下更新指数关联度的均值来看,对林下更新的影响由大到小为大小比数>开敞度>聚集指数>混交度,大小比数对青冈栎林下更新影响最为显著。在促进青冈栎天然次生林林下更新时,应以调整林木大小分化程度为主,综合考虑林木空间分布格局的调整方案。  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