全文获取类型
收费全文 | 2410篇 |
免费 | 119篇 |
国内免费 | 216篇 |
专业分类
林业 | 210篇 |
农学 | 307篇 |
基础科学 | 55篇 |
247篇 | |
综合类 | 1325篇 |
农作物 | 164篇 |
水产渔业 | 61篇 |
畜牧兽医 | 179篇 |
园艺 | 82篇 |
植物保护 | 115篇 |
出版年
2024年 | 9篇 |
2023年 | 42篇 |
2022年 | 97篇 |
2021年 | 96篇 |
2020年 | 92篇 |
2019年 | 107篇 |
2018年 | 79篇 |
2017年 | 92篇 |
2016年 | 112篇 |
2015年 | 106篇 |
2014年 | 134篇 |
2013年 | 122篇 |
2012年 | 178篇 |
2011年 | 197篇 |
2010年 | 163篇 |
2009年 | 148篇 |
2008年 | 144篇 |
2007年 | 122篇 |
2006年 | 116篇 |
2005年 | 85篇 |
2004年 | 81篇 |
2003年 | 42篇 |
2002年 | 31篇 |
2001年 | 35篇 |
2000年 | 35篇 |
1999年 | 28篇 |
1998年 | 25篇 |
1997年 | 25篇 |
1996年 | 22篇 |
1995年 | 26篇 |
1994年 | 18篇 |
1993年 | 22篇 |
1992年 | 34篇 |
1991年 | 19篇 |
1990年 | 19篇 |
1989年 | 21篇 |
1988年 | 11篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1981年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有2745条查询结果,搜索用时 31 毫秒
51.
52.
陕西省长江流域生态地貌功能分区与评价 总被引:2,自引:0,他引:2
采用10个主要生物气候指标,结合地形地貌及相的地带性植被和地带性土壤,对陕西省长江流域31个县(市)进行动态聚类综合分析,本区可归并为2个森林植物气候带,5个生态地貌功能区。即:低山丘陵区北亚热带常绿落叶阔叶林生物气候区,包括安康盆地,汉中盆地,热暖温带针阔混交林、针叶林生物气候区;包括秦岭南坡东部中山,低山区,秦岭南坡中、西部中、高山区和巴山北坡中山、低山。各生态地貌功能区的生物气候特征明显,符 相似文献
53.
54.
Altitudinal gradient incorporates multiple resource gradients, which vary continuously in different fashions. It is important
to study the mountain floristic patterns along altitudinal gradients, which reveal the regular pattern of the flora along
the environmental gradients, the changing trend of biodiversity patterns along the altitudinal gradient, and relevance of
biological fitness. To explore the compositional characteristics and ecological significance of floristic patterns along altitudinal
gradient in China National Nature Reserve of Dawei Mountain in the southeast of Yunnan Province, field investigations have
been made to the flora along the two routes of the southwest slope and the northeast slope of the said reserve, including
a vertical vegetation transect. Meanwhile, further investigations have also been made to the flora of Dawei Mountain, which
has been accounted for in the literature, as Flora Yunnan, The Seed Plant in Yunnan, and so on. The structural characteristics of the flora and the altitudinal distribution pattern of its floristic components
were analyzed. By applying systematic cluster analysis, the altitudinal position of the dividing line of floristic changes
along altitudinal gradient was detected, and the effects of the montane climate on the vertical variation of floristic composition
were studied. Conclusions were put forward. This paper can be summarized as follows: (1) The obvious boundary that differentiates
tropical floristic elements is located at the altitude of approximately 1,500 m; it is reasonable to stipulate the boundary
line between tropical rain forests and the evergreen broad-leaved forests. (2) The vertical vegetation spectrum made by cluster
analysis shows that humid rain forests are below 700 m a.s.l, montane rain forests are between 700 and 1,500 m, monsoon evergreen
broad-leaved forests are between 1,300 and 1,800 m, and montane mossy evergreen broad-leaved forests are above 1,800 m a.s.l.
Nonrepresentative montane mossy dwarf forests (above 2,100 m) in the area are distributed in windward sides and in barren
land on the mountain slopes.
__________
Translated from Acta Phytoecologica Sinica, 2005, 29(6): 894–900 [译自: 植物生态学报] 相似文献
55.
56.
本文对黄土残塬沟壑区人工沙棘林单株根系进行了系统研究。结果表明:1.主根龄是影响沙棘单株根系间模糊聚类的决定性因素,幼龄期单株根系间极易聚合,随着主根龄增加,同龄单株根系间差异逐渐加大而较难聚合。2.用灰色Verhoulst模型对单株沙棘根量、根长的变化过程进行描述和预测,当主根龄达到17龄左右时,根系基本停止生长。3.沙棘根量、根长在土体中的垂直分布规律呈双曲函数关系,各实测值与计算值拟合非常贴切。4.沙棘根系密集层随主根龄的增加而加深,最终稳定在0-50cm左右,粗壮根系多集中在该层。文中还建立了沙棘根量与叶量的线性回归方程。 相似文献
57.
雾灵山自然保护区森林群落的聚类与排序 总被引:3,自引:2,他引:3
采用有聚合的最长距离法、离差平方和法和PCA排序,并结合定性分析,对雾灵山自然保护区的森林植被进行了分类。结果将69个样地划分为12个群系和24个群丛组。 相似文献
58.
What is new about new forest owners? A typology of private forest ownership in Austria 总被引:1,自引:5,他引:1
With structural changes in agriculture, new types of forest owners have become increasingly important. This article develops
an empirically-based typology of forest owners in Austria. Based on a representative survey and by means of cluster analysis,
seven types of forest owners are identified. These types form a sequence, ranging from forest owners with a strong agricultural
background to forest owners with no agricultural background at all. The latter exhibit markedly different behaviour in various
respects, e.g. in their interest in forest-related information. The increasing number of ‘new’ forest owners raises important
questions for forest policy, especially how policy instruments can reach these owners and how extension services can address
them. 相似文献
59.
Different methodological approaches from the field of spatial statistics, the index of cluster size (ICS) and quadrat methods such as the two-term and three-term local quadrat variance (TTLQV and 3TLQV) and the new local variance (NLV) were tested to find a simple spatial measure to classify mixed coniferous uneven-aged, even-aged and conversion stands in the central Black Forest area of Germany. Altogether six stands were analysed with regularly distributed sample plots of 0.25 ha (50×50 m), each subdivided into 25 quadrats of 10×10 m. In each of the quadrats, diameter at breast height (dbh) for trees of the overstory (dbh>7 cm) was assessed and classified into three diameter classes. Height measurements were used to develop specific stand height curves for each stand and to calculate the standing volume per tree and per quadrat. The even-aged stands showed a regular distribution of the standing volume, while the conversion and uneven-aged stands were more clustered. This was detected using ICS, which proved to be a simple but very efficient measure for stand structure. The ICS also showed a highly random distribution of small and medium trees and a regular distribution of large trees of the overstory in the uneven-aged stand. Large and medium trees of one even-aged stand were also regularly distributed while conversion stands showed a regular, random or slightly clustered distribution of these trees. The more uneven the ages in the stands were, the larger were the phases detected by the NLV. The findings of the ICS were generally supported by the TTLQV and 3TLQV. The more uneven the ages in a stand were, the less clustered were the trees of different sizes of the understory. Clustering also decreased with increasing height of understory trees. The patterns detected in the investigated stands were related to the effect of different management regimes. Implications for the management of conversions stands based on the findings of the study are given. 相似文献
60.
本研究采用聚合酶链式反应-变性梯度凝胶电泳(PCR-DGGE)技术结合DGGE图谱条带的克隆和测序比较了绵羊瘤胃、网胃、瓣胃和皱胃菌群的多样性,同时对菌群进行了聚类分析和主成分分析。结果表明:绵羊瘤胃、网胃、瓣胃和皱胃内容物DGGE图谱的平均条带数分别为18、13、16和15条;瘤胃和瓣胃内菌群的多样性指数、均匀度和丰富度均较高,分别为2.83、0.79、17.00和2.82、0.79、16.80,而网胃和皱胃内容物较低,分别为2.52、0.70、12.60和2.73、0.76、15.40;聚类分析结果显示不同胃的内容物菌群具有一定差异,但不同动物个体相同胃的内容物相似性系数均高于0.63;PCR-DGGE图谱中测序的条带大多数归为拟杆菌门(Bacteroidetes)和厚壁菌门(Firmicutes),优势菌群为厚壁菌门(Firmicutes)细菌、拟杆菌门(Bacteroidetes)细菌、未培养瘤胃细菌(uncultured rumen bacterium)、未培养细菌(uncultured bacterium)和韦荣球菌科(Veillonellaceae)细菌,特异性细菌为不动杆菌属(Acinetobacter sp.)细菌和瘤胃球菌属(Ruminococcaceae)细菌。结果提示,绵羊4个胃中均含有种类丰富和数量巨大的细菌,且随着消化道部位由前往后的顺序,菌群的多样性呈现先高后降低再升高的趋势。 相似文献