全文获取类型
收费全文 | 10034篇 |
免费 | 532篇 |
国内免费 | 466篇 |
专业分类
林业 | 3656篇 |
农学 | 327篇 |
基础科学 | 208篇 |
796篇 | |
综合类 | 4311篇 |
农作物 | 341篇 |
水产渔业 | 36篇 |
畜牧兽医 | 316篇 |
园艺 | 791篇 |
植物保护 | 250篇 |
出版年
2024年 | 32篇 |
2023年 | 122篇 |
2022年 | 215篇 |
2021年 | 240篇 |
2020年 | 215篇 |
2019年 | 273篇 |
2018年 | 175篇 |
2017年 | 311篇 |
2016年 | 374篇 |
2015年 | 347篇 |
2014年 | 569篇 |
2013年 | 532篇 |
2012年 | 810篇 |
2011年 | 845篇 |
2010年 | 767篇 |
2009年 | 782篇 |
2008年 | 659篇 |
2007年 | 715篇 |
2006年 | 550篇 |
2005年 | 423篇 |
2004年 | 287篇 |
2003年 | 249篇 |
2002年 | 206篇 |
2001年 | 193篇 |
2000年 | 145篇 |
1999年 | 140篇 |
1998年 | 124篇 |
1997年 | 97篇 |
1996年 | 90篇 |
1995年 | 93篇 |
1994年 | 90篇 |
1993年 | 76篇 |
1992年 | 72篇 |
1991年 | 54篇 |
1990年 | 62篇 |
1989年 | 35篇 |
1988年 | 36篇 |
1987年 | 17篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1983年 | 1篇 |
1978年 | 1篇 |
1963年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Informal green spaces (IGS) such as overgrown vacant lots and urban brownfields constitute a considerable amount of green resources in the city. Given that the increasingly competitive land use of the urban area, enhancing the potential ecosystem services (ES) of IGS through design and management practices is of critical importance. This literature review paper provides an overview of ES delivered by urban IGS: What types of IGS have been discussed? Which ES have been identified in IGS, what interrelationships among different types of ES have been identified, and how can the potential of IGS be enhanced by urban design practices? 112 scientific papers were analyzed for their 1) IGS terms applied, 2) ES studied, 3) current or potential ES discussed, 4) ES trade-offs, and 5) ES assessment methods. Through the review, we found that although different types of ES have been identified in IGS, most studies did not consider ES synergies and trade-offs. The few studies assessing trade-offs of ES in IGS mostly focused on large-scale IGS such as urban brownfields rather than on small-scale IGS such as vacant lands/lots. The literature review highlights two knowledge gaps for future research: the first one is to explore the design and management knowledge that integrate multiple ES in small-scale IGS based on the assessment of potential ES trade-offs and synergies; the second one is to develop the spatial assessment of ES trade-offs and synergies, which is the key to envision design and management interventions that optimize the benefits of IGS. The literature review promotes the acknowledgement of the term IGS through highlighting their value in ES provisioning and further outlines future research directions on small-scale IGS such as vacant lands/lots and patches of spontaneous vegetation. 相似文献
152.
153.
154.
Thomas Thomidis 《Phytoparasitica》2002,30(2):191-193
The virulence ofPhytophthora citrophthora isolated from various host-plants on three peach rootstocks (GF677, PR204, KID I) was examined. There was no significant
difference among the rootstocks with respect to their susceptibility to testedP. citrophthora isolates. The most virulent isolate originated from sycamore (Acer pseudoplatanus); isolates from pistachio trees (Pistacia vera) also showed high virulence but were significantly less virulent than the sycamore isolate. Isolates originating from plum
(Prunus domestica), almond (Prunus amygdalus) and lemon (Citrus limon) trees were moderately virulent on peach rootstocks; those from cyclamen (Cyclamen persicum) showed the lowest virulence of those tested. There was thus great variation in virulence among the testedP. citrophthora isolates. It is possible that the isolates ofP. citrophthora from sycamore, pistachio, plum, almond and lemon trees are a threat to peach trees, whereas the low virulence of the isolates
from cyclamen hosts suggests that these pathogens are not a serious threat to peach trees.
http://www.phytoparasitica.org posting Jan. 3, 2002. 相似文献
155.
孙洪斌 《内蒙古林业调查设计》2013,36(3)
文章在阐述速生树种良种繁育基地建设必要性的基础上,确定了建设目标和建设布局,区划良种培育工程区,规划了主要技术措施。 相似文献
156.
157.
Johnson Chris J. Boyce Mark S. Mulders Robert Gunn Anne Gau Rob J. Cluff H. Dean Case Ray L. 《Landscape Ecology》2004,19(8):869-882
Multiscale analyses are widely employed for wildlife-habitat studies. In most cases, however, each scale is considered discrete and little emphasis is placed on incorporating or measuring the responses of wildlife to resources across multiple scales. We modeled the responses of three Arctic wildlife species to vegetative resources distributed at two spatial scales: patches and collections of patches aggregated across a regional area. We defined a patch as a single or homogeneous collection of pixels representing 1 of 10 unique vegetation types. We employed a spatial pattern technique, three-term local quadrat variance, to quantify the distribution of patches at a larger regional scale. We used the distance at which the variance for each of 10 vegetation types peaked to define a moving window for calculating the density of patches. When measures of vegetation patch and density were applied to resource selection functions, the most parsimonious models for wolves and grizzly bears included covariates recorded at both scales. Seasonal resource selection by caribou was best described using a model consisting of only regional scale covariates. Our results suggest that for some species and environments simple patch-scale models may not capture the full range of spatial variation in resources to which wildlife may respond. For mobile animals that range across heterogeneous areas we recommend selection models that integrate resources occurring at a number of spatial scales. Patch density is a simple technique for representing such higher-order spatial patterns. 相似文献
158.
Chris J. Johnson Mark S. Boyce Robert Mulders Anne Gunn Rob J. Gau H. Dean Cluff Ray L. Case 《Landscape Ecology》2005,19(8):869-882
Multiscale analyses are widely employed for wildlife-habitat studies. In most cases, however, each scale is considered discrete and little emphasis is placed on incorporating or measuring the responses of wildlife to resources across multiple scales. We modeled the responses of three Arctic wildlife species to vegetative resources distributed at two spatial scales: patches and collections of patches aggregated across a regional area. We defined a patch as a single or homogeneous collection of pixels representing 1 of 10 unique vegetation types. We employed a spatial pattern technique, three-term local quadrat variance, to quantify the distribution of patches at a larger regional scale. We used the distance at which the variance for each of 10 vegetation types peaked to define a moving window for calculating the density of patches. When measures of vegetation patch and density were applied to resource selection functions, the most parsimonious models for wolves and grizzly bears included covariates recorded at both scales. Seasonal resource selection by caribou was best described using a model consisting of only regional scale covariates. Our results suggest that for some species and environments simple patch-scale models may not capture the full range of spatial variation in resources to which wildlife may respond. For mobile animals that range across heterogeneous areas we recommend selection models that integrate resources occurring at a number of spatial scales. Patch density is a simple technique for representing such higher-order spatial patterns. 相似文献
159.
Recent research shows that land use history is an important determinant of current ecosystem function. In the United States,
characterization of land use change following European settlement requires reconstruction of the original property mosaic.
However, this task is difficult in unsystematically surveyed areas east of the Appalachian Mountains. The Gwynns Falls watershed
(Baltimore, MD) was originally surveyed in the 1600-1700s under a system of warrants and patents (commonly known as ‘metes
and bounds’). A method for the reconstruction and mapping of warrant and patent properties is presented and used to map the
original property mosaic in the Gwynns Falls watershed. Using the mapped mosaic, the persistence of properties and property
lines in the current Gwynns Falls landscape is considered. The results of this research indicate that as in agricultural areas,
the original property lines in the Gwynns Falls watershed are persistent. At the same time, the results suggest that the property
mosaic in heavily urbanized/suburbanized areas is generally ‘reset.’ Further, trends in surveying technique, parcel size,
and settlement patterns cause property line density and property shape complexity to increase in the less urbanized upper
watershed. The persistence of original patterns may be damping expression of heterogeneity gradients in this urban landscape.
This spatial pattern of complexity in the original mosaic is directly opposite of hypothesized patterns of landscape heterogeneity
arising from urbanization. The technique reported here and the resulting observations are important for landscape pattern
studies in areas settled under unsystematic survey systems, especially the heavily urbanized areas of the eastern United States.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
160.
We evaluated support for four alternate hypotheses explaining the distribution of breeding Brown-headed Cowbirds (Molothrus ater) in forests at varying distances from the forest edge in three Midwestern USA landscapes with varying amounts of forest fragmentation
(core forest area ranged from 5 to 70%). We focused on breeding cowbirds’ use of forest because of the risk of nest parasitism
to forest-dwelling hosts and to identify factors affecting breeding cowbird habitat selection. We compared distances of cowbird
locations in the forest from the forest edge (“edge distances”) to distances of random forest locations in the entire landscape
or within individual cowbird home ranges. We analyzed 1322 locations of 84 cowbirds across three landscapes. We found support
for the landscape context hypothesis that breeding cowbird preference for forest edge varied with landscape context. Ninety
percent of cowbird locations were within 150–350 m of forest edge, despite the overall availability of forest at greater distances
from edge (as far as 500–1450 m) both within cowbird home ranges and the entire forested landscape. Cowbird preference for
edge varied by landscape context largely due to differences in the availability of forest edge. In a highly fragmented forest
cowbirds utilized the entire forest and likely viewed it as “all edge.” In less fragmented forests, cowbirds preferred edge.
We consider how variation in cowbird edge preference might relate to patterns in host abundance, host diversity, and host
quality because cowbird movements indicate they are capable of using forest farther from edges. 相似文献