全文获取类型
收费全文 | 19966篇 |
免费 | 273篇 |
国内免费 | 12篇 |
专业分类
林业 | 3892篇 |
农学 | 1421篇 |
基础科学 | 158篇 |
3490篇 | |
综合类 | 919篇 |
农作物 | 2326篇 |
水产渔业 | 2301篇 |
畜牧兽医 | 2450篇 |
园艺 | 1196篇 |
植物保护 | 2098篇 |
出版年
2024年 | 4篇 |
2023年 | 56篇 |
2022年 | 99篇 |
2021年 | 201篇 |
2020年 | 195篇 |
2019年 | 242篇 |
2018年 | 2869篇 |
2017年 | 2828篇 |
2016年 | 1346篇 |
2015年 | 188篇 |
2014年 | 202篇 |
2013年 | 234篇 |
2012年 | 1073篇 |
2011年 | 2414篇 |
2010年 | 2224篇 |
2009年 | 1396篇 |
2008年 | 1488篇 |
2007年 | 1742篇 |
2006年 | 186篇 |
2005年 | 232篇 |
2004年 | 223篇 |
2003年 | 286篇 |
2002年 | 178篇 |
2001年 | 49篇 |
2000年 | 83篇 |
1999年 | 17篇 |
1998年 | 15篇 |
1997年 | 10篇 |
1996年 | 9篇 |
1995年 | 15篇 |
1994年 | 9篇 |
1993年 | 21篇 |
1992年 | 18篇 |
1991年 | 3篇 |
1990年 | 7篇 |
1989年 | 11篇 |
1988年 | 18篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1983年 | 3篇 |
1981年 | 4篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1972年 | 2篇 |
1971年 | 3篇 |
1968年 | 5篇 |
1964年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Mapping hotspots of multiple landscape functions: a case study on farmland afforestation in Scotland 总被引:4,自引:1,他引:4
Many conservation and restoration efforts in developed countries are increasingly based on the premise of recognising and
stimulating more ‘multi-functionality’ in agricultural landscapes. Public policy making is often a pragmatic process that
involves efforts to negotiate trade-offs between the potentially conflicting demands of various stakeholders. Conservationists’
efforts to influence policy making, can therefore benefit from any tool that will help them to identify other socio-economic
functions or values that coincide with good ecological conservation options. Various types of socio-economic objectives have
in recent years been mapped across landscapes and so there are now important opportunities to explore the spatial heterogeneity
of these diverse functions across the wider landscape in search of potential spatial synergies, i.e. ‘multiple win locations’
or multifunctional ‘hotspots’.
This paper explores the potential occurrence of such synergies within the agricultural landscape of northeast Scotland and
evaluates an existing woodland planting policy using and combining three different policy objectives. Our results show that
there are indeed broad areas of the studied landscape where multiple objectives (biodiversity, visual amenity and on-site
recreation potential) could be achieved simultaneously (hotspots), and that the case study which we evaluate (the Farm Woodland
Premium Scheme) could be much better spatially targeted with regards to each individual objective as well as with regards
to these hotspots of multifunctionality.
相似文献
Dan van der HorstEmail: |
992.
Effects of deficit irrigation strategies on cluster microclimate for improving fruit composition of Moscatel field-grown grapevines 总被引:1,自引:0,他引:1
Tiago Pedreira dos Santos Carlos M. Lopes M. Lucília Rodrigues Claudia R. de Souza Jorge M. Ricardo-da-Silva João P. Maroco João S. Pereira M. Manuela Chaves 《Scientia Horticulturae》2007
The grapevine plays a very important role in the economic, social and cultural sectors of many regions; however vineyards are often grown in regions under stressful conditions and thus they are vulnerable to climate change. The objective of this research was to investigate the effect of partial root-zone drying (PRD) irrigation on vine water relations, vegetative growth, plant microclimate, berry composition and yield components, compared to conventional deficit irrigation (DI, 50% ETc), full irrigation (FI, 100% of ETc) and non-irrigated vines (NI). The study was undertaken in mature ‘Moscatel’ grapevines (Vitis vinifera L.) grown in Pegões, South of Portugal. Compared to the other irrigated treatments, PRD vines showed a better microclimate at the cluster zone with higher incident photosynthetic photon flux density (PPFD). Within the more open canopies of NI and PRD treatments, berry temperatures were higher than those of denser ones (DI and FI). Compared to the conventional irrigation technique the better microclimate observed in PRD vines was a consequence of a reduction in vine growth, where lower values of leaf layer number, leaf area, canopy wideness, water shoots and shoot weight were observed. In PRD vines we observed a tendency to a development of a deeper root system, while DI and FI showed a more homogeneous root distribution throughout the different soil layers. PRD showed an improvement in berry quality with higher values of flavour precursors, and total phenols concentration without any significant yield reduction compared to DI and FI. 相似文献
993.
Combining top-down and bottom-up dynamics in land use modeling: exploring the future of abandoned farmlands in Europe with the Dyna-CLUE model 总被引:9,自引:0,他引:9
Land use change is the result of interactions between processes operating at different scales. Simulation models at regional
to global scales are often incapable of including locally determined processes of land use change. This paper introduces a
modeling approach that integrates demand-driven changes in land area with locally determined conversion processes. The model
is illustrated with an application for European land use. Interactions between changing demands for agricultural land and
vegetation processes leading to the re-growth of (semi-) natural vegetation on abandoned farmland are explicitly addressed.
Succession of natural vegetation is simulated based on the spatial variation in biophysical and management related conditions,
while the dynamics of the agricultural area are determined by a global multi-sector model. The results allow an exploration
of the future dynamics of European land use and landscapes. The model approach is similarly suitable for other regions and
processes where large scale processes interact with local dynamics. 相似文献
994.
Randy T. Larsen John A. Bissonette Jerran T. Flinders Mevin B. Hooten Tammy L. Wilson 《Landscape Ecology》2010,25(1):135-145
Free water is considered important to wildlife in arid regions. In the western United States, thousands of water developments
have been built to benefit wildlife in arid landscapes. Agencies and researchers have yet to clearly demonstrate their effectiveness.
We combined a spatial analysis of summer chukar (Alectoris chukar) covey locations with dietary composition analysis in western Utah. Our specific objectives were to determine if chukars
showed a spatial pattern that suggested association with free water in four study areas and to document summer dietary moisture
content in relation to average distance from water. The observed data for the Cedar Mountains study area fell within the middle
of the random mean distance to water distribution suggesting no association with free water. The observed mean distance to
water for the other three areas was much closer than expected compared to a random spatial process, suggesting the importance
of free water to these populations. Dietary moisture content of chukar food items from the Cedar Mountains (59%) was significantly
greater (P < 0.05) than that of birds from Box Elder (44%) and Keg-Dugway (44%). Water developments on the Cedar Mountains are likely
ineffective for chukars. Spatial patterns on the other areas, however, suggest association with free water and our results
demonstrate the need for site-specific considerations. Researchers should be aware of the potential to satisfy water demand
with pre-formed and metabolic water for a variety of species in studies that address the effects of wildlife water developments.
We encourage incorporation of spatial structure in model error components in future ecological research. 相似文献
995.
996.
Yield management is important subject in modern grape growing and winter pruning and cluster thinning treatments were used for yield control. Standard cluster thinning is generally labor intensive and expensive method and grape growers try to new methods, which are alternative to cluster thinning for reducing berry set. The goal of the present research was to investigate the effects of chemical thinning treatments such as ethephon (ETH), gibberellic acid (GA3), naphthalene acetic acid (NAA) and non-chemical thinning treatments such as early leaf removal (ELR) and standard cluster thinning (SCT) on decreasing grape yield and enhancing berry composition of cv. Shiraz. In this study, percentages of berry set of cv. Shiraz were decreased by both chemical and non-chemical thinning treatments compared to Control. The results from these two different thinning methods shown certainly significant differences in berry composition of cv. Shiraz. Eventually, the best results in improved quality characteristics of cv. Shiraz were obtained from ELR treatment and treatments of ELR, ETH, GA3, NAA and SCT successively resulted in considerable enhancements in quality attributes of cv. Shiraz. 相似文献
997.
Context
Playa wetlands are the primary habitat for numerous wetland-dependent species in the Southern Great Plains of North America. Plant and wildlife populations that inhabit these wetlands are reciprocally linked through the dispersal of individuals, propagules and ultimately genes among local populations.Objective
To develop and implement a framework using network models for conceptualizing, representing and analyzing potential biological flows among 48,981 spatially discrete playa wetlands in the Southern Great Plains.Methods
We examined changes in connectivity patterns and assessed the relative importance of wetlands to maintaining these patterns by targeting wetlands for removal based on network centrality metrics weighted by estimates of habitat quality and probability of inundation.Results
We identified several distinct, broad-scale sub networks and phase transitions among playa wetlands in the Southern Plains. In particular, for organisms that can disperse >2 km a dense and expansive wetland sub network emerges in the Southern High Plains. This network was characterized by localized, densely connected wetland clusters at link distances (h) >2 km but <5 km and was most sensitive to changes in wetland availability (p) and configuration when h = 4 km, and p = 0.2–0.4. It transitioned to a single, large connected wetland system at broader spatial scales even when the proportion of inundated wetland was relatively low (p = 0.2).Conclusions
Our findings suggest that redundancy in the potential for broad and fine-scale movements insulates this system from damage and facilitates system-wide connectivity among populations with different dispersal capacities.998.
Context
The assessment of land-use impacts on biodiversity is one of the central themes of landscape ecology and conservation biology. However, due to the complexity of biodiversity, it is impossible to obtain complete information about the diversity of all species even for small areas, necessitating the selection of individual species or assemblages thereof as species surrogate. In parts of the world where taxonomic expertise is lacking, species identification has hindered progress in biodiversity conservation, and the only practical, relatively-accurate option, is the use of taxonomic minimalism.Objective
We carried out a rapid biodiversity assessment based on three surrogates—land-use (driver-surrogate), terrestrial arthropods (species-surrogate) and morphospecies (taxonomic-surrogate)—to determine the impacts of land-use on biodiversity of the Western Region (Ghana), an area covering ~4 % of the West African biodiversity hotspot.Method
We used diversity profiles to visualize the distribution of a total of 8848 arthropod individuals over seven land-use types which define the complete heterogeneity of the landscape.Results
Here, we present both sample and asymptotic diversity profiles of arthropod morphospecies for each land-use type and the potential of each land-use type for conserving arthropods.Conclusions
We conclude that (1) the morphospecies approach is useful for detecting differences in species diversity of land-use types; (2) the concept of asymptotic diversity may not be necessary for land-use based biodiversity comparison; and (3) maximum diversity profiles are useful for determining the land-use conservation values in cases where pristine areas are not available.999.
Julie Betbeder Marianne Laslier Laurence Hubert-Moy Françoise Burel Jacques Baudry 《Landscape Ecology》2017,32(9):1867-1879
Context
The ability to detect ecological networks in landscapes is of utmost importance for managing biodiversity and planning corridors.Objectives
The objective of this study was to evaluate the information provided by a synthetic aperture radar (SAR) image for landscape connectivity modeling compared to aerial photographs (APs).Methods
We present a novel method that integrates habitat suitability derived from remote sensing imagery into a connectivity model to explain species abundance. More precisely, we compared how two resistance maps constructed using landscape and/or local metrics derived from AP or SAR imagery yield different connectivity values (based on graph theory), considering hedgerow networks and forest carabid beetle species as a model.Results
We found that resistance maps using landscape and local metrics derived from SAR imagery improve landscape connectivity measures. The SAR model is the most informative, explaining 58% of the variance in forest carabid beetle abundance. This model calculates resistance values associated with homogeneous patches within hedgerows according to their suitability (canopy cover density and landscape grain) for the model species.Conclusions
Our approach combines two important methods in landscape ecology: the construction of resistance maps and the use of buffers around sampling points to determine the importance of landscape factors. This study was carried out through an interdisciplinary approach involving remote sensing scientists and landscape ecologists. This study is a step forward in developing landscape metrics from satellites to monitor biodiversity.1000.
Nurcan Erbil Zehra Tugba Murathan Mehmet Arslan Ahmet Ilcim Bilge Sayin 《Erwerbs-Obstbau》2018,60(3):203-209
The pear (Pyrus spp.) is one of the most important fruits consumed in daily life. The aim of this study was to determine the total phenolic and ascorbic acid contents, as well as the antimicrobial, antioxidant, and antimutagenic activities, of various pear cultivars. The fruits of five pear (Pyrus communis L.) cultivars (‘Deveci’, ‘Kizil’, ‘Egirsah’, ‘Gugum’, and ‘Banda’) were used in this study. It was determined that the peel and pulp of the ‘Kizil’ pear had the highest total phenolic content (402.5?mg/100?g and 215.2?mg/100?g, respectively), while those of the ‘Banda’ pear had the lowest total phenolic content (326?mg/100?g and 126.1?mg/100?g, respectively). Additionally, the ‘Kizil’ pear showed the highest antioxidant capacity in the 2,2’-azino-bis(3-ethylbenzothiazoline-6-6-sulphonic acid) (ABTS) and ferric-reducing ability assay (FRAP) (1.72 μmol TE/g FW and 161.25?μmol Fe II/g FW, respectively) and the highest ascorbic acid content (16.02?mg/100?g). The ‘Banda’ pear showed the highest antibacterial activity against the test bacteria. However, none of the pear extracts displayed antifungal activity. While all doses of the ‘Kizil’, ‘Gugum’, and ‘Banda’ pear extracts used in this study, except 80?μL/plate, exhibited antimutagenic activities, only the lowest dose (10?μL/plate) of the ‘Deveci’ pear extract showed the antimutagenic activity against induced mutagenesis in the Salmonella typhimurium TA 98 strain. Consequently, the five Turkish pear cultivars used in this study exhibited different levels of antimicrobial, antioxidant, and antimutagenic activities. 相似文献