全文获取类型
收费全文 | 4381篇 |
免费 | 363篇 |
国内免费 | 416篇 |
专业分类
林业 | 542篇 |
农学 | 227篇 |
基础科学 | 220篇 |
1001篇 | |
综合类 | 2047篇 |
农作物 | 106篇 |
水产渔业 | 551篇 |
畜牧兽医 | 182篇 |
园艺 | 95篇 |
植物保护 | 189篇 |
出版年
2024年 | 65篇 |
2023年 | 97篇 |
2022年 | 153篇 |
2021年 | 172篇 |
2020年 | 209篇 |
2019年 | 243篇 |
2018年 | 121篇 |
2017年 | 187篇 |
2016年 | 217篇 |
2015年 | 182篇 |
2014年 | 297篇 |
2013年 | 315篇 |
2012年 | 400篇 |
2011年 | 463篇 |
2010年 | 331篇 |
2009年 | 301篇 |
2008年 | 257篇 |
2007年 | 252篇 |
2006年 | 166篇 |
2005年 | 123篇 |
2004年 | 103篇 |
2003年 | 84篇 |
2002年 | 70篇 |
2001年 | 60篇 |
2000年 | 56篇 |
1999年 | 50篇 |
1998年 | 34篇 |
1997年 | 31篇 |
1996年 | 26篇 |
1995年 | 21篇 |
1994年 | 17篇 |
1993年 | 9篇 |
1992年 | 14篇 |
1991年 | 11篇 |
1990年 | 10篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1986年 | 1篇 |
排序方式: 共有5160条查询结果,搜索用时 0 毫秒
71.
72.
73.
74.
The importance of the role of local community forestry institutions towards forest conservation is exemplified through a comparison of two adjacent areas within the Central Yucatan Peninsular Region (CYPR) in which Land-Use Cover Change (LUCC) analyses were conducted. We also used logistic regression analyses to examine key environmental, socioeconomic and institutional drivers associated with deforestation. One of the areas, La Montaña (LM) at Hopelchen, Campeche, is part of the northern section and buffer zone of the Calakmul Biosphere Reserve (CBR) as well as part of the Mesoamerican Biological Corridor. LM is an agricultural forest frontier region economically dependent on productive activities. The other study, Zona Maya (ZM), found at the municipality of Felipe Carrillo Puerto in Quintana is characterized by having developed community-based forestry enterprises with world wide recognition. In addition, the major tourism industry in nearby Cancun and Riviera Maya provide an important source of off-farm labor and temporary migration. Results show contrasting annual deforestation rates among the two study areas, being greater in the Campeche site (0.7% from 2000 to 2005) compared to Quintana Roo (−0.002% from 2000 to 2004). Logistic regression results show that the occurrence of land clearing at LM is significantly related to demographic variables as well as soil-environment types and conservation status did not reveal to relate to LUCC processes at all. At Quintana Roo, despite twice the population density, population variables at large did not strongly correlate to forest clearing. Rather forest conservation or maintenance was shown to be influenced by local community forestry institutions and a landscape zoning provided by larger management goals from the part of the communities. Greater availability of wage labor for the prominent tourism economy may also reflect a lesser dependence on agriculture and contribute to the much lower deforestation rate. Nonetheless, results show that community forest management can play an effective role in forest conservation. We argue for a regional land use management approach as a conservation strategy in which local inhabitants are considered key actors. 相似文献
75.
世行货款“贫困地区林业发展项目”是世界银行支持我国中西部地区经济发展的又一个贷款项目。通过对贵州省15个项目县(市)的调查研究,总结分析项目实施中的经验和存在的问题,提出对策建议,供我国中西部12个省(区)在开展贫困地区林业发展项目建设中参考。 相似文献
76.
77.
四川盆地丘陵及盆周低山区杉木实生林受气候等环境条件的影响,速生期早而短,若立地条件差则很多林木因早熟而达不到培育目标。本研究在调查杉木主要分布区林分生长状况基础上,通过树干解析、生长过程对比分析,弄清了该地区杉木林分速生期持续的年限,并提出了以间作、合理施肥、合理密植为提高产量的主要措施。试验证明,这些措施具有明显的效果,既改良了林地土壤的理化性质、提高了土壤肥力,又延长了林分的速生期,提高林分蓄积并改善了木材的规格质量,其经济效益和社会效益都十分明显。 相似文献
78.
Impacts of resource extraction on forest structure and diversity in Bardia National Park,Nepal 总被引:1,自引:0,他引:1
Repeated biomass harvesting for human livelihood needs can cause significant changes in forest structure, composition and diversity. Such impacts have often been associated with the distance to villages and their size, but the effects of individual villages in relation to their characteristics have been little studied. Focussing on the issues around communities’ impacts on the forest, this research aimed to understand how resource extraction by two Buffer Zone villages in Nepal's Bardia National Park affects forest structure and diversity. Tree density, basal area, diameter at breast height (DBH), species richness, two diversity indices and disturbance indices were recorded in 108 plots in 12 transects, from March till May 2005, within the park along the village boundaries, where resource extraction occurs, and in the core park area which is isolated from human disturbance. Forest subject to resource extraction had a lower density of trees, smaller DBH and lower species richness and diversity. Forest disturbance declined with distance from the villages but the two villages differed widely in their impact on the forest. Resource use was more intensive in Shivapur village and percentage of trees lopped, average lopping intensity and numbers of dung piles were statistically significant. This was mainly due to the lack of access to Buffer Zone community Forest for Shivapur households. Interviews with villagers suggested that there have been changes in species availability compared to 5 years ago. Households also underestimated the impact of their resource extraction, which may prevent them from changing their behaviour to benefit conservation. It is clear that understanding rural communities’ needs, their framing of resource extraction and utilisation, and dependence on forest resources will be very helpful for long term conservation measures. 相似文献
79.
Riparian vegetation strongly influences the structure and function of terrestrial and aquatic ecosystems; therefore, harvest of riparian trees can profoundly impact these habitats. Riparian reserves (no harvest zones) are used world-wide to reduce these impacts, including effects on organic matter dynamics. There are, however, few experimental evaluations of their efficacy. Particulate organic matter delivery from riparian vegetation to streams, a critical ecosystem process, was measured over an eight year period after logging in southwestern British Columbia, Canada to quantify the relationship between these litter fluxes and riparian reserves (unlogged controls, 30 and 10-m wide uncut reserves, and clear-cuts). We hypothesized that the timing, composition and quantity of organic matter entering these streams would vary as a function of reserve width, but that these differences would decline with time since logging. We also hypothesized that inter-annual variation in precipitation patterns would partially contribute to litter dynamics. Riparian treatment significantly affected the quantity and composition of litter flux: inputs of needles and twigs were significantly lower and proportion of deciduous litter higher in clear-cut streams compared to other treatments. ANOVA models indicated that treatment differences in litter flux persisted through year 7 for some components of litter, while regression models indicated positive trends between reserve width and litter flux through year 8. This positive slope indicates the clear-cut treatment had yet to recover from logging. Variation in snowfall and plant succession contributed to temporal variation in litter flux; in some cases, litter inputs increased with annual snowfall. Another key finding from this study was that riparian reserves provided total litter inputs to streams that were similar to fully forested controls. 相似文献
80.
[目的] 研究盐胁迫条件下3种海滨植物的生理生态动态响应特征及其耐盐性差异,为盐碱地绿化树种筛选提供科学依据。[方法] 以2年生中山杉、小叶榕和海滨木槿幼苗为材料进行为期56 d的盆栽试验,设置3个盐分梯度对其生长和形态、抗氧化酶系统以及渗透调节系统等指标的动态变化特征进行比较,并通过主成分分析来明确3种植物在耐盐性上的差异。[结果] 短期盐胁迫(14 d)对3种植物存活率、叶形和叶色影响较小,但长期盐胁迫(56 d)对小叶榕产生较大影响,盐胁迫下3种植物固定枝条相对生长率动态特征差异明显。3种植物叶片超氧化物歧化酶(SOD)活性随着盐处理时间增加而呈现降低趋势,过氧化物酶(POD)和抗坏血酸过氧化物酶(APX)活性先增加后降低,丙二醛(MDA)含量呈现总体降低趋势。盐胁迫下3种植物叶片可溶性糖和可溶性蛋白含量均呈现先减少后增加的趋势,叶片相对电导率则随着盐浓度增加而显著提高。盐处理时间和物种的交互作用对SOD和POD活性以及MDA含量有显著影响,3种植物耐盐性主成分有明显差异,表明它们不同的耐盐响应特征。[结论] 3种植物的生态生理响应特征表明盐处理对小叶榕影响最大,中山杉次之,对海滨木槿影响最小,因此海滨木槿和中山杉有优先作为我国东部沿海地区盐碱地绿化树种的潜力。 相似文献