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   102篇
  免费   6篇
  国内免费   2篇
测绘学   10篇
大气科学   20篇
地球物理   16篇
地质学   12篇
天文学   5篇
综合类   4篇
自然地理   43篇
  2023年   2篇
  2022年   3篇
  2021年   1篇
  2020年   4篇
  2019年   6篇
  2018年   2篇
  2017年   4篇
  2016年   4篇
  2015年   4篇
  2014年   3篇
  2013年   17篇
  2012年   2篇
  2011年   2篇
  2010年   4篇
  2009年   1篇
  2008年   2篇
  2007年   4篇
  2006年   8篇
  2005年   5篇
  2004年   3篇
  2003年   6篇
  2002年   3篇
  2001年   2篇
  2000年   3篇
  1999年   2篇
  1998年   8篇
  1997年   1篇
  1991年   1篇
  1990年   1篇
  1986年   1篇
  1984年   1篇
排序方式: 共有110条查询结果,搜索用时 15 毫秒
61.
As land use change continues to increase throughout the Amazon basin, there is a pressing need to accurately map, quantify and assess the effects of different factors on forest cover change (FCC). Land tenure may sometimes have important effects on forest cover, yet such effects remain poorly understood in Amazonia, particularly outside Brazil. In this paper we assess whether significant differences in trends of FCC can be partially explained by different land tenure arrangements, using a case study in southwestern Beni (Bolivian Amazon). We examine spatio-temporal dynamics of FCC across four land tenure systems (indigenous titled territory, protected area, logging concession, and private land) by classifying forests using a time-series of Landsat satellite imagery consisting of four dates (1986, 1996, 2001, 2009). Specifically, we unravel (1) trends in early growth and old-growth forest extent, including changes in total cover area, annual change rates, and spatial change dynamics, and (2) trends in old-growth forest fragmentation. To better understand the association between land tenure and FCC, we qualitatively assess the potential role that other underlying and proximate drivers may have had in FCC over the study period. We found that private lands underwent, by far, the largest FCC, that indigenous territories and the protected area had little FCC, and that logging concessions were responsible for the lowest FCC. Our findings suggest that land tenure played a key role in FCC except in private areas, where many other drivers had operated. Our study sheds light into the potential role of land tenure in FCC and has important implications for public policies aimed at socioeconomic development and environmental conservation in the Amazon. We give some policy recommendations drawn from a biocultural conservation perspective that could contribute to implement more inclusive conservation policies in the region.  相似文献   
62.
The sediment flux data, measured from a dry-hot valley of the Longchuan River, a tributary of the lower Jinsha River, were analyzed with Mann-Kendall test, Seasonal Mann-Kendall test and Sen‘s test. In both the upper reaches (Xiaohekou) and the lower reaches (Xiaohuangguayuan), the sediment fluxes showed a significant increase from 1970 to 2001, despite the fact that the water discharge did not change significantly during the period and numerous reservoir constructions which contribute to the trap of sediment. This can be attributed to the intensification of human activities, especially the activities related to land surface disturbances such as deforestation and afforestation, expansion of agriculture land, and road constructions. This increase is more significant in the lower reaches of the river observed at the place of Xiaohuangguayuan due to the dry-hot climate. The profound increase in sediment flux has significant implications for effective management of the sedimentation problems of the on-going Three Gorges Reservoir.  相似文献   
63.
Central America's tropical forests have been felled more rapidly than those of any other world region during the latter half of the twentieth century. During this time, nearly half of Guatemala's forests were eliminated. Most of this deforestation has been concentrated in the northern department of Petén. The remaining forests in Petén are now mainly concentrated in the Maya Biosphere Reserve (MBR), the heart of the largest lowland tropical forest in Central America. The pace, magnitude, and geography of this trend is of critical importance to forest conservation and rural development efforts. This article examines socioeconomic, political, demographic, and ecological factors behind settler land use and forest clearing among 241 farm households in the Sierra de Lacandón National Park (SLNP), a core conservation zone of the MBR. Some of the factors positively related to forest clearing were household size, Q'eqchí Maya ethnicity, land owned in the previous residence, farm size, land title, and the cropping of velvet bean as a soil amendment. Education, off‐farm employment, and farm distance to a road were negatively related to farm‐level deforestation.  相似文献   
64.
《Climate Policy》2013,13(3):306-315
Consideration of incentives for reducing emissions from deforestation and forest degradation (REDD) is now formally part of the post-2012 climate change negotiations. A significant amount of financing will be required to make REDD a success, but the design of the REDD architecture can determine the availability of capital. Therefore, in negotiations this should be considered at the same time and on an equal basis with methodological and political considerations. Detailed consideration is given to the type of commitment, the financing mechanism, the level of incentive allocation, and the fungibility of carbon credits, in the context of experience from existing carbon markets. We conclude that a financially successful REDD mechanism would be based on a strong regulatory framework with mandatory targets, market-based, with some degree of project-level crediting, creating fungible REDD credits, subject to a cap.  相似文献   
65.
REDD projects have received considerable attention for their potential to mitigate the effects of climatic change. However, the existing literature has been slow to assess the impacts of proposed REDD projects on the livelihoods of forest communities in the developing world, or the implications of these local realities for the success of REDD+ initiatives in general. This study presents ethnographic research conducted with communities within the April-Salomei pilot REDD+ Project in Papua New Guinea (PNG). Several cases of institutional biases and uneven power relationships have been exploited by local elites to prevent landowners from making free and informed choices about their involvement in the project, although landowners and local communities are well positioned to capture forthcoming project benefits. By underestimating the scale and impact of traditional shifting cultivation practices, the credibility of the REDD+ project design and the value of any future carbon credits are critically undermined. Based on the actual practices found in PNG, the authors' radical proposal is to call for a halt on REDD development in PNG while institutional enabling conditions are improved, comprehensive landowner consultations conducted, and detailed mapping and genealogical surveys of landowners completed. Without these developments, future REDD+ projects in PNG are unlikely to benefit either the global climate or local development.  相似文献   
66.
The Ecuadorian Amazon, one of the richest reserves of biodiversity in the world, has faced one of the highest rates of deforestation of any Amazonian nation. Most of this forest elimination has been caused by agricultural colonization that followed the discovery of oil fields in 1967. Since the 1990s, an increasing process of urbanization has also engendered new patterns of population mobility within the Amazon, along with traditional ways by which rural settlers make their living. However, while very significant in its effects on deforestation, urbanization and regional development, population mobility within the Amazon has hardly been studied at all, as well as the distinct migration patterns between men and women. This paper uses a longitudinal dataset of 250 farm households in the Northern Ecuadorian Amazon to understand differentials between men and women migrants to urban and rural destinations and between men and women non-migrants. First, we use hazard analysis based on the Kaplan–Meier (KM) estimator to obtain the cumulative probability that an individual living in the study area in 1990 or at time t, will out-migrated at some time, t+n, before 1999. Results indicate that out-migration to other rural areas in the Amazon, especially pristine areas is considerably greater than out-migration to the growing, but still incipient, Amazonian urban areas. Furthermore, men are more likely to out-migrate to rural areas than women, while the reverse occurs for urban areas. Difference-of-means tests were employed to examine potential factors accounting for differentials between male and female out-migration to urban and rural areas. Among the key results, relative to men younger women are more likely to out-migrate to urban areas; more difficult access from farms to towns and roads constrains women's migration; and access to new lands in the Amazon–an important cause of further deforestation–is more associated with male out-migration. Economic factors such as engagement in on-farm work, increasing resource scarcity–measured by higher population density at the farm and reduction in farm land on forest and crops–and increase in pasture land are more associated with male out-migration to rural areas. On the other hand, increasing resource scarcity, higher population density and weaker migration networks are more associated with female out-migration to urban areas. Thus, a “vicious cycle” is created: Pressure over land leads to deforestation in most or all farm forest areas and reduces the possibilities for further agricultural extensification (deforestation); out-migration, especially male out-migration, occurs to other rural or forest areas in the Amazon (with women being more likely to choose urban destinations); and, giving continuing population growth and pressures in the new settled areas, new pressures promote further out-migration to rural destinations and unabated deforestation.  相似文献   
67.
This paper presents new data on the rate and patterns of loss of tropical moist forest cover to agriculture in Cross River State, Nigeria. Data from sequential aerial photographs and field surveys are used to derive an estimate of forest loss of 0.6 per cent per year. This rate is relatively modest, yet current conservation and forestry policy in the area is based on assumptions of rapid conversion of forest. The implications of these findings for debates about conservation and policy are discussed.  相似文献   
68.
Abstract

This article examines two key aspects of land-cover change in the south of the Chocó region. First, it assesses and compares the local impact on forest condition of labor-intensive and capital-intensive commercial logging. Second, it assesses the regional significance and permanency of these changes. Studies of land-cover change associated with commercial logging have focused almost exclusively on capital-intensive extraction and have assumed that after logging, degraded forests are transformed into agricultural cover. This study shows that both capital- and labor-intensive logging result in similar land-cover changes (i.e., forest degradation) if the timber sought is the same. However, labor-intensive loggers also seek timber species not sought by capital-intensive loggers, and this impact is statistically different from the impact of the extraction of the first group of species. Results also show that only a small fraction (20–30 percent) of the area logged is later converted to agricultural cover types. The persistence of logged forests means that up to 20 percent of the remaining forest cover could correspond to forests with significant and lasting levels of degradation. Furthermore, the different production requirements for each group of species also mean that there is a spatial differentiation in the impact of logging in the region. Logged forests are arranged into two consecutive corridors on each side of access routes (e.g., rivers). The first corridor corresponds to a narrow (approximately 1-km) band of high-intensity degradation. The second, broader (approximately 2-km) forest band, with lower levels of degradation, extends inland along first-tier corridors. A key factor determining the permanency of this land-cover pattern is the strong control local communities have over the land in the region. This limits the spread of patterns observed in other frontier areas, especially the conversion of logged forests into agricultural cover.  相似文献   
69.
Abstract

We simulated snow processes in a forested region with heavy snowfall in Japan, and evaluated both the regional-scale snow distribution and the potential impact of land-use changes on the snow cover and water balances over the entire domain. SnowModel reproduced the snow processes at open and forested sites, which were confirmed by snow water equivalent (SWE) measurements at two intensive observation sites and snow depth measurements at the Automated Meteorological Data Acquisition System sites. SnowModel also reproduced the observed snow distribution (from the MODIS snow cover data) over the simulation domain during thaw. The observed SWE was less at the forested site than at the open site. The SnowModel simulations showed that this difference was caused mainly by differences in sublimation. The type of land use changed the maximum SWE, onset and duration of snowmelt, and the daily snowmelt rate due to canopy snow interception.

Citation Suzuki, K., Kodama, Y., Nakai, T., Liston, G. E., Yamamoto, K., Ohata, T., Ishii, Y., Sumida, A., Hara, T. & Ohta, T. (2011) Impact of land-use changes in a forested region with heavy snowfall in Hokkaido, Japan. Hydrol. Sci. J. 56(3), 443–467.  相似文献   
70.
The presence of Eurasian lynx as a former native species in Britain during the Holocene is known from bones recovered from several sites. AMS radiocarbon dating of lynx bone recovered from two sites in the Craven area of northern England gave 1842 ± 35 14C yr BP and 1550 ± 24 14C yr BP, together representing the youngest dates for lynx from England, and in the case of the latter, the youngest for Britain as a whole. These dates support the view that the game animal whose occurrence in the nearby Lake District is described in the early 7th century Cumbric text Pais Dinogad, and whose translation to date has been problematic, is a lynx. The occurrence of lynx in early medieval Britain shows that earlier periods of climate change, previously blamed for the species' extinction in Britain, were not responsible. Instead, anthropogenic factors such as severe deforestation, declining deer populations, and persecution, are likely to have caused the extirpation of lynx in Britain. Consequently, the lynx qualifies as a candidate for reintroduction. Large‐scale reafforestation, the growth of deer populations, and more positive attitudes towards carnivores in modern society, could permit the restoration of lynx to Britain, particularly in Scotland. Copyright © 2005 John Wiley & Sons, Ltd.  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