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   53篇
  免费   1篇
  2023年   1篇
  2020年   4篇
  2019年   3篇
  2018年   1篇
  2017年   2篇
  2016年   2篇
  2015年   2篇
  2014年   2篇
  2013年   11篇
  2012年   3篇
  2011年   3篇
  2010年   1篇
  2009年   5篇
  2008年   4篇
  2007年   2篇
  2006年   3篇
  2000年   3篇
  1998年   1篇
  1985年   1篇
排序方式: 共有54条查询结果,搜索用时 15 毫秒
1.
A problem which has arisen in discussions of an elm-disease explanation for the Neolithic elm decline has been the lack of fossil records of Scolytus scolytus (F.) (elm bark beetle), which is the main carrier of the fungus which causes the present epidemic of Dutch Elm Disease. Now, the discovery of wing cases of two individuals from elm decline deposits at Hampstead Heath, London, proves that the beetle was present in Britain during this episode. The deposits in which it occurs are described, and the various implications for elm disease causing, aiding or at least occurring during this stage of the early Neolithic are discussed.  相似文献   
2.
Isolation effects on the distributions of plant species in fragmented forests appear to be weak over tens to hundreds of years and strong over geological eras. The 250 km wide, 6.5 millennia old Bass Strait, and other millennium-scale disjunctions in the range of Eucalyptus regnans forests, were used to determine the effects of intermediate periods of isolation on plant species occurrence and composition. Three of six floristic communities were found on both sides of Bass Strait. The residuals from multiple regression models using climatic variables on the latitude vector were not explained by latitude, indicating negligible isolation effects from Bass Strait. However, there was a lesser compositional effect of disjunctions within land masses than between land masses, suggesting some effect of the larger barrier. No species that commonly occurred with E. regnans and were largely confined to wet forest exhibited absences from any region where the climate and soils were within their range. If areas isolated from each other for millennia can maintain their vascular plant biota, the expenditure of conservation funds on creating corridors to connect large areas separated by anthropogenic landscape modification might require more justification than it is currently afforded.  相似文献   
3.
We have investigated the environmental history of human occupation and the development of agriculture in the eastern interior Lake District of Finland. The material consists of archaeological data, which is reviewed in topographical and agrogeological context, and pollen analytical evidence of agricultural indices from eight precisely dated (varved) lake sediment sequences. Before the Viking Age, archaeological evidence, consisting of stray finds, dwelling sites, and graves, is very scarce. Iron Age finds are clearly confined to the lowland environs with silty and clayey soils. During the Viking Age, the number of stray finds multiplies and the first cemeteries are established. Comparison between Viking and Crusade Period finds reveals a topographic shift toward higher locations and morainic soils. Most of the cup-stones are located on upland sites—that is, not in connection with known Iron Age sites. These are interpreted as medieval indicators of slash-and-burn farming of the fertile but stony supraaquatic morainic soils. There is pollen analytical evidence of sporadic cultivation in the area from the Bronze Age onward. Afterca. AD 700, the occurrence of cereal pollen grains becomes regular but remains discontinuous at each site until after the turn of the millennium. There is then an exponential rise in the cereal pollen rain, indicating a fully agricultural population.  相似文献   
4.
个旧锡业开发与生态环境变迁(1890-1949)   总被引:2,自引:0,他引:2  
随着1889年蒙自的开关,尤其是1910年滇越铁路的全线通车,极大地改善了云南的对外交通运输条件,推动了个旧锡业的迅速发展。在1890年至1949年的60年间,个旧共生产大锡323042吨,个旧锡业也取代铜业成为近代云南的矿业支柱。个旧锡业在推动云南矿业发展的同时,也对个旧及周边地区生态环境产生了巨大的影响。在1890年至1949年的60年间,个旧地区仅土法炼锡就至少消耗45万吨以上木炭。个旧、蒙自、建水、开远和石屏五县地区天然林平均覆盖率从清初的90%锐减至1940年的2.98%,近三百年间减少了80%以上,个旧锡业开发显然是五县天然林平均覆盖率急剧减少的重要原因。同时,锡矿的大量粗放开采和冶炼,也污染了个旧地区的土壤、水源和空气环境,带来了环境污染问题,影响了当地民众尤其是矿工的身体健康。因此,锡业开发是导致近代个旧及周边地区生态环境变迁的重要原因。  相似文献   
5.
The Canadian government and the Meadow Lake Tribal Council sponsored a forest extraction corporation in eastern Nicaragua that restructured 16 Miskitu and Mayangna villages and transformed local human‐environment interactions. The Central American aid project demonstrated paternalistic and interventionist tendencies and exposed biases in inter‐Indigenous aid that rendered it inseparable from conventional aid. This case encourages reflection on social and ecological impacts from the marketing of collective resources, the creation of Indigenous development corporations, and the decision‐making criteria and processes driving foreign aid. The case study demonstrates how foreign aid programs targeting Indigenous Peoples may actually thwart the self‐determination that they set out to encourage. Aid agencies and business partners, who had limited knowledge of local cultures and institutions, created externally defined rules that instigated resource conflicts and undermined the authority of customary leaders without resolving poverty or uneven development.  相似文献   
6.
Forest transition theory and the reforesting of Scotland   总被引:1,自引:0,他引:1  
During the 20th century, the forest area in Scotland expanded threefold, after many centuries of decline. Similar trends of forest expansion following deforestation have operated in many other developed countries. The passage from net deforestation to net reforestation is defined as the forest transition, and over the last few years a body of theory relating to its drivers has evolved. The case of Scotland is considered in order to contribute to this growing body of theory.  相似文献   
7.
A detailed study of charcoal burning activities has been made for the Zoersel forest (Northern Belgium) based on an inventory of charcoal kiln remains, the analyses of wood charcoal and other charred botanical macroremains and radiocarbon dating of 10 of these kilns. Age determination of the youngest kilns was refined by dendrochronological analysis of trees growing on top of these kiln sites. The results show that, although many kilns were found, charcoal was produced in Zoersel only at certain moments during late and post-medieval times suggesting that charcoal burning was not a part of the regular management practice of these forests. The wood used for the production of charcoal mainly consists of taxa of alluvial (alder) forests while the kilns are situated on sites that currently are rather dry, with an oak dominated vegetation. For the older kilns, this contrast could be explained by an important change in local soil conditions. The younger kilns may be related to the conversion of alder woodland to grassland in the lower parts of the domain, with the kilns being constructed in the nearby dryer woodland areas.  相似文献   
8.
In 1993 the Swedish parliament deregulated national forestry policy and established an environmental goal in parallel with the previous, long-standing goal of high wood production. This paper shows how the change occurred in the context of major changes in Swedish environmental policy during the late 1980s and early 1990s. Within a short time, new environmental legislation and the introduction of legal protection for small forest and agricultural habitats of high ecological value reoriented national forestry policy, away from an overriding focus on wood production to an increased awareness of nature conservation and biodiversity preservation. Reflecting a major compromise with the state, forest owners have gained greater freedom to manage their land, but must also improve environmental conditions while achieving high wood production, a policy known as ‘freedom under responsibility’. The paper explains how both the parliament and industry supported increased nature conservation and biodiversity to maintain forest health and support the forestry industry, by favouring responsible resource use and not simply protection from human influence.  相似文献   
9.
House-building with wood has a long tradition in Austria. In this study, 47 houses from the Austrian Alps and the alpine foothills were analyzed. Dendrochronology was used to date the houses and to extract further information as construction phases within one building and time needed for harvesting. The construction dates range between 1452 and 1934. The focus was set on log constructions and in particular on wooden log walls. All wooden walls were made of softwood, clearly dominated by Norway spruce (Picea abies). A remarkable decline in the utilization of spruce wood as construction timber was found during the 19th century, which might be due to the high charcoal production at that time. By analyzing age, wood species, time of harvesting and way of processing, knowledge about wood utilization and historical forest management can be regained. Furthermore, some social and economical background of the farmers can be lifted. Technical dates as wall thickness and total wood volume required to build a residential building is provided.  相似文献   
10.
A total of 51 ancient oak wood samples originating from various European archaeological sites, dating from the Neolithic period to the 18th century, were assayed for the presence of reproducible chloroplast (cp) DNA sequences. Five polymorphic chloroplast fragments were targeted. Only five of the samples could be fully genetically characterised, revealing four different oak cpDNA haplotypes. In all cases, the haplotypes detected on ancient woods and the haplotypes characterised from fresh samples from the same localities matched. Overall, this congruence is consistent with a genetic continuity between ancient and modern European oaks, confirming the hypothesis that the mapped genetic patterns largely reflect the original structure that established during the post-glacial. This stability of the genetic structure implies that, in the future, the technique could be used to infer or confirm the transport of wood by man, providing interesting perspectives for the genetic analysis of ancient woods.  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