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111.
Charlotte Seidenberg Ole Mertz Morten Bilde Kias 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(2):71-80
Shifting cultivation is often blamed for deforestation in tropical upland areas. Based on a case study of three villages in northern Lao PDR, this paper combines household surveys with a remote sensing based analysis of forest cover, covering the period 1989–1999, in order to analyse changes in shifting cultivation practices and livelihood strategies and the impact of these on deforestation. Due to population pressure and relocation of villages, easily accessed land is increasingly scarce and fallow periods have been shortened during the 1990s. A net annual deforestation of about 1% was found in the area during the study period. This deforestation rate reflects shorter fallow periods in secondary forests rather than encroachment on mature forests, which are not used for cultivation by the farmers in the three villages. Farmers rate scarce labour as a major constraint on shifting cultivation; nonetheless, a tendency towards lower labour input with shorter fallow periods is observed, contradicting conventional intensification theory. Livelihoods are diversifying through the establishment of plantations, cultivation of wet rice and adoption of animal husbandry, but given the socio-economic conditions in the area, shifting cultivation is likely to remain the most suitable farming system in the near future. 相似文献
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Nanina Blank Alan G. Hudson Pascal Vonlanthen Ole Seehausen Chad R. Hammerschmidt David B. Senn 《Aquatic Sciences - Research Across Boundaries》2013,75(2):261-273
Central European lake whitefish (Coregonus spp.) colonized Swiss lakes following the last glacial retreat and have undergone rapid speciation and adaptive radiation. Up to six species have been shown to coexist in some lakes, and individual species occupy specific ecological niches and have distinct feeding and reproductive ecologies. We studied methylmercury (MeHg) accumulation in sympatric whitefish species from seven Swiss lakes to determine if ecological divergence has led to different rates of MeHg bioaccumulation. In four of seven lakes, sympatric species had distinctly different MeHg levels, which varied by up to a factor of two between species. Generally, species with greater MeHg levels were smaller in body size and planktivorous, and species with lower MeHg were larger and benthivorous. While modest disparities in trophic position between species might be expected a priori to explain the divergence in MeHg, δ15N of bulk tissue did not correlate with fish MeHg in five of seven lakes. Results of a nested ANCOVA analysis across all lakes indicated that only two factors (species, lake) explained substantial portions of the variance, with species accounting for more variance (52 %) than inter-lake differences (32 %). We suggest that differences in MeHg accumulation were likely caused by diverging metabolic traits between species, such as differences in energy partitioning between anabolism and catabolism, potentially interacting with species-specific prey resource utilization. These results indicate substantial variability in MeHg accumulation between closely related fish species, illustrating that ecological speciation in fish can lead to divergent MeHg accumulation patterns. 相似文献
114.
Estimates of surface drifter trajectories in the equatorial Atlantic: a multi-model ensemble approach 总被引：3，自引：3，他引：0
Robert Bruce Scott Nicolas Ferry Marie Drévillon Charlie N. Barron Nicolas C. Jourdain Jean-Michel Lellouche Edward Joseph Metzger Marie-Hélène Rio Ole Martin Smedstad 《Ocean Dynamics》2012,62(7):1091-1109
We compared the estimates of surface drifter trajectories from 1 to 7?days in the equatorial Atlantic over an 18-month period with five eddying ocean general circulation model (OGCM) reanalyses and one observational product. The cumulative distribution of trajectory error was estimated using over 7,000?days of drifter trajectories. The observational product had smaller errors than any of the individual OGCM reanalyses. Three strategies for improving trajectory estimates using the ensemble of five operational ocean analysis and forecasting products were explored: two methods using a multi-model ensemble estimate and also spatial low-pass filtering. The results were insensitive to the method used to create the ensemble estimates, and by most measures, the results were better than the observational product. Comparison of relative skill of the various OGCM reanalyses suggested promising avenues for exploration for further improvements: forcing with higher frequency wind stress and quality control of input data. One of the lowest horizontal resolution OGCMs, with 1/4° longitude horizontal resolution, made the best trajectory estimates. The individual OGCMs were dominated by errors at spatial scales smaller than about 100 to 200?km, i.e., less than the local deformation radius. But buried in those errors were valuable signals that could be retrieved by combining all the OGCM velocity fields to produce a multi-model ensemble-based estimate. This estimate had skill down to spatial scales about 75?km. Results from this study are consistent with previous work showing that ensemble-mean forecast skill is superior to individual forecasts. 相似文献
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Duanne A. White Ole Bennike Simon L. Harley Kevin Kiernan Bernd Wagner 《Quaternary Research》2009,72(1):80-90
The presence of glacial sediments across the Rauer Group indicates that the East Antarctic ice sheet formerly covered the entire archipelago and has since retreated at least 15 km from its maximum extent. The degree of weathering of these glacial sediments suggests that ice retreat from this maximum position occurred sometime during the latter half of the last glacial cycle. Following this phase of retreat, the ice sheet margin has not expanded more than ∼ 1 km seaward of its present position. This pattern of ice sheet change matches that recorded in Vestfold Hills, providing further evidence that the diminutive Marine Isotope Stage 2 ice sheet advance in the nearby Larsemann Hills may have been influenced by local factors rather than a regional ice-sheet response to climate and sea-level change. 相似文献
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In the Precambrian gneisses of SE Norway, several generations of amphibolitised basic dykes/sills occur. While prekinematically emplaced dykes and sills show normal (but complex) fold structures, other metabasic intrusions have imposed a deformation on to the host rock. Apparent fold structures which have developed due to fabric control exerted by the host rocks, are recorded from late kinematic intrusions. Attention is drawn to the problem of distinguishing primary intrusive structures from simple and multiple fold-structures.
Zusammenfassung In die präkambrischen Gneise Südost-Norwegens sind mehrere Generationen amphibolisierter basischer Gänge bzw. Sills eingedrungen. Während präkinematisch angelegte Gänge und Sills normale (aber komplexe) Faltenstrukturen zeigen, haben andere metabasische Intrusionen das Nebengestein beansprucht. Deutliche Faltenstrukturen, die sich in der Gefügekontrolle des Nebengesteins manifestieren, werden von spätkinematischen Intrusionen berichtet. Besondere Beachtung wird der Unterscheidung primärer Intrusivstrukturen von einfachen und multiplen Faltenstrukturen geschenkt.
Résumé Dans le SE de la Norvège, les gneiss Précambriens contiennent plusieurs générations de dykes et sills basiques amphibolitisés. Les dykes et sills prétectoniques montrent des structures de plissement, normales quoique compliquées. Par contre, les autres intrusions basiques ont déformé les roches encaissantes. Localement, des intrusions tardi-tectoniques présentent des plissements apparents controlés et provoqués par la structure de la roche encaissante. L'attention est portée sur le problème de distinction entre des structures intrusives primaires et des structures de plissement, simples et multiples.
- . , , -, ( ) , , , . , , , . — .相似文献
120.
Ole Graversen 《International Journal of Earth Sciences》1974,63(1):335-340
A mullion structure is described from a several meter thick amphibolite from the Precambrian bedrock of south-eastern Norway. The mullion axes deviate about 60–700 from the fold axis defined in the amphibolite by the constructed intersection of foliation planes. The fold axis is also reflected in the lineated hornblende grains. The orientation of the mullion axes is parallel to the trace of the amphibolite foliation on the mullioned surface. It is concluded that the mullions were formed on a discordant surface (probably a fracture surface) in responce to movement along the intersecting foliation planes at the same time as the hornblende lineation was developed.
Zusammenfassung Aus dem südostnorwegischen präkambrischen Grundgebirge wird aus einem mehrere Meter mächtigen verfalteten Amphibolit eine Mullion-Struktur beschrieben. Die Achsen der Mullions weichen 60–700 von der Faltenachse ab, welche aufgrund der Biotitfoliationen im Amphibolit konstruiert werden konnte. Diese Faltenachse wird auch von den linearen Hornblenden nachgezeichnet. Es wird angenommen, daß sich die Mullions auf einer diskordanten Fläche (wahrscheinlich einer Bruchfläche) gebildet haben. Auf dieser Fläche verlaufen die Mullion-Achsen parallel der Spur der Amphibolit-Foliation. Die Mullions entstanden gleichzeitig mit den linearen Hornblenden infolge einer Bewegung entlang der sich schneidenden Foliationsebenen.
Résumé Une structure en «mullion» a été étudiée dans des amphibolites, d'âge Précambrien, en Norvège. Les mullions proviennent d'une couche discordante, qui a été déformée avec les amphibolites. On observe que les axes des mullions font une angle de 60 à 70 degré avec la linéation (prismes de hornblende) qui est parallèle aux axes des plis; les axes des mullions et la linéation sont situés dans le plan de la foliation des amphibolites.
Mullion-Struktur - . 60–70 , . . , (, ). . .相似文献