| 1. |
- Power, M. J., et al.
(författare)
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Changes in fire regimes since the Last Glacial Maximum : an assessment based on a global synthesis and analysis of charcoal data
- 2008
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 30:7-8, s. 887-907
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Tidskriftsartikel (refereegranskat)abstract
- Fire activity has varied globally and continuously since the last glacial maximum (LGM) in response to long-term changes in global climate and shorter-term regional changes in climate, vegetation, and human land use. We have synthesized sedimentary charcoal records of biomass burning since the LGM and present global maps showing changes in fire activity for time slices during the past 21,000 years (as differences in charcoal accumulation values compared to pre-industrial). There is strong broad-scale coherence in fire activity after the LGM, but spatial heterogeneity in the signals increases thereafter. In North America, Europe and southern South America, charcoal records indicate less-than-present fire activity during the deglacial period, from 21,000 to ∼11,000 cal yr BP. In contrast, the tropical latitudes of South America and Africa show greater-than-present fire activity from ∼19,000 to ∼17,000 cal yr BP and most sites from Indochina and Australia show greater-than-present fire activity from 16,000 to ∼13,000 cal yr BP. Many sites indicate greater-than-present or near-present activity during the Holocene with the exception of eastern North America and eastern Asia from 8,000 to ∼3,000 cal yr BP, Indonesia and Australia from 11,000 to 4,000 cal yr BP, and southern South America from 6,000 to 3,000 cal yr BP where fire activity was less than present. Regional coherence in the patterns of change in fire activity was evident throughout the post-glacial period. These complex patterns can largely be explained in terms of large-scale climate controls modulated by local changes in vegetation and fuel load.
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| 2. |
- Hannon, G E, et al.
(författare)
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Climate change and human settlement as drivers of late-Holocene vegetational change in the Faroe Islands
- 2005
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Ingår i: The Holocene. - : SAGE Publications. - 0959-6836 .- 1477-0911. ; 15:5, s. 639-647
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Tidskriftsartikel (refereegranskat)abstract
- Changes in Faroese land surfaces during the late Holocene reflect intimate interactions between cultural and environmental development. Analyses of fossil wood, pollen and plant macrofossils indicate that the present open landscape replaced shrubby vegetation that was present from c. 6000 BC Up to c. AD 660. Conditions altered during the late Holocene, with loss of woody vegetation and increasing erosion: trends that were initiated prior to human settlement. AMS dating of sub-fossil Betula, Salix and Juniperus found buried in peat profiles from the islands of Suouroy, Sandoy, Eysturoy, Vagar and Streymoy, revealed that the islands had at least partial woody vegetation cover up to the time of continuous settlement. The settlement horizon, identified in a lacustrine sequence on the island of Eysturoy, dated to c. AD 570. It comprised pollen evidence for the cultivation of Hordeum, cultural macrofossil assemblages, charcoal fragments, diatom assemblage changes indicating lake nutrient enrichment and physical measurements showing increased sedimentation rates. The pollen record showed that heathland development was initiated prior to anthropogenic impact. The ecosystem impacts of settlement were therefore superimposed on landscape changes that began around AD 250. The earlier changes were most likely forced by increased storminess and declining atmospheric temperatures.
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| 3. |
- Bradshaw-Hajek, B. H., et al.
(författare)
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Dynamic Dielectric Response of Concentrated Colloidal Dispersions: Comparison between Theory and Experiment
- 2009
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Ingår i: Langmuir. - : ACS American Chemical Society. - 0743-7463 .- 1520-5827. ; 25:4, s. 1961-1969
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Tidskriftsartikel (refereegranskat)abstract
- The cell-model electrokinetic theory of Ahualli et al. Langmuir 2006, 22, 704 1; Ahualli et al. J. Colloid Interface Sci. 2007, 309, 342; and Bradshaw-Hajek et al. Langmuir 2008, 24, 4512 is applied to a dense suspension of charged spherical particles, to exhibit the systems dielectric response to an applied electric field as a function of solids volume fraction. The models predictions of effective permittivity and complex conductivity are favorably compared with published theoretical calculations and experimental measurements on dense colloidal systems. Physical factors governing the volume fraction dependence of the dielectric response are discussed.
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| 4. |
- Bradshaw-Hajek, B.H., et al.
(författare)
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Frequency-dependent electrical conductivity of concentrated dispersions of spherical colloidal particles
- 2008
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 24:9, s. 4512-4522
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Tidskriftsartikel (refereegranskat)abstract
- This paper outlines the application of a self-consistent cell-model theory of electrokinetics to the problem of determining the electrical conductivity of a dense suspension of spherical colloidal particles. Numerical solutions of the standard electrokinetic equations, subject to self-consistent boundary conditions, are implemented in formulas for the electrical conductivity appropriate to the particle-averaged cell model of the suspension. Results of calculations as a function of frequency, zeta potential, volume fraction, and electrolyte composition, are presented and discussed. © 2008 American Chemical Society.
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| 5. |
- Bradshaw, Richard H. W., et al.
(författare)
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Long-term succession in a Danish temperate deciduous forest
- 2005
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Ingår i: Ecography. - : John Wiley & Sons. - 0906-7590 .- 1600-0587. ; 28:2, s. 157-164
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Tidskriftsartikel (refereegranskat)abstract
- Forest successional trajectories covering the last 2000 yr from a mixed deciduous forest in Denmark show a gradual shift in dominance from Tilia cordata to Fagus sylvatica and a recent increase in total forest basal area since direct management ceased in 1948. The successions are reconstructed by combining a fifty-year record of direct tree observations with local pollen diagrams from Draved Forest, Denmark. Five of the seven successions record a heathland phase of Viking Age dating from 830 AD. The anthropogenic influence is considerable throughout the period of study even though Draved contains some of the most pristine forest stands in Denmark. Anthropogenic influence including felling masks the underlying natural dynamics, with the least disturbed sites showing the smallest compositional change. Some effects of former management, such as loss of Tilia cordata dominance, are irreversible. Artificial disturbance, particularly drainage, has accelerated and amplified the shift towards Fagus dominance that would have occurred on a smaller scale and at a slower rate in the absence of human intervention. Copyright © Ecography 2005.
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| 6. |
- Giesecke, Thomas, et al.
(författare)
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Towards an understanding of the Holocene distribution of Fagus sylvatica L.
- 2007
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Ingår i: Journal of Biogeography. - : Wiley. - 1365-2699 .- 0305-0270. ; 34:1, s. 118-131
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Tidskriftsartikel (refereegranskat)abstract
- Aim Understanding the driving forces and mechanisms of changes in past plant distribution and abundance will help assess the biological consequences of future climate change scenarios. The aim of this paper is to investigate whether modelled patterns of climate parameters 6000 years ago can account for the European distribution of Fagus sylvatica at that time. Consideration is also given to the role of non-climatic parameters as driving forces of the Holocene spread and population expansion of F. sylvatica. Location Europe. Methods European distributions were simulated using a physiologically-based bioclimatic model (STASH) driven by three different atmospheric general circulation model (AGCM) outputs for 6000 years ago. Results The three simulations generally showed F. sylvatica to have potentially been as widespread 6000 years ago as it is today, which gives a profound mismatch with pollen-based reconstructions of the F. sylvatica distribution at that time. The results indicate that drier conditions during the growing season 6000 years ago could have caused a restriction of the range in the south. Poorer growth conditions with consequently reduced competitive ability were modelled for large parts of France. Main conclusions Consideration of the entire European range of F. sylvatica showed that no single driving force could account for the observed distributional limits 6000 years ago, or the pattern of spread during the Holocene. Climatic factors, particularly drought during the growing season, are the likely major determinants of the potential range. Climatic factors are regionally moderated by competition, disturbance effects and the intrinsically slow rate of population increase of F. sylvatica. Dynamic vegetation modelling is needed to account for potentially important competitive interactions and their relationship with changing climate. We identify uncertainties in the climate and pollen data, as well as the bioclimatic model, which suggest that the current study does not identify whether or not climate determined the distribution of F. sylvatica 6000 years ago. Pollen data are better suited for comparison with relative abundance gradients rather than absolute distributional limits. These uncertainties from a study of the past, where we have information about plant distribution and abundance, argue for extreme caution in making forecasts for the future using equilibrium models.
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| 7. |
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| 8. |
- Hannon, Gina E, et al.
(författare)
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The Bronze Age landscape of the Bjare peninsula, southern Sweden, and its relationship to burial mounds
- 2008
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Ingår i: Journal of Archaeological Science. - : Elsevier BV. - 1095-9238 .- 0305-4403. ; 35:3, s. 623-632
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Tidskriftsartikel (refereegranskat)abstract
- Palaeoecological analyses from a small fen deposit, combined with pollen analysis from buried soil profiles under prehistoric burial mounds, have been used to investigate the timing and vegetation change associated with the Holocene development of a cultural landscape in southern Sweden. Traditional pollen analysis is complemented with plant macrofossil analysis and soil pollen analysis from within and in close proximity to the burial mounds in the coastal Bjare peninsula, well known for its high density of well-preserved Bronze Age monuments. The vegetation development is linked to the construction of the burial mounds. A marked increase of cultural impact on the landscape is recorded during the Neolithic-Bronze Age transition and estimates of landscape openness suggest that by the onset of the Bronze Age, forest cover was only 20-40%, falling to 10% in the immediate vicinity of the burial mounds themselves. The coastal strip appears to have been affected by human activity to a greater extent and at an earlier date than sites from further inland in southern Sweden and the Bronze Age burial mounds were most likely designed to be visible in a largely deforested landscape.
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| 9. |
- Hedman, Jenny E., et al.
(författare)
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Fate of contaminants in Baltic Sea sediments : role of bioturbation and settling organic matter
- 2008
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Ingår i: Marine Ecology Progress Series. - : Inter-Research Science Center. - 0171-8630 .- 1616-1599. ; 356, s. 25-38
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Tidskriftsartikel (refereegranskat)abstract
- This experimental study examined the interactive effects of bioturbation and settling organic matter (OM) on the fate (burial and remobilisation) of 2 surface-deposited contaminants in Baltic Sea sediment: the metal Cd and a hydrophobic organic pollutant, the flame retardant BDE-99. Three macrofaunal species with diverse feeding and bioturbation strategies were used: the amphipod Monoporeia affinis, the clam Macoma balthica and the polychaete Marenzelleria spp. Radiolabelled contaminants were added to the sediment surface in association with 3 different OM types: (1) phytoplankton, (2) terrestrial lignin and (3) Baltic sediment. Bioturbation by all species increased the retention of both contaminants in the sediment, most effectively M. affinis and M. balthica. A decoupled transport of Cd and BDE-99 by Marenzelleria was observed. Generally, Marenzelleria buried the highest amount of Cd into the sediment but also caused the highest remobilisation to the water, indicating an effective transport of (soluble) Cd over the sediment-water interface via bio-irrigation. Lack of the highly hydrophobic and mainly particle-associated BDE-99 below the sediment surface suggests that Marenzelleria caused no significant particle mixing. The addition of various OM types significantly affected the distribution of Cd, but not of BDE-99. There was an interactive effect between bioturbation (species) and OM type, generally showing an increased burial and release of Cd when associated with phytoplankton in the presence of Marenzelleria. Our results emphasise the importance of understanding the complex interactions between ecological (e.g. infaunal feeding and bioturbation activities) and physiochemical processes (contaminant speciation and sorption kinetics) when assessing the fate of contaminants in aquatic ecosystems.
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| 10. |
- Miller, Paul, et al.
(författare)
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Exploring climatic and biotic controls on Holocene vegetation change in Fennoscandia
- 2008
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Ingår i: Journal of Ecology. - : Wiley. - 1365-2745 .- 0022-0477. ; 96:2, s. 247-259
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Tidskriftsartikel (refereegranskat)abstract
- # 1. We investigated the potential drivers of Holocene vegetation changes recorded at four Scandinavian pollen sites, two in Sweden and two in Finland, at a time when they were largely free of anthropogenic influence. # 2. We used the generalized dynamic vegetation model LPJ-GUESS forced with climate anomaly output from an atmospheric general circulation model to simulate tree species dynamics from 10 000 years ago to the present. The model results were compared to high-resolution pollen accumulation rates gathered at the sites. # 3. Our results indicate that both the observed northern distributional limits of temperate trees, and the limits of Pinus sylvestris and Alnus incana at the tree line, are a result of millennial variations in summer and winter temperatures. The simulation of several distinct trends in species occurrence observed in the pollen record indicates that a time lag due to the slow spreading of species need not be invoked for most species. # 4. Sensitivity studies indicate that competition, natural disturbance and the magnitude of interannual variability play key roles in determining the biomass, establishment and even the presence of species near their bioclimatic limits. However, neither disturbance due to fire nor limits on establishment due to drought were likely to have been major determinants of the observed trends on the timescales considered. # 5. We were unable to limit the modelled occurrence of Picea abies at the study sites to the periods at which it was observed in the pollen records, indicating that we have still not completely understood the driving or limiting factors for Holocene changes in Picea abies abundance. # 6. Synthesis. This study shows that by combining quantitative vegetation reconstructions with a modern, process-based dynamic vegetation model, we may gain new insights into the potential biotic and abiotic drivers of Holocene vegetation dynamics, and their relative importance. This knowledge will be crucial in enabling us to assess more confidently the response of northern European vegetation to future climate change.
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