| 1. |
- Fang, Keyan, et al.
(författare)
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How robust are Holocene treeline simulations? A model-data comparison in the European Arctic treeline region
- 2013
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Ingår i: Journal of Quaternary Science. - : Wiley. - 1099-1417 .- 0267-8179. ; 28:6, s. 595-604
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Tidskriftsartikel (refereegranskat)abstract
- Treeline encroachments and retractions can provide global-scale feedbacks to the climate system, and treeline dynamics are therefore of great relevance for understanding global climate variability. To assess the accuracy of long-term treeline simulations based on the generalized dynamic vegetation model LPJ-GUESS, we simulate European Arctic treeline dynamics over the past 9000 years and compare the results with fossil-based reconstructions. The results show that while LPJ-GUESS is limited in its ability to capture species-level current treeline patterns and past dynamics, it is generally able to realistically simulate the Holocene coniferous treeline changes with a cutoff biomass carbon of 2 C kg m(-2). The model captures the northward expansion of the boreal forest during the mid Holocene and correctly simulates a treeline retreat in response to climate cooling during the last 3000 years. However, there are data-simulation disagreements particularly during the early Holocene, which mainly result from the differences between the two palaeoclimate model scenarios used to drive the simulations. We suggest that the spatial accuracy of the model could be improved by incorporating the influence of topographic features, the extent of the Arctic peatlands, the tree species life-history characteristics, microclimate and other ecological factors. Copyright (c) 2013 John Wiley & Sons, Ltd.
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| 2. |
- Salonen, J. Sakari, et al.
(författare)
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Pollen-based palaeoclimate reconstructions over long glacialinterglacial timescales : methodological tests based on the Holocene and MIS 5dc deposits at Sokli, northern Finland
- 2013
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Ingår i: Journal of Quaternary Science. - : Wiley. - 0267-8179 .- 1099-1417. ; 28:3, s. 271-282
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Tidskriftsartikel (refereegranskat)abstract
- Palaeoclimatic reconstructions over long glacialinterglacial timescales present major methodological challenges, as calibration methods based on modern climate and vegetation patterns may yield biased reconstructions from non-analogue palaeoclimates, such as severely continental glacial periods. In this work, we present pollen-based summer temperature reconstructions based on Holocene and early Weichselian (Marine Isotope Stages (MIS) 5dc) deposits at Sokli, northern Finland. We attempt to improve the robustness of reconstructions of continental palaeoclimates by using two calibration sets, with one located in the region surrounding the fossil site and another in a more continental region in northwestern Russia. We assess the reliability of the reconstructions by estimating the compositional fit between fossil and calibration samples and by comparison with independent, plant macrofossil-based reconstructions. We find the fossil samples to fit the extra-regional, high-continentality calibration set better during the early Holocene and MIS 5dc. Especially in MIS 5dc, however, even the high-continentality calibration set fits the fossil samples poorly compared to the fit achieved in the mid and late Holocene. Thus our results highlight the problem of finding applicable calibration data for pre-Holocene periods. We finally discuss the relative strengths and vulnerabilities of different calibration methods in reconstruction of non-analogue palaeoclimates.
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| 3. |
- Salonen, J. Sakari, et al.
(författare)
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The effect of calibration data set selection on quantitative palaeoclimatic reconstructions
- 2013
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Ingår i: The Holocene. - : SAGE Publications. - 0959-6836 .- 1477-0911. ; 23:11, s. 1650-1654
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Tidskriftsartikel (refereegranskat)abstract
- Quantitative palaeoclimatic reconstructions based on biological fossils are a major source of information on long-term climatic variability. Such reconstructions typically use some kind of a modern calibration data set describing the variation of the studied biological group in present-day climate space. Here, we explore the effect of calibration data set selection on palaeoclimatic reconstructions, by creating alternate calibration data sets via stratified random sampling to reconstruct mean July temperature (T-jul) for four fossil pollen sequences from northern Europe. We show that palaeoclimatic reconstructions using methods based on taxon-response models can be highly sensitive to the calibration data set used. In particular, the absolute reconstructed temperatures show great sensitivity to calibration data selection, which suggests that the absolute values of palaeoclimatic reconstructions may not be robust. By contrast, we find the relative shapes of the reconstructed curves to be more robust to calibration data selection because taxa tend to occupy similar relative locations along the sampled gradient regardless of calibration data set location. Based on this robustness of relative palaeoclimate curves, we suggest a debiasing procedure in which palaeoclimate values are estimated by fixing the relative curve with the modern observed value, thus correcting biases resulting from calibration data selection.
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| 4. |
- Shala, Shyhrete, et al.
(författare)
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Evaluating environmental drivers of Holocene changes in water chemistry and aquatic biota composition at Lake Loitsana, NE Finland
- 2014
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Ingår i: Journal of Paleolimnology. - : Springer Science and Business Media LLC. - 0921-2728 .- 1573-0417. ; 52:4, s. 311-329
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Tidskriftsartikel (refereegranskat)abstract
- This study presents a detailed analysis of geochemical and biotic proxies in a lake sediment profile to assess the effects of local and regional environmental drivers on the Holocene development of Lake Loitsana, situated in the northern boreal forest of NE Finland. Multi-proxy studies, in particular those that include a detailed plant macrofossil record, from the part of the northern boreal zone of Fennoscandia which has not been affected by treeline fluctuations, are scarce and few of these records date back to the earliest part of the Holocene. A 9-m sediment sequence of gyttja overlying silts representing the last c. 10,700 cal year, allowed for a high-resolution study with emphasis on the early to mid-Holocene lake history. The lacustrine sediments were studied using lithology, loss-on-ignition and C/N ratios, micro- and macro-fossils of aquatic and wetland taxa, diatoms, chironomids and accelerator mass spectrometry C-14 dating on terrestrial plant macrofossils. Our study shows that the local development at Loitsana was complex and included a distinct glacial lake phase and subsequent drainage, a history of fluvial input affected by nearby wetland expansion, and lake infilling in an eventual esker-fed shallow lake. Enhanced trophic conditions, due to morphometric eutrophication, are recorded as Glacial Lake Sokli drained and open water conditions became restricted to a relatively small Lake Loitsana depression. pH appears to have been stable throughout the Holocene with a well-buffered lake due to the local carbonatite bedrock (Sokli Carbonatite Massif). The fossil assemblage changes are best explained by a complex mixture of drivers, including water-body conditions (i.e. depth, turbidity and turbulence), rate of sediment input, and the general infilling of the lake, highlighting the need to carefully evaluate the possible influence of such local factors as palaeoenvironmental conditions are reconstructed based on aquatic proxies.
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| 5. |
- Valiranta, Minna, et al.
(författare)
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Scattered late-glacial and early Holocene tree populations as dispersal nuclei for forest development in north-eastern European Russia
- 2011
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Ingår i: Journal of Biogeography. - : Wiley. - 0305-0270 .- 1365-2699. ; 38:5, s. 922-932
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Tidskriftsartikel (refereegranskat)abstract
- Aim Concepts about patterns and rates of post-glacial tree population migration are changing as a result of the increasing amount of palaeobotanical information being provided by macroscopic plant remains. Here we combine macrofossil, pollen and stomata records from five sites in north-eastern European Russia and summarize the results for the late-glacial-early Holocene transition. The late-glacial-early Holocene transition encompasses the first indications of trees (tree-type Betula, Picea abies, Abies sibirica and Larix sibirica) and subsequent forest development. Considerable time-lags between the first macrobotanical and/or stomata finds of spruce (Picea abies) and the establishment of a closed forest are reconsidered. Location Pechora basin, north-eastern European Russia. Methods We used plant macrofossil, stomata, pollen and radiocarbon analyses to reconstruct late-glacial and early Holocene tree establishment and forest development. The data were derived from lake sediment and peat archives. Results Palaeobotanical data reveal an early Holocene presence (11,500-10,000 cal. yr bp) of arboreal taxa at all five sites. One site presently located in the northernmost taiga zone, shows the presence of spruce and reproducing tree birch during the late-glacial. Given the current view of post-glacial population dynamics and migration rates, it seems likely that the source area of these early tree populations in north-eastern European Russia was not located in southern Europe but that these populations had local origins. Results thus support the emerging view that the first post-glacial population expansions in non-glaciated regions at high latitudes do not reflect migration from the south but were a result of an increase in the size and density of small persisting outlying tree populations. Main conclusions Results suggest that the area east of the margin of the Scandinavian ice sheet to the Ural Mountains had isolated patches of trees during the late-glacial and early Holocene and that these small populations acted as initial nuclei for population expansion and forest development in the early Holocene.
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