Refine
Year of publication
Language
- English (37)
Is part of the Bibliography
- yes (37)
Keywords
- Larix (4)
- climate change (3)
- metabarcoding (3)
- pollen (3)
- sedimentary ancient DNA (3)
- treeline (3)
- Environmental DNA (2)
- Holocene (2)
- Holocene environmental history (2)
- Larix gmelinii (2)
Institute
- Institut für Biochemie und Biologie (37) (remove)
Recent and subfossil pollen spectra from the Alashan Plateau are presented in order to provide information on desert plant representation and on recent changes in vegetation and climate in this remote area in northern China. The desert vegetation composition is faithfully represented by the surface pollen spectra. The comparison of the desert plant species to the related pollen taxa yielded the following sequence from over-representation to under- representation: Chenopodiaceae, Artemisia, Ephedra fragilis-type s.l., Reaumuria, Nitraria and Calligonum. A 72 cm long sediment record from a small hydrologically-closed inter-dune lake (SE Badan Jilin Sand Sea, southern Alashan Plateau) covering the past similar to 160 years (dated by(137)Cs) was analysed palynologically. Intervals of denser Artemisia coverage on the sand dunes around the lake, indicating wetter climate, occurred from the mid-1850s to the mid-1870s, during the first two decades of the 20th century and from the late 1930s to the beginning of the 1960s
A 13.94-m-long sediment core, collected from a medium-sized lake in the Qilian Mountains (NE Tibetan Plateau, China), was analysed palynologically at 81 horizons. The interpretation of indicator taxa yielded various vertical shifts of the vegetation belts. These palaeovegetation results have been checked with lake surface pollen spectra from 8 takes representing different altitudinal vegetation belts. Our main findings are the following: A short period of the late Marine Isotope Stage 3 (around similar to 46,000 yr ago) was characterized by interglacial temperature conditions with a tree line above its present-day altitude. During the LGM, the vicinity of the lake was not covered by ice but by sparse alpine vegetation and alpine deserts, indicating that the climate was colder by similar to 4-7 degrees C than today Markedly higher temperatures were inferred from higher arboreal pollen frequencies between similar to 13,000 and similar to 7000 yr ago with a Holocene temperature optimum and a maximal Picea-Betula mixed-forest expansion between similar to 9000 and similar to 7000 yr ago, when temperatures exceeded the present-day conditions by at least 1-2 degrees C. Alpine steppes and meadows and sub-alpine shrub vegetation dominated around the lake since the middle Holocene, suggesting that vegetation and climate conditions were exceptionally stable in comparison to previous periods.
The late-Quaternary climate history of monsoonal Central Asia was inferred from 75 palaeoclimatic records which provide information about moisture conditions in the last 50 ka (or part of this period). Wet conditions occurred during middle and late Marine Isotope Stage 3, while the Last Glacial Maximum (LGM) was characterized by dry climate conditions in the region. A stepwise climate amelioration is suggested by the climate records following the LGM. Several climate signals of this period, which were reported from high-latitude ice core records, are preserved in archives from monsoonal Central Asia as well. During the early Holocene, high effective moisture was inferred from most records from the area dominated by the Indian Monsoon (e.g. the Tibetan Plateau) suggesting that Holocene optimal climate conditions occurred there during this period. In contrast, areas which are dominated by the South-East Asian monsoon (SE Monsoon) and the Westerlies (in north-western and north-central China, Mongolia) do not uniformly show an early Holocene climate optimum. For this area optimal conditions prevailed during the mid-Holocene. These apparent contradictions can possibly be explained by the regional uplift and descent of air masses in the Holocene. During the early Holocene, strengthened insolation possibly caused an enhanced low-level convergence over the Tibetan Plateau which led to the intensification of the summer monsoon. The strong air uplift caused intensified precipitation and air divergence in the upper troposphere over the Tibetan Plateau. The areas adjacent to the north therefore experienced an intensified descent of air masses and consequently increased aridity. The majority of the palaeoclimatic records suggest reduced effective moisture since the late Holocene in the region.
Simulating biome distribution on the Tibetan Plateau using a modified global vegetation model
(2011)
We used a regionally modified global vegetation model (BIOME4-Tibet) to simulate biome distribution on the Tibetan Plateau under current climate conditions derived from regional meteorological observations. The bioclimatic limits (mean temperatures of the coldest and warmest months, minimum temperature, growing degree-days on 5 degrees C and 0 degrees C bases) for some key alpine plant functional types (temperate deciduous and conifer trees, boreal deciduous and conifer trees, desert woody plants, tundra shrubs, cold herbaceous plants, and lichens/forbs) were redefined based on regional vegetation-climate relationships. Modern vegetation maps confirmed that the BIOME4-Tibet model does a better job of simulating biome patterns on the plateau (gridcell agreement 52%) than the original BIOME4 model (35%). This improved model enhanced our ability to simulate temperate conifer forest, cool conifer and mixed forest, evergreen taiga, temperate xerophytic shrubland, temperate grassland and desert, and steppe and shrub tundra biomes, but made a negligible or reduced difference to the prediction of temperate deciduous forest, warm-temperate mixed forest, and three tundra biomes (erect dwarf-shrub tundra, prostrate dwarf-shrub tundra, and cushion forb, lichen, and moss tundra). Future modification of the vegetation model, by increasing the number of shrub and herb plant functional types, re-parameterization of more precise bioclimatic constraints, and improved representation of soil, permafrost, and snow processes, will be needed to better characterize the distribution of alpine vegetation on the Tibetan Plateau.
Patterns of past vegetation changes over time and space can help facilitate better understanding of the interactions among climate, ecosystem, and human impact. Biome changes in China over the last 22,000 yr (calibrated radiocarbon date, a BP) were numerically reconstructed by using a standard approach of pollen-plant functional type-biome assignment (biomization). The biomization procedure involves pollen data from 2434 surface sites and 228 fossil sites with a high quality of pollen count and C-14 dating, 51 natural and three anthropogenic plant functional types (PFTs), as well as 19 natural and one anthropogenic biome. Surface pollen-based reconstruction of modern natural biome patterns is in good agreement (74.4%) with actual vegetation distribution in China. However, modem large-scale anthropogenic biome reconstruction has not been successful based on the current setup of three anthropogenic PFTs (plantation, secondary, and disturbed PFT) because of the limitation of non-species level pollen identification and the difficulty in the clear assignment of disturbed PFTs. The non-anthropogenic biome distributions of 44 time slices at 500-year intervals show large-scale discrepant and changed vegetation patterns from the last glacial maximum (LGM) to the Holocene throughout China. From 22 ka BP to 19 ka BP, temperate grassland, xerophytic shrubland, and desert dominated northern China, whereas cold or cool forests flourished in central China. Warm-temperate evergreen forests were restricted to far southern China, and tropical forests were absent During 18.5 ka BP to 12 ka BP, cold, cool, and dry biomes extended to some parts of northern, westem, and eastern China. Warm-temperate evergreen and mixed forests gradually expanded to occupy the whole of southern China. A slight northward shift of forest biomes occurred from 15 ka BP to 12 lea BP. During 11.5 ka BP to 9 ka BP, temperate grassland and shrubland gradually stretched to northern and western China. Cold and cool forests widely expanded into northern and central China, as well as in the northern margin of South China along with temperate deciduous forest. Since the early mid-Holocene (approximately 8.5 ka BP to 5.5 ka BP), all forest biomes shifted northward at the expense of herbaceous and shrubby biomes. Simultaneously, cold and cool forest biomes occupied the marginal areas of the Tibetan Plateau and the high mountains in western China. During the middle to late Holocene, from 5 ka to the present, temperate grassland and xerophytic shrubland expanded to the south and east, whereas temperate deciduous forests slightly shifted southward. After 3 lea BP, forest biomes were absent in western China and on the Tibetan plateau surface. Dramatic biome shifts from the LGM to the Holocene were observed in the forest-grassland ecotone and transitional zones between temperate and subtropical climates, between subtropical and tropical regions, and in the mountainous margins of the eastern Tibetan Plateau. Evidence showed more human disturbances during the late Holocene. More pollen records and historical documents are therefore further needed to understand fully the human disturbance-induced large-scale forest changes. In addition, more classifications of anthropogenic biome or land cover, more distinct assignment of pollen taxa to anthropogenic PFTs, and more effective numerical and/or mechanistic techniques in building large-scale human disturbances are required. (C) 2014 Elsevier B.V. All rights reserved.
Diatom diversity in lakes of northwest Yakutia (Siberia) was investigated by microscopic and genetic analysis of surface and cored lake sediments, to evaluate the use of sedimentary DNA for paleolimnological diatom studies and to identify obscure genetic diversity that cannot be detected by microscopic methods. Two short (76 and 73 bp) and one longer (577 bp) fragments of the ribulose 1,5-bisphosphate carboxylase/oxygenase (rbcL) gene, encoding the large subunit of the rbcL, were used as genetic markers. Diverse morphological assemblages of diatoms, dominated by small benthic fragilarioid taxa, were retrieved from the sediments of each lake. These minute fragilarioid taxa were examined by scanning electron microscopy, revealing diverse morphotypes in Staurosira and Staurosirella from the different lakes. Genetic analyses indicated a dominance of haplotypes that were assigned to fragilarioid taxa and less genetic diversity in other diatom taxa. The long rbcL_577 amplicon identified considerable diversification among haplotypes clustering within the Staurosira/Staurosirella genera, revealing 19 different haplotypes whose spatial distribution appears to be primarily related to the latitude of the lakes, which corresponds to a vegetation and climate gradient. Our rbcL markers are valuable tools for tracking differences between diatom lineages that are not visible in their morphologies. These markers revealed putatively high genetic diversity within the Staurosira/Staurosirella species complex, at a finer scale than is possible to resolve by microscopic determination. The rbcL markers may provide additional reliable information on the diversity of barely distinguishable minute benthic fragilarioids. Environmental sequencing may thus allow the tracking of spatial and temporal diversification in Siberian lakes, especially in the context of diatom responses to recent environmental changes, which remains a matter of controversy.
Genetic investigations on eukaryotic plankton confirmed the existence of modern biogeographic patterns, but analyses of palaeoecological data exploring the temporal variability of these patterns have rarely been presented. Ancient sedimentary DNA proved suitable for investigations of past assemblage turnover in the course of environmental change, but genetic relatedness of the identified lineages has not yet been undertaken. Here, we investigate the relatedness of diatom lineages in Siberian lakes along environmental gradients (i.e. across treeline transects), over geographic distance and through time (i.e. the last 7000 years) using modern and ancient sedimentary DNA. Our results indicate that closely-related Staurosira lineages occur in similar environments and less-related lineages in dissimilar environments, in our case different vegetation and co-varying climatic and limnic variables across treeline transects. Thus our study reveals that environmental conditions rather than geographic distance is reflected by diatom-relatedness patterns in space and time. We tentatively speculate that the detected relatedness pattern in Staurosira across the treeline could be a result of adaptation to diverse environmental conditions across the arctic boreal treeline, however, a geographically-driven divergence and subsequent repopulation of ecologically different habitats might also be a potential explanation for the observed pattern.
We investigated 4.84-m-long sediment record spanning over the Late Glacial and Holocene from Lake Donggi Cona to be able to reconstruct circulation pattern on the Tibetan Plateau (TP). Presently, Lake Donggi Cona is located at the boundaries of Westerlies and Asian monsoon circulations in the northeastern TP. However, the exact timing and stimulating mechanisms for climatic changes and monsoon shifts in this region are still debated. We used a 19-ka-long stable isotope record of sedimentary n-alkanes to address this discrepancy by providing insights into paleohydrological conditions. The SD of nC(23) is influenced by lake water evaporation; the BD. values of sedimentary nC(29) are mainly controlled by moisture source and temperature changes. Long-chain n-alkanes dominate over the core whereas three mean clusters (i.e. microbial, aquatic and terrestrial) can be inferred. Multi-proxies suggest five major episodes in the history of Lake Donggi Cona. The Lake Donggi Cona record indicates that the Late Glacial(18.4-14.8 cal ka BP) was dominated by low productivity of mainly microbial and aquatic organisms. Relatively low delta D values suggest low temperatures and moist conditions eventually caused by stronger Westerlies, winter monsoon and melt-water influence. Likely, the shift (similar to 17.9 cal ka BP) from microbial to enhanced aquatic input suggests either a change from deep to shallow water lake or a break in local stratification. Between 14.8 and 13.0 cal ka BP, variable climatic conditions prevailed. Although the Westerlies weekend, the increase in temperature enhanced the permafrost and snow melting (displayed by a high sedimentary accumulation rate). Higher delta D values indicate increasingly arid conditions with higher temperatures which eventually lead to high evaporative conditions and lowest lake levels. Low vegetation cover and high erosion rates led to high sediment accumulation resulting in stratification followed by anoxia in the terminal lake. From 13.0 to 9.2 cal ka BP, lowered values of 813 along with high contents of terrestrial organic matter marked the early-Holocene warming indicating a further strengthening of summer precipitation and higher lake levels. A cooling trend was observed in the mid-Holocene between 9.2 and 3.0 cal ka BP accompanied by higher moisture availability (displayed by lowered SD values) caused by reduced evaporative conditions due to a drop in temperature and recovering Westerlies. After 3.0 cal ka BP, a decrease in lake productivity and cold and semi-arid conditions prevailed suggesting lower lake levels and reduced moisture from recycled air masses and Westerlies. We propose that the summer monsoon was the predominant moisture source during the Belling-Allered warm complex and early -Holocene followed by Westerlies in mid-to-late Holocene period. Stable carbon isotope values-32%o indicate the absence of C-4 -type vegetation in the region contradicting with their presence in the Lake Qinghai record. The 81) record from lake Donggi Cona highlights the importance of the interplay between Westerlies and summer monsoon circulation at this location, which is highly dynamic in northeastern plateau compared to the North Atlantic circulation and insolation changes. Consequently lake Donggi Cona might be an important anchor point for environmental reconstructions on the Tibetan Plateau. (C) 2017 The Authors. Published by Elsevier Ltd.
Pollen-based quantitative reconstructions of past climate variables is a standard palaeoclimatic approach. Despite knowing that the spatial extent of the calibration-set affects the reconstruction result, guidance is lacking as to how to determine a suitable spatial extent of the pollen-climate calibration-set. In this study, past mean annual precipitation (P-ann) during the Holocene (since 11.5 cal ka BP) is reconstructed repeatedly for pollen records from Qinghai Lake (36.7 degrees N, 100.5 degrees E; north-east Tibetan Plateau), Gonghai Lake (38.9 degrees N, 112.2 degrees E; north China) and Sihailongwan Lake (42.3 degrees N, 126.6 degrees E; north-east China) using calibration-sets of varying spatial extents extracted from the modern pollen dataset of China and Mongolia (2559 sampling sites and 168 pollen taxa in total). Results indicate that the spatial extent of the calibration-set has a strong impact on model performance, analogue quality and reconstruction diagnostics (absolute value, range, trend, optimum). Generally, these effects are stronger with the modern analogue technique (MAT) than with weighted averaging partial least squares (WA-PLS). With respect to fossil spectra from northern China, the spatial extent of calibration-sets should be restricted to radii between ca. 1000 and 1500 km because small-scale calibration-sets (<800 km radius) will likely fail to include enough spatial variation in the modern pollen assemblages to reflect the temporal range shifts during the Holocene, while too broad a scale calibration-set (>1500 km radius) will include taxa with very different pollen-climate relationships. (C) 2017 Elsevier Ltd. All rights reserved.