TY  - JOUR
A1  - Herzschuh, Ulrike
A1  - Ni, Jian
A1  - Birks, H. John B.
A1  - Böhner, Jürgen
T1  - Driving forces of mid-Holocene vegetation shifts on the upper Tibetan Plateau, with emphasis on changes in atmospheric CO2 concentrations
JF  - Quaternary science reviews : the international multidisciplinary research and review journal
N2  - Numerous pollen records across the upper Tibetan Plateau indicate that in the early part of the mid-Holocene, Kobresia-rich high-alpine meadows invaded areas formerly dominated by alpine steppe vegetation rich in Artemisia. We examine climate, land-use, and CO2 concentration changes as potential drivers for this marked vegetation change. The climatic implications of these vegetational shifts are explored by applying a newly developed pollen-based moisture-balance transfer-function to fossil pollen spectra from Koucha Lake on the north-eastern Tibetan Plateau (34.0 degrees N; 97.2 degrees E; 4540 m a.s.l.) and Xuguo Lake on the central Tibetan Plateau (31.97 degrees N; 90.3 degrees E; 4595 m a.s.l.), both located in the meadow-steppe transition zone. Reconstructed moisture-balances were markedly reduced (by similar to 150-180 mm) during the early mid-Holocene compared to the late-Holocene. These findings contradict most other records from the Indian monsoonal realm and also most non-pollen records from the Tibetan Plateau that indicate a rather wet early- and mid-Holocene. The extent and timing of anthropogenic land-use involving grazing by large herbivores on the upper Tibetan Plateau and its possible impacts on high-alpine vegetation are still mostly unknown due to the lack of relevant archaeological evidence. Arguments against a mainly anthropogenic origin of Kobresia high-alpine meadows are the discovery of the widespread expansion of obviously 'natural' Kobresia meadows on the south-eastern Tibetan Plateau during the Lateglacial period indicating the natural origin of this vegetation type and the lack of any concurrence between modern human-driven vegetation shifts and the mid-Holocene compositional changes. Vegetation types are known to respond to atmospheric CO2 concentration changes, at least on glacial-interglacial scales. This assumption is confirmed by our sensitivity study where we model Tibetan vegetation at different CO2 concentrations of 375 (present-day), 260 (early Holocene), and 650 ppm (future scenario) using the BIOME4 global vegetation model. Previous experimental studies confirm that vegetation growing on dry and high sites is particularly sensitive to CO2 changes. Here we propose that the replacement of drought-resistant alpine steppes (that are well adapted to low CO2 concentrations) by mesic Kobresia meadows can, at least, be partly interpreted as a response to the increase of CO2 concentration since 7000 years ago due to fertilization and water-saving effects. Our hypothesis is corroborated by former CO2 fertilization experiments performed on various dry grasslands and by the strong recent expansion of high-alpine meadows documented by remote sensing studies in response to recent CO2 increases.
KW  - Tibetan Plateau
KW  - Pollen
KW  - Holocene
KW  - Transfer function
KW  - Kobresia meadow
KW  - Atmospheric CO2 concentration
Y1  - 2011
U6  - https://doi.org/10.1016/j.quascirev.2011.03.007
SN  - 0277-3791
VL  - 30
IS  - 15-16
SP  - 1907
EP  - 1917
PB  - Elsevier
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Wischnewski, Juliane
A1  - Mackay, Anson W.
A1  - Appleby, Peter G.
A1  - Mischke, Steffen
A1  - Herzschuh, Ulrike
T1  - Modest diatom responses to regional warming on the southeast Tibetan Plateau during the last two centuries
JF  - Journal of paleolimnolog
N2  - A general mean annual temperature increase accompanied with substantial glacial retreat has been noted on the Tibetan Plateau during the last two centuries but most significantly since the mid 1950s. These climate trends are particularly apparent on the southeastern Tibetan Plateau. However, the Tibetan Plateau (due to its heterogeneous mountain landscape) has very complex and spatially differing temperature and precipitations patterns. As a result, intensive palaeolimnological investigations are necessary to decipher these climatic patterns and to understand ecological responses to recent environmental change. Here we present palaeolimnological results from a (210)Pb/(137)Cs-dated sediment core spanning approximately the last 200 years from a remote high-mountain lake (LC6 Lake, working name) on the southeastern Tibetan Plateau. Sediment profiles of diatoms, organic variables (TOC, C:N) and grain size were investigated. The (210)Pb record suggests a period of rapid sedimentation, which might be linked to major tectonic events in the region ca. 1950. Furthermore, unusually high (210)Pb supply rates over the last 50 years suggest that the lake has possibly been subjected to increasing precipitation rates, sediment focussing and/or increased spring thaw. The majority of diatom taxa encountered in the core are typical of slightly acidic to circumneutral, oligotrophic, electrolyte-poor lakes. Diatom species assemblages were rich, and dominated by Cyclotella sp., Achnanthes sp., Aulacoseira sp. and fragilarioid taxa. Diatom compositional change was minimal over the 200-year period (DCCA = 0.85 SD, p = 0.59); only a slightly more diverse but unstable diatom assemblage was recorded during the past 50 years. The results indicate that large-scale environmental changes recorded in the twentieth century (i.e. increased precipitation and temperatures) are likely having an affect on the LC6 Lake, but so far these impacts are more apparent on the lake geochemistry than on the diatom flora. Local and/or regional peculiarities, such as increasing precipitation and cloud cover, or localized climatic phenomena, such as negative climate feedbacks, might have offset the effects of increasing mean surface temperatures.
KW  - Diatoms
KW  - Tibetan Plateau
KW  - Mountain lake
KW  - Climate change
KW  - Lake sediments
KW  - Palaeolimnology
Y1  - 2011
U6  - https://doi.org/10.1007/s10933-011-9533-x
SN  - 0921-2728
VL  - 46
IS  - 2
SP  - 215
EP  - 227
PB  - Springer
CY  - Dordrecht
ER  - 
TY  - JOUR
A1  - Wischnewski, Juliane
A1  - Kramer, Annette
A1  - Kong, Zhaochen
A1  - Mackay, Anson W.
A1  - Simpson, Gavin L.
A1  - Mischke, Steffen
A1  - Herzschuh, Ulrike
T1  - Terrestrial and aquatic responses to climate change and human impact on the southeastern Tibetan Plateau during the past two centuries
JF  - Global change biology
N2  - Rapid population growth and economic development have led to increased anthropogenic pressures on the Tibetan Plateau, causing significant land cover changes with potentially severe ecological consequences. To assess whether or not these pressures are also affecting the remote montane-boreal lakes on the SE Tibetan Plateau, fossil pollen and diatom data from two lakes were synthesized. The interplay of aquatic and terrestrial ecosystem response was explored in respect to climate variability and human activity over the past 200 years. Nonmetric multidimensional scaling and Procrustes rotation analysis were undertaken to determine whether pollen and diatom responses in each lake were similar and synchronous. Detrended canonical correspondence analysis was used to develop quantitative estimates of compositional species turnover. Despite instrumental evidence of significant climatic warming on the southeastern Plateau, the pollen and diatom records indicate very stable species composition throughout their profiles and show only very subtle responses to environmental changes over the past 200 years. The compositional species turnover (0.36-0.94 SD) is relatively low in comparison to the species reorganizations known from the periods during the mid-and early-Holocene (0.64-1.61 SD) on the SE Plateau, and also in comparison to turnover rates of sediment records from climate-sensitive regions in the circum arctic. Our results indicate that climatically induced ecological thresholds are not yet crossed, but that human activity has an increasing influence, particularly on the terrestrial ecosystem in our study area. Synergistic processes of post-Little Ice Age warming, 20th century climate warming and extensive reforestations since the 19th century have initiated a change from natural oak-pine forests to seminatural, likely less resilient pine-oak forests. Further warming and anthropogenic disturbances would possibly exceed the ecological threshold of these ecosystems and lead to severe ecological consequences.
KW  - climate change
KW  - compositional species turnover
KW  - diatoms
KW  - human impact
KW  - pollen
KW  - Procrustes rotation
KW  - Tibetan Plateau
Y1  - 2011
U6  - https://doi.org/10.1111/j.1365-2486.2011.02474.x
SN  - 1354-1013
VL  - 17
IS  - 11
SP  - 3376
EP  - 3391
PB  - Wiley-Blackwell
CY  - Malden
ER  - 
TY  - JOUR
A1  - Wang, Yongbo
A1  - Herzschuh, Ulrike
T1  - Reassessment of Holocene vegetation change on the upper Tibetan Plateau using the pollen-based REVEALS model
JF  - Review of palaeobotany and palynology : an international journal
N2  - Previous studies based on fossil pollen data have reported significant changes in vegetation on the alpine Tibetan Plateau during the Holocene. However, since the relative proportions of fossil pollen taxa are largely influenced by individual pollen productivities and the dispersal characteristics, such inferences on vegetation have the potential to be considerably biased. We therefore examined the modern pollen-vegetation relationships for four common pollen species on the Tibetan Plateau, using Extended R-value (ERV) models. Assuming an average radius of 100 m for the sampled lakes, we estimated the relevant source area of pollen (RSAP) to be 2200 m (which represents the distance from the lake). Using Poaceae as the reference taxa (Pollen Productivity Estimate, PPE = 1), ERV Submodel 2 derived relative high PPEs for the steppe and desert taxa: 2.079 +/- 0.432 for Artemisia and 5.379 +/- 1.077 for Chenopodiaceae. Low PPEs were estimated for the Cyperaceae (1.036 +/- 0.012). whose plants are characteristic of the alpine Kobresia meadows. Applying these PPEs to four fossil pollen sequences since the Late Glacial, the plant abundances on the central and north-eastern Tibetan Plateau were quantified using the "Regional Estimates of Vegetation Abundance from Large Sites" (REVEALS) model. The proportions of Artemisia and Chenopodiaceae were greatly reduced compared to their original pollen percentages in the reconstructed vegetation, owing to their high productivities and their dispersal characteristics, while Cyperaceae showed a relative increase in the vegetation reconstruction. The reconstructed vegetation assemblages of the four pollen sequence sites always yielded smaller compositional species turnovers than suggested by the pollen spectra, as revealed by Detrended Canonical Correspondence Analyses (DCCA) of the Holocene sections. The strength of the previously reported vegetation changes may therefore have been overestimated, which indicates the importance of taking into account pollen-vegetation relationships when discussing the potential drivers (such as climate, land use, atmospheric CO(2) concentrations) and implications (such as for land surface-climate feedbacks, carbon storage, and biodiversity) of vegetation change.
KW  - pollen productivity
KW  - vegetation reconstruction
KW  - ERV model
KW  - REVEALS model
KW  - Holocene
KW  - Tibetan Plateau
Y1  - 2011
U6  - https://doi.org/10.1016/j.revpalbo.2011.09.004
SN  - 0034-6667
VL  - 168
IS  - 1
SP  - 31
EP  - 40
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - THES
A1  - Wang, Yongbo
T1  - Late glacial to Holocene climate and vegetation changes on the Tibetan Plateau inferred from fossil pollen records in lacustrine sediments
T1  - Pollenanalytische Ableitung der spätglazialen und holozänen Klima- und Vegetationsveränderungen auf dem tibetischen Hochland anhand von Seesedimenten
N2  - The past climate in central Asia, and especially on the Tibetan Plateau (TP), is of great importance for an understanding of global climate processes and for predicting the future climate. As a major influence on the climate in this region, the Asian Summer Monsoon (ASM) and its evolutionary history are of vital importance for accurate predictions. However, neither the evolutionary pattern of the summer monsoon nor the driving mechanisms behind it are yet clearly understood. For this research, I first synthesized previously published Late Glacial to Holocene climatic records from monsoonal central Asia in order to extract the general climate signals and the associated summer monsoon intensities. New climate and vegetation sequences were then established using improved quantitative methods, focusing on fossil pollen records recovered from Tibetan lakes and also incorporating new modern datasets. The pollen-vegetation and vegetation-climate relationships on the TP were also evaluated in order to achieve a better understanding of fossil pollen records. The synthesis of previously published moisture-related palaeoclimate records in monsoonal central Asia revealed generally different temporal patterns for the two monsoonal subsystems, i.e. the Indian Summer Monsoon (ISM) and East Asian Summer Monsoon (EASM). The ISM appears to have experienced maximum wet conditions during the early Holocene, while many records from the area affected by the EASM indicate relatively dry conditions at that time, particularly in north-central China where the maximum moisture levels occurred during the middle Holocene. A detailed consideration of possible driving factors affecting the summer monsoon, including summer solar insolation and sea surface temperatures, revealed that the ISM was primarily driven by variations in northern hemisphere solar insolation, and that the EASM may have been constrained by the ISM resulting in asynchronous patterns of evolution for these two subsystems. This hypothesis is further supported by modern monsoon indices estimated using the NCEP/NCAR Reanalysis data from the last 50 years, which indicate a significant negative correlation between the two summer monsoon subsystems. By analogy with the early Holocene, intensification of the ISM during coming decades could lead to increased aridification elsewhere as a result of the asynchronous nature of the monsoon subsystems, as can already be observed in the meteorological data from the last 15 years. A quantitative climate reconstruction using fossil pollen records was achieved through analysis of sediment core recovered from Lake Donggi Cona (in the north-eastern part of the TP) which has been dated back to the Last Glacial Maximum (LGM). A new data-set of modern pollen collected from large lakes in arid to semi-arid regions of central Asia is also presented herein. The concept of "pollen source area" was introduced to modern climate calibration based on pollen from large lakes, and was applied to the fossil pollen sequence from Lake Donggi Cona. Extremely dry conditions were found to have dominated the LGM, and a subsequent gradually increasing trend in moisture during the Late Glacial period was terminated by an abrupt reversion to a dry phase that lasted for about 1000 years and coincided with the first Heinrich Event of the northern Atlantic region. Subsequent periods corresponding to the warm Bølling-Allerød period and the Younger Dryas cold event were followed by moist conditions during the early Holocene, with annual precipitation of up to about 400 mm. A slightly drier trend after 9 cal ka BP was then followed by a second wet phase during the middle Holocene that lasted until 4.5 cal ka BP. Relatively steady conditions with only slight fluctuations then dominated the late Holocene, resulting in the present climatic conditions. In order to investigate the relationship between vegetation and climate, temporal variations in the possible driving factors for vegetation change on the northern TP were examined using a high resolution late Holocene pollen record from Lake Kusai. Moving-window Redundancy Analyses (RDAs) were used to evaluate the correlations between pollen assemblages and individual sedimentary proxies. These analyses have revealed frequent fluctuations in the relative abundances of alpine steppe and alpine desert components, and in particular a decrease in the total vegetation cover at around 1500 cal a BP. The climate was found to have had an important influence on vegetation changes when conditions were relatively wet and stable. However, after the 1500 cal a BP threshold in vegetation cover was crossed the vegetation appears to have been affected more by extreme events such as dust storms or fluvial erosion than by the general climatic trends. In addition, pollen spectra over the last 600 years have been revealed by Procrustes analysis to be significantly different from those recovered from older samples, which is attributed to an increased human impact that resulted in unprecedented changes to the composition of the vegetation. Theoretical models that have been developed and widely applied to the European area (i.e. the Extended R-Value (ERV) model and the Regional Estimates of Vegetation Abundance from Large Sites (REVEALS) model) have been applied to the high alpine TP ecosystems in order to investigate the pollen-vegetation relationships, as well as for quantitative reconstructions of vegetation abundance. The modern pollen–vegetation relationships for four common pollen species on the TP have been investigated using Poaceae as the reference taxa. The ERV Submodel 2 yielded relatively high PPEs for the steppe and desert taxa (Artemisia Chenopodiaceae), and low PPEs for the Cyperaceae that are characteristic of the alpine Kobresia meadows. The plant abundances on the central and north-eastern TP were quantified by applying these PPEs to four post-Late Glacial fossil pollen sequences. The reconstructed vegetation assemblages for the four pollen sequences always yielded smaller compositional species turnovers than suggested by the pollen spectra, indicating that the strength of the previously-reported vegetation changes may therefore have been overestimated. In summary, the key findings of this thesis are that (a) the two ASM subsystems show asynchronous patterns during both the Holocene and modern time periods, (b) fossil pollen records from large lakes reflect regional signals for which the pollen source areas need to be taken into account, (c) climate is not always the main driver for vegetation change, and (d) previously reported vegetation changes on the TP may have been overestimated because they ignored inter-species variations in pollen productivity.
N2  - Das Paläoklima in Zentralasien, besonders in der Hochebene von Tibet (HT), ist von großer Bedeutung um globale Klimaprozesse zu verstehen und mögliche Voraussagung für die zukunft zu treffen. Als wichtigstes Klimaphänomen nehmen der asiatische Sommermonsun (ASM) und seine Entwicklungsgeschichte eine Schlüsselposition ein. Dennoch sind derzeit weder das Entwicklungsschema noch der antreibende Vorgang ausreichend verstanden. Dies gilt insbesondere für das Holozän, für welches große Kimaschwankungen und regionale Diskrepanzen weithin belegt sind. Deshalb habe ich zuerst holozäne Klimadaten zusammengefasst. Bereits veröffentlichte Publikationen aus den Monsungebieten Zentralasiens dienten als Grundlage, um die wichtigsten Klimasignale und die zugehörigen Intensitäten des Sommermonsuns heraus zu arbeiten. Anhand von Pollensequenzen aus tibetischen Seen erzeugte ich neue Klima- und Vegetationssequenzen, welche auf verbesserten quantitativen Methoden und rezenten Datensätzen beruhen. Außerdem wurden die Verhältnisse Pollen-Vegetation und Vegetation-Klima bewertet, um Schlussfolgerungen fossiler Pollensequenzen zu verbessern. Die Zusammenfassung der zuvor veröffentlichten, niederschlagsbezogenen Paläoklimadaten im Monsungebiet Zentralasiens ergab generell unterschiedliche Muster für die zwei Teilsysteme des ASMs, den Indischen Sommermonsun (ISM) und den Ostasiatischen Sommermonsun (OASM). Der ISM weist maximale feuchte Bedingungen während des frühen Holozöns auf, während viele Datensätze aus dem Gebiet des OASMs einen relativ trockenen Zustand anzeigen, besonders im nördlichen Zentralchina, wo maximale Niederschläge während des mittleren Holozäns registriert wurden. Genaue Betrachtungen der Antriebsfaktoren des Sommermonsuns ergaben, dass der ISM hauptsächlich durch Veränderungen der Sonneneinstrahlung auf der Nordhemisphäre angetrieben wird, während der OASM potentiell durch den ISM beherrscht wird - dies führt zu asynchronen Entwicklungen. Diese Hypothese wird durch rezente Monsunindizes gestützt. Sie weisen eine signifikant negative Korrelation zwischen den beiden Sommermonsun-Teilsystemen auf. Für die quantitative Klimarekonstruktion von Pollensequenzen wurde ein Sedimentkern aus dem See Donggi Cona im Nordosten der HT analysiert, der bis zum letzten glazialen Maximum (LGM) zurückdatiert wurde. Aufgrund der Tatsache, dass Donggi Cona ein relativ großer See ist, wird hiermit ein neuer Pollen-Klima-Kalibrierungsdatensatz auf Grundlage großer Seen in ariden und semiariden Regionen Zentralasiens vorgelegt. Das Konzept des Pollenherkunftsgebietes wurde in diese rezente, pollenbasierte Klimakalibrierung eingebracht und auf die Pollensequenz von Donggi Cona angewendet. Die Auswertung ergab, dass extrem trockene Bedingungen während des LGM (ca. 100 mm/yr) vorherrschten. Ein ansteigender Trend von Niederschlägen während des späten Glazials wurde durch einen abrupten Rückgang zu einer etwa 1000-jährigen Trockenphase beendet, welche mit Heinrich-Ereignis 1 in der Nordatlantik-Region übereinstimmt. Danach entsprechen die Klimaperioden dem warmen Bølling/Allerød und dem Kälteereignis der Jüngeren Dryas. Anschließend herrschten feuchte Bedingungen im frühen Holozän (bis zu 400 mm/yr). Ein etwas trockenerer Trend nach dem Holozänen Klimaoptimum wurde dann von einer zweiten Feuchtphase abgelöst, welche bis 4,5 cal. ka vor heute andauerte. Relativ gleichmäßige Bedingungen dominierten das späte Holozän bis heute. Die Klimadynamik seit dem LGM wurde vor allem durch Entgletscherung und Intensitätsschwankungen des ASM bestimmt. Bei der Betrachtung des Vegetation-Klima-Verhältnisses habe ich die zeitlichen Variationen der bestimmenden Faktoren hinsichtlich der Vegetationsdynamik auf der nördlichen HT untersucht. Dabei wurden hochauflösende holozäne Pollendaten des Kusai-Sees verwendet. Eine Redundanzanalyse (RDA) wurde angewendet um die Korrelation zwischen Pollenvergesellschaftungen und individuellen sedimentären Klimaanzeigern als auch die damit verbundene Signifikanz zu bewerten. Es stellte sich heraus, dass das Klima einen wichtigen Einfluss auf den Veränderungen in der Vegetation besaß, wenn die Bedingungen relativ warm und feucht waren. Trotzdem scheint es, dass, dass die Vegetation bei zu geringer Bedeckung stärker durch Extremereignisse wie Staubstürme oder fluviale Erosion beeinflusst wurde. Pollenspektren der vergangen 600 Jahre erwiesen sich als signifikant unterschiedlich verglichen mit den älterer Proben, was auf verstärkten anthropogenen Einfluss hindeutet. Dieser resultierte in einem beispiellosen Wandel in der Zusammensetzung der Vegetation. In Hinsicht auf das Pollen-Vegetation-Verhältnis und der quantitativen Rekonstruktion der Vegetationshäufigkeit habe ich theoretische Modelle, welche für europäische Regionen entwickelt und weithin angewendet wurden, respektive die Modelle "Extended R-Value" (ERV) sowie "Regional Estimates of Vegetation Abundance from Large Sites" (REVEALS), auf die hochalpinen Ökosysteme der HT überführt. Dafür wurden rezente Pollen-Vegetations-Verhältnisse von vier weit verbreiteten Pollen-Arten der HT überprüft. Poaceae wurden als Referenztaxa verwendet. Bei der Anwendung dieser Verhältnisse auf vier Pollensequenzen, welche die Paläoumweltbedingungen seit dem letzten Glazial widerspiegeln, wurden die Häufigkeiten von Pflanzen auf der zentralen und nordöstlichen HT quantifiziert. Anteile von Artimisia und Chenopodiaceae waren dabei im Vergleich zu ihren ursprünglichen Pollenprozenten deutlich verringert. Cyperaceae hingegen wies eine relative Zunahme in dieser Vegetationsrekonstruktion auf. Die rekonstruierten Vegetationsvergesellschaftungen an den Standorten der vier Pollensequenzen ergaben stets geringere Umwälzungen in der Artenzusammensetzung, als durch die Pollenspektren zu vermuten gewesen wäre. Dies kann ein Hinweis darauf sein, dass die Intensität der bislang angenommenen Vegetationsveränderungen überschätzt worden ist. Zusammengefasst sind die Hauptresultate dieser Dissertation, dass (a) die zwei ASM Teilsysteme asynchrone Muster während des Holozäns und heute aufweisen, dass (b) fossile Pollensequenzen großer Seen regionale Klimasignale widerspiegeln sofern die Herkunftsgebiete der Pollen berücksichtigt werden, dass (c) Klima nicht immer der Haupteinflussfaktor für Vegetationswandel ist und dass (d) das Ausmaß von Vegetationsveränderungen in zuvor veröffentlichten Studien auf der Hochebene von Tibet überschätzt worden sein kann, weil Diskrepanzen der Pollenproduktivität zwischen den Arten nicht einbezogen wurden.
KW  - Asiatischer Sommermonsun
KW  - ASM
KW  - Holozän
KW  - Seesedimente
KW  - Pollen
KW  - Hochland von Tibet
KW  - Asian Summer Monsoon
KW  - Holocene
KW  - Lake sediments
KW  - Pollen
KW  - Tibetan Plateau
Y1  - 2011
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-63155
ER  - 
TY  - THES
A1  - Wischnewski, Juliane
T1  - Reconstructing climate variability on the Tibetan Plateau : comparing aquatic and terrestrial signals
T1  - Klimarekonstruktionen auf dem Tibet Plateau : aquatische und terrestrische Signale im Vergleich
N2  - Spatial and temporal temperature and moisture patterns across the Tibetan Plateau are very complex. The onset and magnitude of the Holocene climate optimum in the Asian monsoon realm, in particular, is a subject of considerable debate as this time period is often used as an analogue for recent global warming. In the light of contradictory inferences regarding past climate and environmental change on the Tibetan Plateau, I have attempted to explain mismatches in the timing and magnitude of change. Therefore, I analysed the temporal variation of fossil pollen and diatom spectra and the geochemical record from palaeo-ecological records covering different time scales (late Quaternary and the last 200 years) from two core regions in the NE and SE Tibetan Plateau. For interpretation purposes I combined my data with other available palaeo-ecological data to set up corresponding aquatic and terrestrial proxy data sets of two lake pairs and two sets of sites. I focused on the direct comparison of proxies representing lacustrine response to climate signals (e.g., diatoms, ostracods, geochemical record) and proxies representing changes in the terrestrial environment (i.e., terrestrial pollen), in order to asses whether the lake and its catchments respond at similar times and magnitudes to environmental changes. Therefore, I introduced the established numerical technique procrustes rotation as a new approach in palaeoecology to quantitatively compare raw data of any two sedimentary records of interest in order to assess their degree of concordance. Focusing on the late Quaternary, sediment cores from two lakes (Kuhai Lake 35.3°N; 99.2°E; 4150 m asl; and Koucha Lake 34.0°N; 97.2°E; 4540 m asl) on the semi-arid northeastern Tibetan Plateau were analysed to identify post-glacial vegetation and environmental changes, and to investigate the responses of lake ecosystems to such changes. Based on the pollen record, five major vegetation and climate changes could be identified: (1) A shift from alpine desert to alpine steppe indicates a change from cold, dry conditions to warmer and more moist conditions at 14.8 cal. ka BP, (2) alpine steppe with tundra elements points to conditions of higher effective moisture and a stepwise warming climate at 13.6 cal. ka BP, (3) the appearance of high-alpine meadow vegetation indicates a further change towards increased moisture, but with colder temperatures, at 7.0 cal. ka BP, (4) the reoccurrence of alpine steppe with desert elements suggests a return to a significantly colder and drier phase at 6.3 cal. ka BP, and (5) the establishment of alpine steppe-meadow vegetation indicates a change back to relatively moist conditions at 2.2 cal. ka BP. To place the reconstructed climate inferences from the NE Tibetan Plateau into the context of Holocene moisture evolution across the Tibetan Plateau, I applied a five-scale moisture index and average link clustering to all available continuous pollen and non-pollen palaeoclimate records from the Tibetan Plateau, in an attempt to detect coherent regional and temporal patterns of moisture evolution on the Plateau. However, no common temporal or spatial pattern of moisture evolution during the Holocene could be detected, which can be assigned to the complex responses of different proxies to environmental changes in an already very heterogeneous mountain landscape, where minor differences in elevation can result in marked variations in microenvironments. Focusing on the past 200 years, I analysed the sedimentary records (LC6 Lake 29.5°N, 94.3°E, 4132 m asl; and Wuxu Lake 29.9°N, 101.1°E, 3705 m asl) from the southeastern Tibetan Plateau. I found that despite presumed significant temperature increases over that period, pollen and diatom records from the SE Tibetan Plateau reveal only very subtle changes throughout their profiles. The compositional species turnover investigated over the last 200 years appears relatively low in comparison to the species reorganisations during the Holocene. The results indicate that climatically induced ecological thresholds are not yet crossed, but that human activity has an increasing influence, particularly on the terrestrial ecosystem. Forest clearances and reforestation have not caused forest decline in our study area, but a conversion of natural forests to semi-natural secondary forests. The results from the numerical proxy comparison of the two sets of two pairs of Tibetan lakes indicate that the use of different proxies and the work with palaeo-ecological records from different lake types can cause deviant stories of inferred change. Irrespective of the timescale (Holocene or last 200 years) or region (SE or NE Tibetan Plateau) analysed, the agreement in terms of the direction, timing, and magnitude of change between the corresponding terrestrial data sets is generally better than the match between the corresponding lacustrine data sets, suggesting that lacustrine proxies may partly be influenced by in-lake or local catchment processes whereas the terrestrial proxy reflects a more regional climatic signal. The current disaccord on coherent temporal and spatial climate patterns on the Tibetan Plateau can partly be ascribed to the complexity of proxy response and lake systems on the Tibetan Plateau. Therefore, a multi-proxy, multi-site approach is important in order to gain a reliable climate interpretation for the complex mountain landscape of the Tibetan Plateau.
N2  - Die räumlichen und zeitlichen Temperatur- und Feuchtigkeitsmuster auf dem Tibet-Plateau sind sehr komplex. Im Einzugsbereich der asiatischen Monsune sind insbesondere der Beginn und das Ausmaß des Klimaoptimums während des Holozäns von wissenschaftlichem Interesse, da diese Periode oft als Analogie für die derzeitige globale Klimaerwärmung herangezogen wird. In Hinblick auf sich teilweise widersprechende Paläoklima- und Umweltrekonstruktionen für das Tibet-Plateau, ist es mein Ziel, die bestehenden Unstimmigkeiten bezüglich des Zeitpunktes und des Ausmaßes des Umweltwandels zu erklären. Dafür wurden von mir zeitliche Variationen fossiler Pollen- und Diatomeenspektren und geochemische Untersuchungen an Seesedimenten unterschiedlicher Zeitskalen (Spätquartär und die letzten 200 Jahre) aus zwei Kernregionen auf dem NO und SO Tibet-Plateau analysiert. Zur Unterstützung der Interpretation wurden die hier erhobenen Daten mit bereits vorhandenen paläoökologischen Aufzeichnungen der Lokalitäten kombiniert, um Datensätze der entsprechenden aquatischen und terrestrischen Proxy-Daten (Stellvertreterdaten) zweier Seenpaare aus den beiden Regionen gegenüberstellen zu können. Hierbei konzentrierte ich mich auf den direkten Vergleich von Proxies, die die Seenentwicklung reflektieren (z.B. Diatomeen, Ostracoden, geochemische Eigenschaften), mit Proxies, die Veränderungen der terrestrischen Umgebung des Sees beschreiben (terrestrische Pollen). Durch diesen Vergleich lässt sich beurteilen, ob Veränderungen im See selbst mit Umweltveränderungen in dem jeweiligen Einzugsgebiet zeitlich übereinstimmen. Dafür habe ich die bereits etablierte numerische Methode Procrustes-Rotation als neuen Ansatz in der Paläoökologie eingeführt. Damit ist ein quantitativer Vergleich von Rohdaten zweier beliebiger sedimentärer Datensätze möglich, um den Grad der Übereinstimmung zu prüfen. Um die in dieser Arbeit rekonstruierten Umwelt- und Klimaereignisse des nordöstlichen Tibet-Plateaus in einen größeren Zusammenhang hinsichtlich holozäner Klimaentwicklung des gesamten Plateaus setzen zu können, und um schlüssige zeitliche und räumliche Klimatrends auf dem Plateau erkennen zu können, habe ich auf alle vorhandenen Paläoklimadatensätze einen Fünf-Skalen Feuchtigkeitsindex und eine Clusteranalyse angewandt. Es konnten jedoch keine einheitlichen zeitlichen und räumlichen Trends der holozänen Klimaentwicklung nachgewiesen werden, was meiner Analyse entsprechend, auf die komplexen Reaktionen verschiedener Proxies auf Umweltveränderungen in einer ohnehin sehr heterogen Berglandschaft, zurückgeführt werden kann. Die Ergebnisse des numerischen Proxy-Vergleichs beider Seenpaare zeigen, dass die Verwendung von verschiedenen Proxies und die Arbeit mit paläo-ökologischen Datensätzen unterschiedlicher See-Typen zu abweichenden Klimaableitungen führen können. Unabhängig vom untersuchten Zeitraum (Holozän oder die letzten 200 Jahren) oder der Region (SO oder NO Tibet-Plateau), ist die Übereinstimmung zweier Datensätze hinsichtlich der Richtung, des Zeitpunktes und des Ausmaßes der abgeleiteten Paläo-Umweltverhältnisse in der Regel zwischen den entsprechenden terrestrischen Datensätzen besser als zwischen den entsprechenden lakustrinen Datensätzen. Die derzeitige Uneinigkeit über stimmige zeitliche und räumliche Klimatrends auf dem Tibet-Plateau kann daher teilweise der Komplexität der verschieden Proxies und ihrer individuellen Empfindlichkeiten gegenüber Umweltveränderungen sowie der unterschiedlichen Reaktionsweise verschiedenartiger See-Systeme auf dem Plateau zugeschrieben werden. Meine Ergebnisse zeigen, dass ein „Multi-Proxy-Multi-Site-Ansatz“ für zuverlässige Paläoklimaableitungen für das Tibet-Plateau von zentraler Bedeutung ist.
KW  - Tibet Plateau
KW  - Holozän
KW  - Pollen
KW  - Diatomeen
KW  - Prokrustes Analyse
KW  - Tibetan Plateau
KW  - Holocene
KW  - Pollen
KW  - Diatoms
KW  - Procrustes rotation analysis
Y1  - 2011
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-52453
ER  -