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Lacustrine fossil pollen records have been widely used to reconstruct palaeovegetation and palaeoclimate changes on the Tibetan Plateau (TP). However, little is known about the vegetation representation of modern lacustrine pollen assemblages there. This paper presents the results of modern pollen investigation on 63 surface sediments from the lake basin and 37 topsoil samples from the drainage area of a large lake, Nam Co, located in the central TP. It aims to assess quantitatively the influences on lacustrine pollen assemblages of the pollen sources and sedimentary processes, and to establish vegetation representations for modern lacustrine pollen assemblages. Modern pollen assemblages from topsoils of different vegetation had diagnostic features in terms of their composition and pollen percentage. The spatial variabilities and results of principal component analysis suggested that lacustrine pollen assemblages were influenced by both the regional/local source vegetation and sedimentary processes. The lacustrine pollen assemblages were mainly homogeneous due to in-lake sedimentary processes (mixing and redistribution). An accumulation zone for lacustrine pollen assemblages was found in the deep lake basin (depth > 60 m) due to sediment focusing. The results of boosted regression tree analysis further confirmed that source vegetation was the predominant factor (85.8%) responsible for the vegetation representation of lacustrine pollen assemblages, while sedimentary processes accounted for only 14.2%. The results of discriminant analysis indicated that most lacustrine pollen assemblages (90.5%) were representative for the regional vegetation of alpine steppe in the Nam Co catchment and central TP, while only 9.5% were representative for the local meadow vegetation. Therefore, it is recommended that lacustrine pollen assemblages from deep lake basin of accumulation zone in large lakes of the TP can be used to retrieve efficiently the signals from regional vegetation and climate changes. [ABSTRACT FROM AUTHOR]

This paper describes 2007/2008 inter-annual changes in runoff from the Zhadang Glacier located on the northern slope of Nyainqêntanglha Range, Tibet, and analyzes their causes. Precipitation increased by 17.9% in summer months of 2008 compared with the same period in 2007, drainage basin runoff decreased by 33.3%, and glacial meltwater decreased by 53.8%. Change in positive accumulated air temperature explained approximately half of the inter-annual difference in glacial meltwater using a degree-day model. This suggests that the glacier is extremely sensitive to changes in air temperature. Energy balance analysis showed that change in glacier surface albedo, considered to be caused by difference in precipitation form, resulted in the large inter-annual difference in glacial meltwater. It was shown statistically that precipitation form in the summer months of 2007 was mainly rainfall which comprised 71.5% of total precipitation, while during the same period in 2008 rainfall accounted for 30.7%, with the majority of precipitation falling as snow. Precipitation form should be considered an independent factor when analyzing glacier sensitivity to climate change or forecasting the runoff from certain glaciers. [ABSTRACT FROM AUTHOR]

Fossil pollen from Nam Co and modern pollen from altitudinal vegetation belts around the lake are investigated to reveal alpine vegetation succession in response to climate changes during the Holocene in the central Tibetan Plateau. The discriminant analysis on 37 topsoil samples shows that pollen samples from alpine steppe at lower elevations (<4800 m) and alpine meadow on upper slopes (4800–5200 m) can be distinguished by their pollen assemblages. Samples from alpine steppe contain more Artemisia (25.1%) and Poaceae pollen (11.5%), whereas those from alpine meadow are dominated by Cyperaceae pollen (>60%). Our result indicates that the pollen ratio of Artemisia to Cyperaceae (A/Cy) can be used as an indicator of the vertical shift of vegetation belts and temperature changes in the central Tibetan Plateau as suggested by previous studies. A history of the vertical shift of vegetation belts on the northern slope of Nyainqentanglha Mountains and climate changes since 8.4 kyr BP are thus recovered by 198 fossil pollen assemblages from a 332 cm core of Nam Co. Paleovegetation reconstructed from fossil pollen assemblages through discriminant analysis shows a general downward shift of altitudinal vegetation belts, suggesting a decline in the temperature trend since 8.4 kyr BP. This result is consistent with the reduction of A/Cy ratios. The fossil pollen record also reveals warm and wet climate during the early to mid Holocene, and cold and dry conditions during the late Holocene in the Nam Co area. A comparison of Holocene climatic reconstructions across the Plateau indicates that termination of maximum moisture at around 6–5.5 kyr BP in our record is associated with the southeastward retreat of the Southwest Monsoon. [ABSTRACT FROM AUTHOR]