Lingyu Tang

Changes in South Asian monsoon: New high-resolution paleoclimatic records from Tibet, China

High-resolution pollen records from 6 small lakes in the Tibetan Plateau provided the details of evolution of South Asian monsoon since the Last Glacial Maximum. Prior to 16 kaBP, the region was a desert-steppe characterized by cold and dry climates, the January temperature was 7 −10°C lower than that of present and the annual precipitation only accounted for 40% of the present. The temperature and precipitation increased gradually and trees began to live in the region after 12 kaBP, but during the interval from 9.2 to 6.3 kaBP, forest and forest-meadow appeared occasionally. From 8 to 5 kaBP, both January and July temperature was 2—3°C higher and annual precipitation was also about 200 mm higher than that of the present. After 5 kaBP, temperature and precipitation decreased linearly and steppe vegetation began to degenerate.

Ecotone shift and major droughts during the mid-late Holocene in the central Tibetan Plateau.

A well-dated pollen record from a large lake located on the meadow-steppe ecotone provides a history of ecotone shift in response to monsoonal climate changes over the last 6000 years in the central Tibetan Plateau. The pollen record indicates that the ecotone shifted eastward during 6000-4900, 4400-3900, and 2800-1600 cal. yr BP when steppes occupied this region, whereas it shifted westward during the other intervals when the steppes were replaced by meadows. The quantitative reconstruction of paleoclimate derived from the pollen record shows that monsoon precipitation fluctuated around the present level over the last 6000 years in the central Tibetan Plateau. Three major drought episodes of 5600-4900, 4400-3900, and 2800-2400 cal. yr BP are detected by pollen signals and lake sediments. Comparison of our record with other climatic proxy data from the Tibetan Plateau and other monsoonal regions shows that these episodes are three major centennial-scale monsoon weakening events.

Numerical Analysis of Modern and Fossil Pollen Data from the Tibetan Plateau

This study applies a number of multivariate numerical techniques in the analysis of modern and fossil pollen data for the purpose of paleovegetational reconstruction in the Tibetan Plateau. Modern pollen spectra from 227 sampling sites in the Tibetan Plateau were classified into groups using cluster analysis and detrended correspondence analysis. By comparing the quantitatively derived groups with local vegetation types at sampling locations, the modern pollen spectra were assigned into five major vegetation types—shrubland, forest, meadow, steppe, and desert. These were used as a priori groups for discriminant analysis. A set of discriminant functions was derived that correctly classifies 93.3 percent of the surface samples. These functions were then applied to a fossil pollen record from Yidun Glacial Lake near the Sichuan–Tibet border to reconstruct the major pattern of vegetational changes since the last glacial maximum. The results show that steppe and meadow prevailed around the site during the Late Glacial period (17.3–11.5 ka BP). Regional vegetation changed from meadow to forest during the transitional period of 11.5–9.2 ka BP. Forests have persisted in the Yidun area since 9.2 ka BP. A comparison of our reconstructed paleovegetation with other proxy data suggests that the transition to forest around 9.2 ka BP followed the early Holocene intensification of the southwest Asian monsoon. Our study also demonstrates that discriminant analysis is a useful technique for reliable reconstruction of paleovegetation in the Tibetan Plateau.

Holocene Environmental Change in the Himalayan-Tibetan Plateau Region: Lake Sediments and the Future

The South Asian Monsoon system is one of the most important and influential of the Earth’s major climate systems. The people of the most heavily populated Asian countries have adapted many aspects of their society to the subtleties of the monsoon rains, and are thus highly susceptible to small changes in the timing and intensity of monsoon precipitation. A monsoon failure can have disastrous effects, and flooding related to extreme monsoon rains has proven to be one of the most deadly of natural catastrophes (e.g. in Bangladesh, China, India and Nepal). These vulnerabilities are likely to increase in the future with continued population growth, intensified land-use and sea-level rise. Although there is a growing effort to improve seasonal to interannual monsoon prediction skills via new research, the largest threats to human health and livelihood could come from unanticipated decade- and longer-scale extremes in monsoon. A major goal of this paper is to summarize the state-of-the-art regarding century to millennium-scales of monsoon variability, and to identify the paleoenvironmental research that is most urgently needed in the Himalayan-Tibetan Plateau if society is to be served effectively in the 21st century.

Relationship between surface pollen assemblages and vegetation in Luonan Basin, Eastern Qinling Mountains, Central China

The catchment of South Luohe River in Central China is an important region for investigating modern pollen-environment relationship, because it is located in the transitional zone between south and north China, an environment which is sensitive to climate changes. In this study, 40 surface samples under ten vegetation types were collected to reveal the relationship between pollen assemblages and vegetation. The results show that the surface pollen assemblages reflect the vegetation quite well. In forest topsoils, the average of arboreal pollen content is greater than 40%, and the Selaginella sinensis spore is high. As to sparse forest grassland and shrub community, the average arboreal pollen is 13.2% and 16.6% respectively, and the shrub pollen is relatively higher than that of grassland samples. The grassland and farmland are characterized by low percentage of tree and shrub pollen (<10% and <1%), and high percentage of herbs (>80%). Pinus, Quercus and some other arboreal pollen can indicate the regional vegetation because of their dispersal ability. Quercus pollen is under-representative and so is Pinus. Artemisia pollen is significantly over-represented, has poor correlation with the plant coverage, and may reflect human disturbance. Gramineae can indicate plant quite well, but with low representation. High content of Chenopodiaceae probably suggests human impact. Predominant Selaginella sinensis can be used as an indicator of forest environment. Cluster analysis and principal components analysis of pollen assemblages can distinguish forest and non-forest vegetation well. The former method is better at separating pine and mixed forests, while the latter is more stable and could better differentiate farmland and other non-forest area. The first axis of PCA mainly reflects the humidity.