VEGETATION AND CLIMATE CHANGES IN SOUTHWEST CHINA DURING THE LAST MILLENNIUM REVEALED BY A PALYNOLOGICAL RECORD FROM LAKE YIHAI
The variations in Asian Summer Monsoon and corresponding climate fluctuations are essentially importation for regional ecosystem composition and socioeconomic development,especially for the period of last millennium which has attracted wide attentions.Fossil pollen preserved in lake sediments can effectively reflect the evolution history of regional vegetation and reveal the evolution history of regional climate and environment.Lake Yihai(28.73° N,102.24° E;2280 m a.s.l.)is located in the Hengduan Mountain region of Southwest China and affected by the southwest monsoon.In this study,we conducted palynological analysis of sediment core from Lake Yihai in the Hengduan Mountainous region.A total of 151 terrestrial pollen species were identified from 36 samples in a 71 cm long sediment core,among which 13 types with proportions of over 1％in at least 2 samples were selected for data analysis.Combining with 210Pb/137Cs and radiocarbon dating results,the history of vegetation and environment changes in Southwest China during the past 1000 years were reconstructed.Based on the numerical analysis of pollen composition,including CONISS cluster analysis and principal component analysis(PC A),the environmental evolution of the study area was divided into five stages.In stage YH-1(1050～1320 A.D.),the pollen assemblage was dominated by arboreal pollen species,which indicated a briefly warm and wet medieval climate condition with strong Asian summer monsoon.In stages YH-2(1320～1570 A.D.)and YH-3(1570～1680 A.D.),the decrease in broad-leaved forest components(such as Alnus and Cyclobalanopsis),along with the gradual increase in herbaceous pollen taxa dominated by Artemisia,indicated increased landscape openness following the decline in regional moisture during the Little Ice Age(LIA).In stage YH-4(1680～1950 A.D.),the migration during the Qing Dynasty led to a great increase in regional population and corresponding human activities.Accordingly,selective deforestation process has possibly caused the significant increase in the proportion of Alnus pollen as preferred fuelwood.The stage YH-5(from 1950 A.D.to present)was characterized by an abnormal increase in the proportions of Pinus and Alnus pollen,which represented regional afforestation through the aerial seeding process in western Sichuan.The reconstructed climate variations based on the fossil pollen assemblage from Lake Yihai expresses a broadly consistent pattern with published climate records in Southwest China,while solar activity fluctuation and migration of intertropical convergence zone is considered to be the dominant factor driving summer monsoon variations over the past millennium.During the Middle Ages,strong total solar irradiance(TSI)increased the meridional circulation while increasing the land-sea heat correlation,and the northward shift of the intertropical convergence zone(ITCZ)transferred more water vapor to the Northern Hemisphere,which together drove the increase in Asian summer monsoon precipitation.Conditions were reversed during the Little Ice Age(LIA).
万方数据
维普
