Vegetation And Climate Changes During The Younger Dryas Chron In The Catchment Of Mengsuo Longtan Lake,Southwest Yunnan

Located near the border region between Yunnan and Myanmar,Mengsuo Longtan Lake is deeply affected by the South Asian monsoon,and thus the vegetation evolution in its catchment mainly reflects the change of the South Asian monsoon.In this study,modern pollen data from surface samples of 75 lakes in Southwest Yunnan were firstly used to develop modern pollen-climate transfer functions.The redundancy analysis(RDA)was employed to determine the relationships between pollen and climatic parameters,the weighted average of the partial least squares(WA-PLS)model was finally selected to develop modern pollen-climate transfer functions.Pollen/charcoal analyses of fossil pollen samples from 1530-1400-cm of MSLT-1 core taken from Mengsuo Longtan Lake were then conducted.The depth-age model was established using 6 ¹⁴C dates from this part of the core to determine its age.Modern pollen-climate transfer functions were finally applied to these fossil pollen data to reconstruct paleoclimate quantitatively,and charcoal data was analyzed to reveal the forest fire history.Vegetation,climate,and forest fire history in the catchment of Mengsuo Longtan Lake during 13.5—11.2 cal.ka BP(dominated by the Younger Dryas chron)were thus reconstructed qualitatively and quantitatively.Main results and conclusions obtained by this study are as follows:(1)Pollen analysis of 75 lake surface samples in Southwest Yunnan shows that pollen spectra were dominated by arboreal pollen(64.7%),which was followed by herbaceous pollen(23.2%)and fern spores(12.1%).Dominant arboreal pollen taxa include Pinus,Alnus,Lithocarpus/Castanopsis,evergreen Quercus,Cyclobalanopsis,Betula,Castenea,and Ulmus.Common herbaceous pollen taxa are Poaceae,Artemisia,Asteraceae,Polygonaceae,Cruciferae,and Cyperaceae.These pollen spectra from lake surface samples generally reflect regional vegetation in the catchment of sampling lake.(2)The result of RDA on modern pollen and climate data showed that mean annual temperature(MAT)and annual precipitation(MAP)were major climatic parameters controlling the modern pollen rains in Southwest Yunnan.Among them,annual precipitation was more important than others.(3)Using C2 software,8 models were firstly selected to build modern pollen-climate functions with the test of left-one cross-validation method.The WA-PLS model was finally selected to establish the modern pollen-climate transfer functions in Southwest Yunnan in terms of the determination coefficient(R²),maximum bias(max-bias),and root mean square error(RMSEP).(4)Paleoclimate in the catchment of Mengsuo Longtan Lake during the period of13.5-11.2 cal.ka BP was reconstructed quantitatively using fossil pollen data of 1530-1400-cm from the MSLT-1 core and pollen-climate transfer functions established from WA-PLS model.The results are:from 13.5 to 11.2 cal.ka BP,the average of MAT was ca.0.12?lower than that of the present,and the average of MAP was ca.400 mm less than that of the today.However,paleoclimate fluctuated in this Younger Dryas chron.At 13.5-12.9 cal.ka BP,MAT was ca.0.17?higher than that of the present whereas MAP was ca.301 mm less than that of the present.During the period of 12.9-12.6 cal.ka BP,Temperature and precipitation decreased nonlinearly to the present level.At12.6-11.6 cal.ka BP,MAT declined persistently to be 19.32?,and MAP decreased to be ca.1380 mm;this period was the coldest and driest one of the Younger-Dryas chron.During the period of 11.6-11.2 cal.ka BP,MAT and MAP increased gradually,and MAT rose to be 19.57?and MAP increased to be 1485 mm.(5)During the period 13.5-11.2 cal.ka BP,forest fires showed a temporal patten of less?more?the highest?less,which was associated with MAP decrease and thus its induced increase of the amount of combustible material.