Mid-late Holocene Vegetation History And Climate Change Of Southern Zhejiang Province Recorded By Pollens

Eastern China monsoon region is China's largest population density area.Now that people's living environment have been threatening by the increasingly serious climate change,it is a must to study intensively the climate variability in the past in order to predict the future trend in climate change.The research of climate change during the Holocene has become the focus among numerous academic topics in the 21St century,the key points of which are the starting and ending time,temperature humidity configuration and driving mechanism in each period of Holocene.This paper uses the pollen analysis method based on the precise chronological framework and the statistical methods of cluster analysis,principal component analysis(PCA),etc.to compare the Wangdongyang wetland with the Lanjianghu wetland in the southern Zhejiang province,monsoon region.The research results of other alpine marsh wetlands in the adjacent regions have been discussed in detail the history of vegetation evolution and climate change in southern Zhejiang province since the Mid-Late Holocene.The main results and conclusions are listed as follows:(1)The climatic records and vegetation evolution history of the Mid-Late Holocene in the southeast of Zhejiang Province in the observation of speleothems of the Wangdongyang wetland and the Lanjianghu wetland have good consistency with the pollen records of the Lantianyan wetland and the Daiyunshan wetland in the same region.The Holocene suitable period in the south of Zhejiang Province ended at about 2.1 ka B.P..(2)During the Holocene best period(8.1-2.1 ka B.P.),the sporo-pollen assemblage recorded in the Wangdongyang wetland were Cunninghamia,Quercus-evergreen,Alnus,Ranunculaceae,with an average pollen concentration of 2,436,153 granules/g.The sporo-pollen assemblages in the Lanjinghu wetland during this period were Cunninghamia,Quercus-evergreen,Cyperaceae,Ranunculaceae,fern with an average pollen concentration of 122,736 granules/g.The zonal vegetation was the evergreen coniferous broad-leaved mixed forest dominated by Chinese fir forest,accompanied by deciduous broad-leaved trees and shrubs.During this period,the forest coverage rate in the mountains of southern Zhejiang province was high,and the climate was warm and humid with abundant rainfall.The most suitable period lasted for more than 5,000 years.Slightly different,the records of the most suitable period in Holocene in Wangdongyang wetlands was relatively stable,while the record of the Lanjianghu wetlands shows that during the period of 8.1-6.5 ka B.P.,the contents of Cupressaceae,Poaceae and Polypodiaceae were relatively high.The content of Quercus-evergreen gradually increased,indicating a gradual increase in temperature.After 6.5 ka B.P.,the climate was stable and the temperature and humidity were optimally configured.(3)The late Holocene cooling period(2.1 ka B.P.?present),the sporo-pollen assemblages recorded in Wangdongyang wetland were Pinus,Quercus-evergreen,Alnus,Poaceae,Artemisia,Ranunculaceae,fern,average pollen concentration reached 1,228,676 grains/g.The sporo-pollen assemblages recorded in the Lanjianghu wetland were Cunninghamia,pinus,Quercus-evergreen,Poaceae,Artemisia,fern with an average pollen concentration of 500,840 grains/g.The zonal vegetation was evergreen coniferous broad-leafed and deciduous mixed forests.The drought-tolerant herb expanded slightly,and the cool and humid fern flourishes.The content of the warm and humid genus Cunninghamia and Alnus were the lowest.The sporo-pollen assemblage indicates that after the Holocene suitable period,the climate turned cold and the humidity changes frequently.In addition,the 1.4 ka B.P.cold event has a good correspondence in the pollens record of the Lanjianghu wetland.Since 1.0 ka B.P.,with clear evidence of human activities,climate change and human activities have influenced the changes in vegetation and the ecological environment in this region.(4)The abrupt change in the pollens content of Alnus in mountainous regions in southern Zhejiang province may be a record of forest fire incidents in this region.The change of percentage content of Alnus in the pollens records and the analysis of micro-carbon deposits in the sediments show that the pollen content of Alnus in the Wangdongyang wetland was higher than 2.1 ka B.P.and the change was stable since the late Holocene.After the 2.1 ka B.P.,the volatility declined,and there were two peaks around 2.1?1.9 ka B.P.and 1.6 ka B.P..It was speculated that there were two large-scale fire incidents in the Wangdongyang wetland.2.1?1.8 ka B.P.was the transitional period of the climate and environment transition.The content of Alnus recorded in the Wangdongyang wetland fluctuates sharply in a short time,with two peaks appearing,and it also occured in the vicinity of 4.0 ka B.P.in the Lantianyan wetland.The increase of the pollen content of Alnus may be related to the rapid expansion of Alnus secondary forest after the fire was destroyed.