Histories Of Vegetation,Climate And Forest Fire Over The Last 13300 Years In The Watershed Of Fuxian Lake

Fuxian Lake is a well-known rift deep-water lake in the Yunnan-Guizhou Plateau;its sediments contain abundant environmental information in its drainage region during geologic times.A 900-cm core as the object of this study was taken from the southern part of the lake under 81.2 m of water depth,and its age-depth model was established using 7 AMS ¹⁴C dates of lake sediments.A total of 91pollen/charcoal samples were obtained at 10-cm intervals with an average resolution of ca.150 a.Pollen/charcoal analyses on these samples and numerical analysis of pollen data?cluster analysis and principal component analysis?were conducted to reveal histories of vegetation,climate,and forest fire in the watershed of Fuxian Lake during the last 13300 years.The results of pollen/charcoal analysis and principal component analysis on pollen data show that the watershed of Fuxian Lake experienced six phases of vegetation succession,climate change,and forest fires over the last 13300 years.?1?Phase ??13300¹3000 cal.a BP?:the vegetation was dominated by pine forests,accompanied by dark coniferous forests,indicating cold and wet conditions;the relatively high charcoal influxes indicate high frequent forest fires.?2?Phase ??13000¹0400 cal.a BP?:the vegetation was still dominated by pine forests,but the evergreen broadleaved elements increased obviously;the climate was still cold,but the temperature was higher and the humidity was lower than the previous stage;meanwhile,forest fires still occurrd frequently.?3?Phase ??10400⁶800 cal.a BP?:the shrinkage in pine forest,expansion in evergreen broadleaved forest,and appearance of deciduous broadleaved forest indicate relatively warm and dry conditions;forest fires occurred frequently like the previous stage as climatic conditions became warm and dry at the early period of this stage,until 9500 cal.a BP when relative humidity increased forest fires decreased obviously.?4?Phase ??6800¹700 cal.a BP?:pine and evergreen broadleaved forests still dominated,but Liquidambar forests favoring warm and hot climate first appeared,showing warm and wet conditions;The overall charcoal influxes remained low,indicating a lower frequency of fires.?5?Phase ??1700¹100 cal.a BP?:the expansion of pine forest and the shrinkage of deciduous broadleaved forest with an increase of upland herbaceous plants indicate relatively cold and dry conditions;forest fires were more than the former stage but less than the earliest two stages.?6?Phase ??1100 cal.a BP to the present?:the shrinkage of pine forests and a significant increase of upland herbs show cool and dry conditions,and suggest increasing human activities.In summary,the pollen record reveals such a vegetation succession as pine forest+montane dark coniferous forest?pine forest+evergreen broadleaved forest?the shrinkage of pine forest+evergreen broadleaved forest?pine forest+evergreen broadleaved forest+deciduous broadleaved tree?the expansion of pine forest?the shrinkage of pine forest+the expansion of herbaceous plants in the watershed of Fuxian Lake during the past 13,300 years;this vegetation succession reflects a corresponding evolution of climatic conditions,i.e.cold and wet?cold and relatively dry?relatively warm and dry?warm and humid?cold and dry?cool and drier.Charcoal record indicates that the cold climate is one of climatic forcings for frequent forest fires in Yunnan.