Histories Of Vegetation,Climate And Forest Fire Over The Last 15400 Years In The Yilong Lake Catchment Of Southern Yunnan

Yilong Lake is a typical semi-closed faulted lake in Yunnan Plateau.The vegetation,climate and environment in the Yilong Lake catchment are strongly affected by the southwest monsoon,so its vegetation,climate and forest fire history will provide detailed information on the evolution of the southwest monsoon.This study takes the upper 261cm of the Core of YLH-1 at the water depth of 3.6m in the middle of Yilong Lake as the study object.The age-depth model was established using AMS ¹⁴C dating data of 8 pollen samples.102 pollen samples were collected at an interval of 2 cm for pollen/charcoal analysis to reconstruct vegetation succession,climate change and forest fire histories in the Yilong Lake catchment over the last15400 years.Their histories showed following five stages:The first stage(261?219 cm;15400?14200 cal.a BP):The vegetation was dominated by pine forest as well as evergreen and deciduous broad-leaved mixed forest(forest components mainly were Pinus,deciduous Quercus,Cyclobalanopsis,Castanopsis/Lithocarpus,and evergreen Quercus).This stage was characterized by mild and humid climate.High charcoal influx at this stage indicated frequent occurrence of forest fire.The second stage(219?140 cm;14200?9400 cal.a BP):This stage is characterized by an obvious expansion of pine forest.The deciduous broad-leaved forest maintained a similar size as the previous stage,in which deciduous Quercus increased at the cost of Betula,Corylus/Carpinus,and Pterocarya.The evergreen broad-leaved forest decreased with a major drop mainly in evergreen Quercus and Castanopsis/Lithocarpus.In this stage,the temperature decreased but the humidity increased exhibiting a cool and wet climate,however,the climate at the end of this stage showed a warming and wetting trend.Charcoal influxes of all sizes reached their maxima of the profile,indicating that this stage was the most frequent forest fires over the last 15400 years.The third stage(140?94 cm;9400?5800 cal.a BP):This stage is characterized by a significant shrink of pine forest and an obvious expansion of deciduous broad-leaved forest and evergreen broad-leaved forest.Deciduous broad-leaved components are mainly deciduous Quercus,Betula,Ulmus and Liquidambar,while evergreen broad-leaved components are mainly Cyclobalanopsis,evergreen Quercus,and Castanopsis/Lithocarpus.The marked development of Liquidambar forest favoring warm-hot climate as well as the increase of philotherms such as Macaranga and Rubiaceae indicated a hot and humid climate.Charcoal influx with the second lowest value in the whole profile indicated a low fire frequency.The fourth stage(94?40 cm;5800?3000 cal.a BP):Pine forest slightly expanded,deciduous broad-leaved forest kept a similar size as the previous stage.The latter components were mainly deciduous Quercus,Alnus,Ulmus,Betula and Pterocraya.The evergreen broad-leaved forest greatly decreased as its dominants such asevergreen Quercus,Cyclobalanopsis,Castanopsis/Lithorcarpus significantly reduced.Vegetation indicated a decreased temperature and humidity exhibiting a warm and dry climate.Charcoal influx increased significantly in the early stage and decreased gradually in the middle and late stage,reflecting the increase of forest fires caused by relatively dry climate conditions.The fifth stage(40?0 cm;3000 cal.a BP to the present):This stage is characterized by a significant expansion of pine reaching its maximum of the profile.Deciduous broad-leaved forest shrunk significantly along with is dominants such as deciduous Quercus and Liquidambar,but the Alnus,the dominant of the secondary vegetation,showed an increasing trend.Evergreen broad-leaved forest also shrunk significantly along with is dominants such as evergreen Quercus,Cyclobalanopsis,and Castanopsis/Lithorcarpus.At this stage,the temperature gradually decreased,and the humidity became drier and drier.Low charcoal influx indicated low frequent forest fire.In summary,the climate over the last 15400 years in the Yilong Lake catchment experienced the change of mild and wet?cool and wet?warm-hot and wet?warm and dry?mild and dry.The temperature roughly experienced a series of change,i.e.,mild?rise?high?drop?drop further.The humidity showed an increasingly dry trend.The climate over the last 15400 years of in the Yilong Lake catchment suggested that the southwest monsoon evolved over the past 15400 years as following:Strengthened around 12250 cal.a BP,peaked at 9400?5800 cal.a BP,and then weakened during 5800?3000 cal.a BP,a modern monsoon environment was established after 3000 cal.a BP.