Vegetation Succession,Climate Change,and Forest Fire At 38.2-23.0 Ka BP In The Catchment Of Lugu Lake

In this paper,pollen/charcoal analysis was carried out on samples from 785-1300 cm of the LGH2 core taken from Lugu Lake to study vegetation succession,climate change and forest fire histories at 38.2-23.0 ka BP in the catchment of Lugu Lake of northwest Yunnan.The dating control model of this section of sediments was established in two steps.First,6 dating control points were determined by a comparison of the carbonate content between two cores(LGH1 and LGH2 cores)and 14 C dates from the LGH1 core;and then the age of each depth level was obtained by linear interpolation according to these control points.A total of 104 pollen/charcoal samples were subsampled at an interval of 5-cm.The pollen/charcoal data,including pollen percentage,concentration and influx,were processed by the TILIA software to make pollen diagrams and the division of pollen zones.The pollen percentage data were analyzed using principal component analysis(PCA)in the CANOCO software.Pollen/charcoal analysis,PCA of pollen percentage data,total organic carbon(TOC)and carbonate content revealed that the catchment of Lugu Lake experienced four stages of changes in vegetation,climate and fire regime at 38.2 – 23.0 ka BP.Stage ?(38.2-35.24 ka BP;1300-1200 cm): The catchment of Lugu Lake was mainly occupied by subalpine dark coniferous forest with Picea and Abies as constructive species and pine forest with Pinus as constructive species.Sclerophyllous oaks(Quercus(E))and shrubs of Rosaceae occurred as accompanying species in forests,herbs of Poaceae,Artemisia,Cyperaceae grew under forests and around lake.PCA result shows that the first and second principal components are the proxies of temperature and humidity respectively;the temperature is relatively cool and the humidity is dry at this stage.Stage ?(35.2—29.3 ka BP;1200-1000 cm): Subalpine dark coniferous forest greatly expanded and pine forest gradually shrank,and other tree and shrub elements changed little and main herbaceous plants decreased.PCA result indicates a decrease in temperature and humidity,and thus cold and dry climatic conditions.The forest fire frequency is extremely high with a peak around 34.6 ka BP.Stage ?(29.3—25.7 ka BP;1000-860 cm): Subalpine dark coniferous forest composed of spruce and fir maintained the coverage at the end of previous stage,and pine forest continued to shrink,whereas understory and lakeside herbaceous plants increased.PCA result exhibits the highest temperature and the lowest humidity at the period from 38.2 to 23.0 ka BP,and thus cold and wet climatic conditions.Forest fires as suggested by charcoal data showed a decreasing trend in fire frequency.Stage ?(25.7—23.0 ka BP;860-785 cm): Subalpine dark coniferous forest obviously shrank,pine forest and sclerophyllous oak forest expanded,and understory and lakeside herbaceous plants further increased.PCA result shows an increase in temperature and a decrease in humidity,and thus warm and relatively wet climatic conditions.Charcoal data indicates low frequent forest fire.Pollen spectra from 785-1300 cm of the LGH2 core in Lugu Lake generally reflect the vegetation at 38.2-23.0 ka BP in the catchment of Lugu Lake,dominated by subalpine dark coniferous forest and pine forest.The vegetation in the catchment of the lake experienced four stages of changes,i.e.subalpine dark coniferous forest + pine forest expansion in subalpine dark coniferous forest + gradual shrinkage in pine forest subalpine dark coniferous forest remained high coverage + pine forest continued to shrink obvious shrinkage in subalpine dark coniferous forest + rapid expansion in pine forest and sclerophyllous oak forest.Meanwhile,the climate in the catchment of Lugu Lake changed from cool and dry,to cold and dry,to cold and wet,to cool and relatively wet conditions.Forest fires experienced four stages in the occurrence of fire frequency,i.e.frequent,very frequent,low frequent,and low frequent again.