Vegetation And Climate Changes Over The Last 3000 Years Recorded By Sediments From West Lake In Dali,West Yunnan

The late Holocene climate has always been an important issue of paleoclimate study because it was affected by both natural and anthropogenic forcings.In this study,80 pollen/charcoal samples were collected at 4 cm interval from the upper 317 cm of DLXH1 core in West Lake,Dali of West Yunnan.The depth-age model for this part of the core was established using 5 AMS 14 C dates.Pollen/charcoal analysis as well as cluster analysis and principal component analysis(PCA)of fossil pollen data were conducted to reconstruct vegetation succession,climate evolution,and forest fire history over the last 3000 years in the catchment of West Lake in Dali,West Yunnan.Results of pollen/charcoal analysis and numerical analysis of fossil pollen data showed that vegetation,climate,and forest fire experienced following six-phase changes over the last 3000 years in the catchment of West Lake:The first phase(3100-2430 cal.a BP): This phase is characterized by high arboreal pollen percentages.Arboreal pollen was dominated by Pinus,together with some evergreen broadleaved trees.The herbaceous layer was mainly composed of Poaceae,Cyperaceae,Asteraceae,and Artemisia.Pollen percentages of Betula and evergreen Quercus reached their maxima of the whole section.Low charcoal influx implied low frequency of forest fire.Pollen assemblages,charcoal,and PCA results reflected cold and dry climatic conditions in the catchment of West Lake.The second phase(2430-1730 cal.a BP): Arboreal pollen increased significantly at the cost of herbaceous pollen.Pine forest favoring warm climatic conditions expanded rapidly,evergreen broadleaved trees decreased,suggesting warm climatic conditions.Charcoal influx increased,suggesting more frequent forest fires.Pollen assemblages and PCA scores showed that the climate at this phase was gradually warmer and relatively wetter compared with the previous phase.The third phase(1730-1060 cal.a BP): This phase is characterized by a significant decrease in arboreal pollen and a dramatic increase in herbaceous pollen.Herbaceous pollen percentages reached its maxima of the whole section.Pine forest shrank to its minimum in size,whereas Alnus forest expand Artemisia pollen increased significantly at 1410 cal.a BP.However,charcoal concentrations and influx decreased,indicating low frequent forest fires.The result of PCA on fossil pollen data indicated cold and wet climatic conditions in the catchment of West Lake.The fourth phase(1060-590 cal.a BP): Arboreal pollen increased significantly,and herbaceous pollen decreased.Pine forest expanded largely,together with a decrease in Picea/Abies trees.Pollen assemblages suggested a rise of temperature and a decrease of humidity.Charcoal influx increased,indicating an increasing frequency of forest fires.The fifth phase(590-140 cal.a BP): This phase is characterized by a significant decrease in arboreal pollen and a marked increase in herbaceous pollen.Pine forest shrank,but evergreen broadleaved trees increased.Picea/Abies trees occurred rarely,but reached their maxima of the whole section.The climate changed from warm and dry to relatively cold and dry.PCA result showed decreases in temperature and precipitation.This phase was correspondent to the Little Ice Age.The sixth phase(140 cal.a BP to the present): Arboreal pollen increased to its maximum of the whole section,whereas herbaceous pollen decreased rapidly to its minimum of the whole section.Pine forest expanded dramatically together with some evergreen broadleaved tress.Poaceae and Asteraceae pollen decreased to their minima of the whole section.Pollen assemblages and PCA result indicated warm-dry climatic conditions.Charcoal influx of different scales increased greatly to their maxima of the section,suggesting a dramatic increase in forest fire frequency.Especially,charcoal influx at scale of <50 ?m reached as high as 19193particle/(cm~2·a),indicating frequent occurrence of regional forest fires,partly due to increased human activities,partly due to warm-dry climatic conditions.In summary,vegetation in the catchment of West Lake experienced six-phase succession,i.e.,evergreen broadleaved trees + pine forest ? pine forest expansion ?pine forest shrink + Alnus trees ? dramatic expansion of pine forest ? pine forest shrink + evergreen broadleaved trees ? pine forest expansion + evergreen broadleaved trees.Climate in the catchment of West Lake experienced six-phase evolution,i.e.,cold and dry ? warm and wet ? cold and wet ? warm and dry? cold and dry ? warm and dry.