Quantitative Reconstruction Of Palaeo-vegetation Distribuction Pattern In Qinghai-Tibet Plateau And Its Surrounding Areas Over The Past 20,000 Years

The quantitative reconstruction of palaeo-vegetation is one of the important methods of understanding the law of climate changes,inverting the paleo-geochemical cycle,and exploring the relationship between atmosphere,biosphere and human activities.The Qinghai-Tibet Plateau is the hub of vegetation zone in Eurasia because of its complex climate and diverse vegetation types.However,the single vegetation structure and weak soil erosion resistance,adding the dynamic influence of Southwest monsoon,East Asian monsoon and westerlies make the Plateau particularly sensitive to monsoon evolution and climate changes.Therefore,the Qinghai-Tibet Plateau is an important area for studying climate changes,vegetation changes,and multi-layer coupling mechanism.Due to the limitations of sampling conditions and other factors,the sampling points in previous studies is small and uneven.Based on pollen data,this study aims to quantitatively reconstruct the geographical distribution pattern of paleo-vegetation on the Qinghai-Tibet Plateau and surrounding areas over the past 20,000 years,to explore the process of paleo-vegetation distribution changes on the Qinghai-Tibet Plateau,analyzing how vegetation changes in different areas of the Qinghai-Tibet Plateau under the background of rapid climate transition,and could help us better understand the laws of climate changes,finally could be beneficial to explore the relationship between atmosphere,biosphere and human activities.Pollen is an ideal material of reconstructing ancient vegetation,with the advantages of large yield,easy dissemination and easy preservation.This study integrates the original pollen data published from 1960 to 2020,the numerical data of the pollen maps in the literature and a small part of unpublished data,a total of 4,987 sampling points of modern pollen records and 162 sampling points of fossil pollen Record.The palynology database was established through standardizing palynology data and years.Biomization is the most stable and common method for quantitative reconstruction of large-scale palaeo-vegetation,and has a good performance.According to Biomization,we first assign the pollen groups to the corresponding plant functional types(PFTs),and then use the combination of key plant functional types to define the biome area,thereby establish the correspondence between the pollen groups and the biome area,and realize the quantitative reconstruction of palaeo-vegetation.Based on the global and Chinese plant functional classification schemes,this study made some adjustments to the classification of plant functional types,and assigned the pollen groups to the corresponding plant functional types by referring to the updated plant and ecology information in the flora.In terms of the division of biomes,in order to more clearly embody the macro-change characteristics of each biome,this study combines biomes in the same bioclimatic zone into mega-biome in the discussion.Based on pollen data,this study verified the applicability of the improved biomization scheme in China,and used biomization to quantitatively reconstruct the geographical distribution pattern of palaeo-vegetation in the Qinghai-Tibet Plateau and its surrounding areas in the past 20,000 years.The main conclusions are as follows:(1)The reconstruction results based on surface pollen show that there is a good consistency between the reconstructed modern biome and the potential vegetation biome,and 61.8%of the sample sites are correctly allocated,which indicates that the improved biomization scheme is suitable for China and the plateau region,the results of the reconstruction of the plateau vegetation are reliable.(2)The results of quantitative reconstruction of palaeo-vegetation based on fossil pollen of the Qinghai-Tibet Plateau and surrounding areas indicate that the distribution of biomes in the Qinghai-Tibet Plateau varies in space and time during the past 20,000 years,and sometimes varies greatly.Last Glacial Maximum(20 ka-19 ka):Affected by cold and dry climate,the plateau biome in this period was dominated by cold-temperate forests,temperate grasslands and temperate deserts.Cold-temperate forests were mainly concentrated in the eastern plateau and southeastern marginal area of Qinghai-Tibet Plateau.The temperate grassland occupied a vast area of the Qinghai-Tibet Plateau from east to west.The temperate deserts had expanded comparing with today and distributed in the plateau area in scattered spots.Cold-temperate forests appeared briefly in the eastern part of the Plateau,and tundra appeared in the southeastern margin of the Plateau at 20 ka.There were no temperate deciduous broad-leaved forests,warm-temperate forests and tropical rain forests in the Qinghai-Tibet Plateau during the last Glaciation Maximum.Last glacial(18.5 ka-12 ka):Temperate grasslands dominated most of the Qinghai-Tibet Plateau.Deserts occupied the eastern and northern regions of the plateau.After 14 ka,the cold-temperate forest occupied the eastern part of the Plateau steadily,and cool-temperate forests gradually moved from the margin of the plateau to the the central part.The warm temperate forest moved slightly to the north and occupied the southeastern part and the southern margin of the plateau.Tundra was only scattered in the eastern and northern regions of the plateau.There were no temperate deciduous broad-leaved forests and tropical forests during the last glacial.Early Holocene(11.5 ka-9 ka):Temperate grassland still occupied most of the Qinghai-Tibet Plateau,while migrated to the north and west of the plateau.The desert occupied the northern part of the plateau and reached its maximum at 9.5 ka.The tundra migrated south to west and reached its largest area at 9 ka.The cold-temperate forest still occupied the eastern part of the plateau and migrated eastward during 10.5 ka-9.5 ka.Cold-temperate forests expanded to the west,while warm-temperate forests still occupied the southeastern and southern margins of the plateau.The temperate forest biome was not reconstructed in the early Holocene.Mid Holocene(8.5 ka-5.5 ka):The forest biome area expanded to the largest extent.The temperate grassland area shrank in the southern plateau part of the plateau and moved westward as a whole.Tundra experienced a development process of contraction,expansion and contraction,and later only scattered in the southeastern part of the plateau.Temperate deserts spread along the northern margin of the plateau.The cold-temperate forest was reconstructed in the eastern part of the Plateau only at 8 ka and 6 ka,and moved slightly to the west comparing with the early Holocene.The cold-temperate forest biome area occupied more marginal areas of the Qinghai-Tibet Plateau and the high mountains in western China,and the area gradually shrinks after 6 ka.The warm-temperate forest expanded in the southeastern margin of the plateau and moved slightly northward in 7.5 ka.Late Holocene(5 ka-0.5 ka):The pattern of biomes in the late Holocene was similar to the distribution of modern biomes.The forest biomes migrated to the south.The cold-temperate forest was reconstructed in the northeastern part of the plateau only within 0.5 ka,and migrated slightly to the south and east.Cold-temperate forests contracted in the northern and eastern regions of the plateau,and concentrated in the eastern and southeastern margin of the Plateau after 3 ka.Warm temperate forests migrated to the east.After 3 ka,the forest area shrank significantly on the Qinghai-Tibet Plateau.Temperate grasslands was still dominant in the Qinghai-Tibet Plateau.Deserts expanded from 5 ka to 2 ka.But after 1.5 ka,deserts contracted and mainly distributed in the northern and northeastern regions of the plateau.The tundra spread over the Qinghai-Tibet Plateau from 5.5 ka to 2 ka.After 1.5 ka,the area of tundra shrank and concentrated in the eastern part of the plateau.(3)In this study,the transition period from the Last Glacial to the Early Holocene was selected to explore the response of Plateau vegetation to climate change.The results showed that during 11.5 ka to 9 ka,the forest coverage of the Qinghai-Tibet Plateau increased,the temperate grassland area decreased and migrated to the north.The area of tundra expanded,mainly in the eastern and southern parts of the plateau.The desert distributed in the northern part of the plateau.After 10 ka,the desert area expanded and migrated southward.This study shows that the improved biomization scheme has a good applicability in China,and can quantitatively reconstruct the changes of the plateau vegetation zone in the historical period.In the future,the accuracy of the simulation can be further improved by adding the pollen database data and improving plan of plant functional types.