The Relationship Between Phytolith And Vegetation In Temperate Steppe In China And Quantitative Reconstruction Of Paleo-vegetation In Meadow Steppe Since The Late Glacial Period

Grassland ecosystems can sensitively respond to climate changes.Reconstructing palaeovegetation of temperate steppe in China is of great significance for understanding the variations of palaeovegetation in the Eurasian steppe,and also,for knowing its responses to palaeoclimate changes.In this thesis,phytoliths were employed as a vegetation proxy,and phytolith characteristics of 7 Stipa species,157 typical communities,and 154 topsoil samples,which from three different steppe types in temperate grasslands of China,had been carefully investigated.Quantitative relationships between phytoliths and vegetation was established then by WA-PLS method,and these relationships were further applied to phytolith assemblages?n=68?from AGS sand-paleosol sediment sequence to obtain the palaeovegetation conditions in temperate meadow steppe since late glacial period.In addition,this thesis also analyzed the grain size characteristics?n=69?,organic matter content?n=69?and carbonate content?n=69?of AGS to acquire the climate conditions of temperate steppe since the late glacial period.Lastly,palaeovegetation dynamics and climate conditions were compared to test the reliability of the quantitative relationship between phytolith and vegetation.Major conclusions are as follows:?1?13 phytolith morphotypes were identified in 7 Stipa species,and the short-cell grass phytoliths?GSCPs?are all C3 type.The dominant type in phytolith assemblages was rondel flat in most species except Stipa glareosa,which is dominated by elongate echinate.Stipa klemenzii,Stipa breviflora,Stipa glareosa and Stipa gobica,which are adapted to arid conditions,can produce abundant echinate phytolith.Community samples and topsoil samples produced similar phytolith morphotypes,and the dominant type in phytolith assemblages of both the two were rondel flat,which vary within 30⁴2% and 27⁴1%,respectively.Differences appeared in phytolith assemblages among different steppe types.More saddle and papillae echinate occurred in meadow steppe,while more rondel echinate and elongate echinate appeared in desert steppe.The content of most phytolith type in typical steppe were in the middle,which due to its transitional ecological position.Trapeziform sinuate,simple bilobate,Stipa-bilobate,rondel echinate and elongate echinate may indicate arid habitat,and these morphotypes possess relatively high percentages in typical steppe and desert steppe.The differences in phytolith assemblages in three steppe type provide a basis for further application of discriminant analysis,and also for proposing phytolith indices.?2?35 phytolith morphotypes were similar in community samples and topsoil samples,while jigsaw type only occurred in topsoil samples.Both community and topsoil phytolith assemblages can distinguish different steppe type by using discriminant analysis?DA?,and the total accuracies are 82.2% and 89.0%,respectively.Besides,community types also can be identified with the assistance of DA.For example,shrub communities,non-grass communities,C3 grass communities,and C4 grass communities in meadow steppe can be reliably distinguished,and the total accuracies of community samples and topsoil samples were 96.4% and 89.3%,respectively.Also,the total accuracies of different C3 grass community types in typical steppe were 90% and 91.7%,respectively.Discriminant analysis is one of the effective semi-quantitative methods for studying phytolith-vegetation relationships in temperate steppe in China,and it is also of referential significance for establishing quantitative relationship between phytolith and vegetation.?3?Both high value of Ic index and low value of Iw index can indicate cold climate condition in temperate steppe in China,and the two indices were significantly negative correlated,which demonstrates that both of the two showed well suitability in temperate steppe in China.However,Iph index and Fs index,which can indicate moisture conditions,are not applicable in temperate steppe in China due to the lack of phytoliths related to the calculation.Therefore,based on several ecological indicating phytolith types,Id index was proposed to indicate the degree of drought in temperate steppe in China.A high value of Id index indicates a direr environment,otherwise it tends to be humid.Combining these three indices can determine the climatic conditions,and provide a reference for establishing phytolith-vegetation relationship in temperate steppe of China.?4?Four representation phytolith types were determined by comparing the Association index,Over-representation index,Under-representation index,similarity coefficient,correlation coefficient and R value.The 4 types includes over-representative type,moderate-representative type,under representative type and special type.Comparing the root mean square errors of prediction?RMSEP?of the calibrated transfer functions and uncalibrated ones led to a knowledge of phytolith representative bias can be calibrated by R value.The transfer functions of phytolith-total coverage,phytolith-total biomass,phytolith-C3 grass coverage and phytolith-C4 grass coverage constructed by WA-PLS method were reliable(RMSEP_{-total coverage}= 13.82,R²_-boot= 0.76;RMSEP_{-total biomass}= 187.24,R²_-boot= 0.48;RMSEP_{-C3 grass coverage}= 19.60,R²_-boot= 0.24;RMSEP_{-C3 grass coverage}= 12.25,R²_-boot= 0.42).The establishment of quantitative phytolith-vegetation relationship in temperate steppe of China is of great significance for reconstructing the high-precision palaeovegetation composition in temperate steppe,and also can provides an innovatory method for quantitatively reconstructing palaeovegetation by using phytolith.?5?Palaeovegetation in temperate meadow steppe in China since last glacial period was quantitatively reconstructed based on phytolith-vegetation relationships.According to phytolith assemblages in AGS,the palaeovegetation type was meadow steppe and the community types were mainly C3 grass communities and C4 grass communities with several C3 / C4 shifts since last glacial period.The total coverage had fluctuated within the range of 54 82%,with an average value of 69.18%;and the average value of total biomass was 412 g,varied between 314 g and 469g;and the average value of C3 grass coverage was 25.15%,with a minimum of 11% and a maximum of 39%;the average range of C4 grass coverage ranged from 11% to 51%,with an average value of 27.07%.The appearance of palaeovegetation experienced slightly sparse C4 grass community-sparse C3 grass community-flourish C4 grass community-slightly flourish C3 grass community.Moreover,the climate change process indicated by grain size,organic matter content,and carbonate content in the same profile coincided with the above-mentioned vegetation succession sequence.In addition,the palaeovegetation condition of temperate meadow steppe also reflected several extreme climate events.The consistency of multiple proxies proved that transfer functions in this thesis is useful and reliable for the reconstruction of palaeovegetation in temperate steppe in China.The results of this study may provide basic references for palaeovegetation research in temperate steppe.