Relationship Between Pollen Assembblages Of Lake Centre Surface Sediment And Vegetation In Mongolia Plateau And Its Surrounding Areas

The lake surface sediments-based pollen-vegetation relationships provide important references for the interpreting lake core fossil pollen and the reconstructions of paleovegetation and paleoclimate.However,taphonomic processes such as pollen productivity,source area,transportation,suspension and re-deposition,and preservation complicated the lake surface sediment-based pollen-vegetation relationship,resulting in uncertainties of the capabilities to represent regional vegetation and climate,leading to questions concerning the feasibility of lake core pollen to reconstruct past vegetation and climate.In the arid and semi-arid Inner Mongolia plateau,sparsely developed regional vegetation,diversities on meteorological,hydrological,and topographic conditions led to complex taphonomic processes of pollen assemblages of lake sediment and thus complicated the regional pollen-vegetation relationships,calling questions to the capabilities of lake sediment pollen to reflect regional vegetation and climate.Therefore,we surveyed the pollen assemblages of lake centre surface sediments from 92 lakes and reservoirs in the Inner Mongolia Plateau and surrounding areas.We calculated or collected the regional and non-zonal vegetation data,the regional vegetation diversity data,the human activity intensity of land surface（HAILS）data for numerical analysis.In this paper,we studied pollen representations for major pollen types,the indicator pollen taxa for different vegetation types or ecotones.We explored the pollen-vegetation relationships using principal component analysis（PCA）,discriminant analysis（DA）,random forest model（RF）,and support vector machine model（SVM）,and further discussed the feasibility and limitations of using lacustrine pollen to reconstruct paleo-vegetation and paleo-climate.Also,we used Mantel test analysis to reveal the effect of non-zonal vegetation pollen on lake surface sediments pollen-vegetation relationships.In addition,we discussed the relationship between lake sediment pollen diversity index and vegetation diversity index,and the relationship between pollen assemblages and human activities.The primary conclusions are as follows:（1）The results of PCA ordination analysis,indicator species analysis,pollen representation analysis DA,RF,and SVM classification analysis generally reveal positive correlations between lake centre surface sediment-based pollen assemblages and vegetation in arid and semi-arid Inner Mongolia plateau and surrounding areas,suggesting broad capabilities of pollen assemblages to represent regional vegetation.（2）Mantel test analyses between pollen data and vegetation data for samples of dune desert and Gobi desert all yield better correlations for azonal vegetation than regional vegetation,suggesting that the pollen of azonal vegetation distorts the regional pollen-vegetation relationships and thus weakens the capabilities of pollen to represent regional vegetation and climate.（3）PCA plots shows that the sample sites of Gobi desert,desert steppe,and meadow steppe overlap significantly,the DA,RF,and SVM classification results also indicate high error rates of classification,suggesting poor relationship between lake surface sediments pollen assemblages and vegetation and difficulties to distinguish them using pollen assemblages.The similarities of vegetation components at the family or genus level,as well as the over-representation of Artemisia and Amaranthaceae pollen,and the under-representation of forbs pollen types lead to the incapabilities of pollen assemblages to differentiate them effectively.（4）The pollen representation analysis shows that,on the whole,Artemisia,Amaranthaceae,Poaceae-M,Cyperaceae,Nitraria,Ephedra,Aster-type,Rosaceae,Ulmus,Brassicaceae,Taraxacum-type have a strong correlation with vegetation;Pinus,Betula,Quercus,Picea,Juglans have over-representation;Juniperus,Salix,Hippophae,Caryophllyeae,Poaceae-L,Tamaricaeae,Thalictrum,Polygonum,Fagopyrum,Humulus,Linum have weak correlation with vegetation.（5）Broad correspondings existed between pollen diversity and vegetation diversity for samples of forest steppe,meadow steppe,and typical steppe,but not for desert and desert steppe.（6）Four pollen taxa,i.e.,Poaceae-L（R2=0.27,p<0.001）,Humulus（R2=0.17,p<0.001）,Fagopyrum（R2=0.07,p=0.012）,and Linum（R2=0.07,p=0.014）,are positively correlated with HAILS,indicating that they are good indicator species for HAILS.