Pollen Assemblages Of The Surface Soil And Their Relationships With The Modern Vegetation And Climate In Northern Xinjiang

The global environmental changes have become one of the focuses of common concern in all fields of scientific researches,it is crucial to study the history of environmental changes for predicting the future of environment.As an important indicator of the reconstruction of palaeovegetation and palaeoclimate,spore-pollen(the parthenogenetic cells of spore plants and seed plants—spores and pollen)plays an important role for studing the past climate and hydrology changes.Northern Xinjiang,China is located in aird central Asia,with a frangible ecology which is easily influenced by climate change.However,due to the complication of yields,dispersal processes and precipitation environments of diverse pollens,the relationship between pollen assemblages and vegetation is not well constrained,therefore the research of surface pollen assemblages and their relationships with modern vegetation and climate provide a foundation for reconstructing palaeo-environment by fossil pollen.Nowadays,most modern samples for studying pollen assemblages are based on the surface soil pollen samples.The studies on modern spore-pollen are useful to understand the relationship of vegetation,climate and hydrology in Northern Xinjiang areas,and have important significance to guide solutions to environmental change in future.In September 2016,we conducted a detailed field investigation in Northern Xinjiang,and collected 22 topsoil pollen samples,21 cow dung samples as well as vegetation imformation from 23 sample sites,in order to compare these two kinds of modern pollen samples and analyse their relationships with the modern vegetation and climate in northern Xinjiang.By comparing pollen assemblages of surface soil and cow dung samples in the dessert steppe areas and typical steppe areas in northern Xinjiang,we found that the pollen assemblages of these two samples are generally similar with each other,and can both indicate the regional vegetation types.The pollen concentrations of arbors for most samples are lower than 10%,those of shrubs are averaged at 5.6%,and those of herbs are generally higher than 70%.Artemisia and Chenopodiaceae are most important components of the pollen assemblages.The characteristics of pollen assemblages in the different communities show that the Chenopodiaceae plants are more drought-resistant than Artemisia plants,and the former dominates in the arid area whereas the latter is mainly distributedin the semi-arid environment.Cow dung samples have less pollen types than the surface soil samples;arbors content of cow dung(2.04%)is lower than that of surface soil(3.78%).Because the cow dung samples were produced in most recent months while the formation time of the surface soil samples were uncertain,it is likely that the pollen assemblages of cow dung should be more relevant to the modern vegetation types.We thus propose that the cow dung and surface soil can be both used asthe archive of modern vegetation types in the dessert steppe and typical steppe areas.Furthermore,the surface pollen assemblages and cow dung samples have obvious distinctions in the diffenrent ecological regions.The arbor pollen concentration increases and the shrubs and herbs pollen concentrations decrease in surface soil from dessert steppe to typical steppe.Meanwhile,the pollen assemblages of cow dung show an inverse trend,which is probablyrelated with the diet of cows.Moreover,the Principal Component Analysis(PCA)of the main pollen assemblages(Rosaceae,Poaceae,Umbelliferae,Leguminosae,etc.)and environmental factors reveal that the pollen assemblages have the highest correlation with the mean June-August precipitation and the mean annual precipitation,and the secondary high correlation with the mean temperatures of thecoldest and warmest months,but weak correlation with the mean annual temperature.Ephedra saw an increased trend with the precipitation increased.Thus this pollen type represents drier environmental condition.The results of correlation analysis show that the percentage of Artemisia and Chenopodiaceae is negatively related to mean June-August precipitation(r=-0.434),which indicates Artemisia and Chenopodiaceae are drought-resistant,and futher shows close relationship between vegetation communities and precipitation.PCA results indicate that different types of sporo-pollen samples are in quite different humidity ranges: samples from dessert steppe are relatively lower than that from humidity regions,while samples fromtypical steppe areas are relatively higher.The results of this study suggest that the precipitationis likely the most important factor controlling modern vegetationdistribution in northern Xinjiang area.