Differential Responses to Simulated Precipitation Exhibited by a Typical Shrub and a Herb Coexisted in Hunshandak Sandy Land On the Fossil Pediastrum from the Gaoximage Section, Hunshandak Sandy Land and Its Ecological Significance Since 5 000 a BP A Preliminary Investigation on the Holocene Vegetation Changes From Pollen Analysis in the Gaoximage Section, Hunshandak Sandy Land Classification and ordination analysis of plant communities in Inter-dune lowland in Hunshandak Sandy Land