Abstract: In this study, we explored the changes in plant community diversity and their relationship with soil factors under shrub encroachment pressure by selecting four marsh areas in Sanjiang Plain with different degrees of shrub cover (a, 0≤a≤100%), including marsh with no shrub encroachment (a=0), light shrub encroachment (0<a≤30%), medium shrub encroachment (30%<a≤70%), and heavy shrub encroachment (70%<a≤100%). The results showed that shrub encroachment was the main reason for the heterogeneity of plant community composition in marsh. With shrub encroachment, the dominant species Deyeuxia angustifolia was replaced by Spiraea salicifolia, whereas Carex schmidtii disappeared. However, the importance values of Sanguisorba tenuifolia and Stellaria radians in the community significantly increased by 0.8- and 9.0-fold, respectively. Shrub encroachment resulted in changes of plant community diversity in marsh. The highest α and β diversity in plant community were observed under light shrub encroachment area, whereas there were no significant differences in Shannon and Simpson indices between heavy and no shrub encroachment areas, despite their similarity in plant community composition was the lowest. All the studied soil factors were significantly affected by shrub encroachment, except total phosphorus. There were significant reductions in soil moisture, organic carbon, and total nitrogen in heavy marsh shrub encroachment area by 32.6%, 69.3%, and 66.1%, respectively compared with that in no marsh shrub encroachment area, whereas soil total potassium and total magnesium significantly increased by 33.8% and 40.6%, respectively. Results of redundancy analysis showed that soil organic carbon, moisture content, total nitrogen, and N:P were the main factors controlling the diversity changes of marsh plant community under shrub encroachment pressure.

Key words: shrub encroachment, plant community diversity, Sanjiang Plain marsh