Abstract: The groundwater depth can affect the composition and structure of plant communities. However, effects of the groundwater depth on the characteristics of shrub–grass communities in muddy coastal zones remain uncertain. We conducted a field experiment to evaluate effects of the different groundwater depth, which included 0.54 m, 0.83 m, 1.18 m, 1.62 m, and 2.04 m, on soil salinity, soil moisture, community diversity, distribution pattern and growth of the dominant Tamarix chinensis in the muddy coastal zone of Bohai Bay. Our results showed that (1) the soil moisture and salinity gradually decreased with increasing groundwater depth (P<0.001); (2) The Simpson, Shannon‒Wiener, Pielou and Margalef indices of the T. chinensis communities were greatest in the plots with 1.62 m; (3) The dominant shrub T. chinensis population exhibited an aggregated distribution and optimal growth of T. chinensis shrubs emerged in the plots with 1.18 m or 1.62 m; (4) The groundwater depth affected the diversity of the plant community mainly by influencing soil salinity rather than moisture; the dominant shrub T. chinensis promoted diversity of plant community, but this facilitation effect was inhibited by soil salinity. Our results suggest that the optimal groundwater depth for maintaining biodiversity falls within the range of 1.18 to 1.62 m. Shallow groundwater diminishes biodiversity both directly through soil salinization and indirectly by impairing T. chinensis’ facilitation of biodiversity. Therefore, maintaining optimal groundwater levels and protecting T. chinensis are critical for biodiversity conservation and ecosystem recovery in muddy coastal areas.

Key words: Groundwater depth, Soil salinity, Tamarix chinensis, Plant diversity, Coastal tidal flats