Abstract: Groundwater depth is a key environmental factor influencing the composition and structure of plant communities in coastal ecosystems. However, effects of the groundwater depth on the characteristics of shrub-grass communities in muddy coastal zones remain poorly understood. In this study, we conducted a field experiment to evaluate effects of the different groundwater depth (0.54, 0.83, 1.18, 1.62, 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 demonstrated that (1) the soil moisture and salinity gradually decreased with increasing groundwater depth (P < 0.001); Compared to the 0.54 m groundwater depth, soil moisture at depths of 0.83, 1.18, 1.62, and 2.04 m decreased by 16.02%, 24.83%, 54.40%, and 61.24%, and soil salinity decreased by 43.17%, 50.82%, 63.93%, and 73.41%, compared to 0.54 m, respectively. (2) The Simpson, Shannon-Wiener, Pielou and Margalef indices of the T. chinensis communities peaked at the 1.62 m groundwater table depth; (3) The dominant shrub T. chinensis population exhibited an aggregated distribution and optimal growth of T. chinensis shrubs occurring within the groundwater table depth range of 1.18 to 1.62 m; (4) The groundwater depth affected the diversity of the plant community primarily by influencing soil salinity rather than soil 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, regulating optimal groundwater table depth 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