Facilitation or Competition? The Effects of the Shrub Species Tamarix chinensis on Herbaceous Communities are Dependent on the Successional Stage in an Impacted Coastal Wetland of North China

The responses of herbaceous species to shrubs are crucial for community succession, but are not fully understood. Three sites experiencing degraded, early positive and late positive successional stages in Laizhou Bay coastal wetland of North China were selected to investigate herbaceous responses to the dominant shrub Tamarix chinensis at both community and species level. The effects of T. chinensis on plant diversity and growth of herbaceous community were negative in the degraded succession but facilitative in the positive succession. Solar irradiance was the only environmental factor investigated that was significant for species’ proximity to the shrub in the degraded succession, while both aboveground (solar irradiance) and belowground environmental factors (soil electrical conductivity, water content, nitrogen concentration) played an important role in the positive succession. The performances of dominant herbaceous species in interaction with T. chinensis were both species-specific and traits-specific. Of all investigated factors, both solar irradiance and soil electrical conductivity significantly affected by the shrub and were the key factors for plant growth and leaf nutrient concentration at the species level. Thus, T. chinensis played an important role in the succession processes of the coastal wetland, but mainly in the positive succession stage and less in the degraded succession.