Does salt stress affect the interspecific interaction between regionally dominant Suaeda salsa and Scirpus planiculumis?

Plant-plant interactions that change along environmental gradients can be affected by different combinations of environmental characteristics, such as the species and planting density ratios. Suaeda salsa and Scirpus planiculumis are regionally dominant species in the Shuangtai estuarine wetland. Compared with non-clonal S. salsa, clonal S. planiculumis has competitive advantages because of its morphological plasticity. However, salt-tolerant S. salsa may grow faster than S. planiculumis in saline-alkali estuary soil. Whether the interactions between these two species along salinity gradients are affected by the level of salt stress and mixed planting density ratio remains unclear. Thus, to test the effects of salt stress and planting density ratios on the interactions between S. planiculumis and S. salsa in the late growing season, we conducted a greenhouse experiment consisting of 3 salinity levels (0, 8 and 15ppt) and 5 planting density ratios. Our results showed that the promotion of S. salsa growth and inhibition of S. planiculumis growth at low salinity levels (8 ppt) did not alter the interactions between the two species. Facilitation of S. salsa occurred at high salinity levels, and the magnitude of this net outcome decreased with increases in the proportion of S. salsa. These results suggest that competition and facilitation processes not only depend on the combinations of different life-history characteristics of species but also on the planting density ratio. These findings may contribute to the understanding of the responses of estuarine wetland plant-plant interactions to human modifications of estuarine salinity.