Morphological responses to competition modulated by abiotic factors in two monoculture-forming wetland plants

•P. australis, S. deltarum were subjected to environmental stressor treatments.•P. australis, S. deltarum grown in individual monoculture and in mixture.•Treatments may reflect environmental conditions associated with Mississippi River sediment diversions.•Stressors influenced whether competition suppressed productivity.•S. deltarum increased root-to-shoot ratio in brackish water.