Competitive interactions between invasive Phragmites australis and Spartina alterniflora: Mechanisms and management implications for marsh migration

• P. australis is invading high and low marsh in Great Sippewissett Marsh • Shading and elevation effects enable the P. australis invasion • The invasion may impair S. alterniflora landward migration as sea level rises • Repeated clipping of P. australis doubled S. alterniflora cover for 3 yrs. • Clipping may be a safe practical management tool for such invasions. A genetic variant of Phragmites australis (haplotype M) has expanded massively into freshwater wetlands in the northeastern United States and, more recently, into salt marshes where it competes with plant species critical to sustaining ecological services. We documented such vegetation changes, assessed elevation-mediated competition and shading of salt marsh plants by the taller canopy of P. australis , and tested effectiveness of mechanical clipping for reducing P. australis dominance at Great Sippewissett Marsh, Massachusetts. Remote sensing revealed a more than sixfold expansion of P. australis percent area in the high marsh zone between 1979 and 2023. Elevation surveys showed that by 2023, P. australis had occupied high- to mid-elevation range historically dominated by high marsh vegetation in 1984 and extended into areas of low marsh supporting Spartina alterniflora , potentially blocking its landward migration that will be required under accelerating sea-level rise. Light availability measurements demonstrated a six-fold reduction in photosynthetically active radiation beneath P. australis canopies compared to clipped plots, highlighting shading as a strong but underexplored mechanism of competitive suppression. Concern over key salt marsh species and environmental services has prompted interest in management. A high-frequency clipping experiment (weekly cuts, May–October 2023) reduced P. australis canopy cover and allowed a near doubling of S. alterniflora cover within one growing season, with benefits persisting for at least two years post-treatment. Repeated clipping of the P. australis overstory may reduce its competitive edge, facilitating landward expansion and persistence of S. alterniflora and other important salt marsh species.