Comparison of biomass production and decomposition between Phragmites australis (common reed) and Spartina patens (salt hay grass) in brackish tidal marshes of New Jersey, USA

The recent expansion of Phragmites australis (common reed) from the marsh-upland interface into high marsh zones provides an opportunity to assess the impact of individual plant species on biomass production and decomposition in salt marshes. Seasonal harvests of aboveground and belowground biomass demonstrate that annual production of P. australis is approximately three times greater for aboveground biomass, two times greater for belowground biomass, and 30% lower in root: shoot ratio than neighboring populations of S. patens. Whole-plant litter (stems and leaves) also decomposes at a much slower annual rate for P. australis (k=0.25) than S. patents litter (k=0.57). By crossing litter type with site of litter decomposition, I found these plant species to influence decay rates through litter type and not through their effects on marsh surface conditions (e.g., temperature, sedimentation rates). Based on these calculations, annual rates of carbon accumulation in the peat of high marshes are likely to increase 5-fold once P. australis becomes established due to its greater rates of biomass production and residence time in infrequently flooded brackish marshes.