Increased sediment accretion rates following invasion by Phragmites australis:: The role of litter

Negative connotations of invasive plants worldwide have implicated them as the bearers of unfavorable ecosystem change. We contrasted 5-yr-old and 20-yr-old Phragmites populations with pre-invasion areas occupied by Typha spp. and Panicum virgatum in an oligohaline tidal marsh of Chesapeake Bay. Peak live biomass was 3 times greater, while standing dead and litter was twice as great in the 20-yr-old Phragmites. It is this abundance of concentrated litter on the marsh surface of mature Phragmites populations that we implicate as encouraging the trapping of organic and mineral matter. The rate of vertical accretion in 20-yr-old Phragmites populations is 3-4 mm yr(-1) above the adjacent populations. By integrating the constant initial concentration and constant rate of supply models on individual Pb-210 cores, we estimate that Phragmites populations require a minimum of 7-yr post-colonization to enhance rates of accretion in this system. In light of the considerable loss of marsh habitat from relative sea-level rise, this finding contests the view that invasion creates strictly undesirable change at the ecosystem level.