Phragmites australis expansion in Delaware Bay salt marshes

Abstract

Phragmites australis, widespread in tidal marshes of the eastern coast of North America, spreads primarily by the growth of surface runners and underground rhizomes. The distribution of Phragmites, based on maps from aerial photography on Estuary Enhancement Program (EEP) sites of Public Service Enterprise Group (PSEG), was investigated before and after marsh restoration. Initial establishment of Phragmites occurred around 1950 near riverfronts or creek mouths and proceeded upstream. Phragmites expanded to creek bank berms, the tidal/upland edge as well as central marsh plains. Phragmites growth may be characterized into periods of (1) initial establishment, (2) unrestricted expansion, (3) restricted expansion (restricted by open water-creeks, upland borders, or other colonies of Phragmites), and (4) monospecific marsh plains of Phragmites. Intrinsic rate of increase and increase in percent coverage measures for Phragmites growth on the EEP sites are comparable to those of other studies, although some EEP site values are much higher. Describing Phragmites expansion in these terms does not account for the actual distribution and pattern of Phragmites colonies as constrained by tidal creeks and upland boundaries. Therefore, an approach to estimate the actual rate of linear (or “radial”) expansion of Phragmites colonies was developed using standard GIS functions by having colonies represented by an equivalent ellipse having the same area and “diameter” as the colony or polygon. This perimeter expansion rate (PER) appears to estimate the actual growth rate of an advancing front of Phragmites as observed in the field from site maps of Phragmites coverage. This approach, as well as a GIS simulation of Phragmites colony growth, can be used to estimate the density of growing Phragmites rhizome buds (potential for regrowth) following management efforts.