Colonization and expansion of Phragmites australis in upper Chesapeake Baytidal marshes

The common reed, Phragmites australis, has spread throughout Gulf and Atlan tic Coast marshes of the U.S. in the past thirty years. In the Chesapeake B ay area, natural resource managers are uncertain as to the current distribu tion or recent colonization rate of this low-salinity marsh grass. A geogra phic information system (GIS) was used to determine the distribution and ex pansion rate of P. australis within four brackish and three tidal freshwate r marshes in the upper Chesapeake Bay region. Vegetation patterns were mapp ed by interpreting aerial photographs from the 1930s, 1970s, 1980s, and 199 0s. The aerial extent of P. australis stands was measured by digitizing veg etation boundaries, correcting for distortion, and analyzing the data using a GIS. A geometric growth formula documented an intrinsic rate of increase for each P. australis stand. In addition, aboveground biomass was sampled from monotypic stands of P, australis. Results show that P. australis is pr esent in all seven marshes; however, it is most pervasive in the three tida l freshwater marshes. Based on interpretations of photographs, P. australis was present prior to 1938 in these three marshes. In each successive time period, there was a net increase of P, australia in all seven marshes that were examined. In the past ten years, however, the rate of increase has dec lined or stabilized in each marsh in which P. australis was well-establishe d prior to 1985. This slowdown is especially prevalent in the three tidal f reshwater marshes. In contrast, the three brackish marshes most recently co lonized by P. australis showed high annual intrinsic rates of increase (0.0 6 to 0.19 yr(-1) or more). Also, the highest biomass estimates were found i n two of these three marshes.