Disturbance by wrack facilitates spread of Phragmites australis in a coastal marsh

Disturbance is an important factor facilitating the spread of nuisance and invasive species. Over the past century, the grass Phragmites australis has been spreading aggressively throughout coastal marshes of North America and is now considered a nuisance species in many regions. In this study, I examine how natural disturbance by wrack (primarily dead plant material of the native grass Spartina alterniflora) affects the local spread of stands of P. australis in a coastal marsh in southern New England, USA. A survey indicated that wrack covered 8.4% of the marsh and that a disproportionate amount of wrack (73% of wrack cover) is stranded at the highest tidal elevations of the marsh next to stands of P. australis. To test hypotheses that wrack inhibits or facilitates the seaward spread of P. australis, I did two experiments where I manipulated the presence or absence of wrack next to stands of P. australis and then monitored the performance of P. australis: (1) wrack was either removed or not removed for two growing seasons from areas where wrack had naturally stranded adjacent to stands of P. australis, and (2) wrack was either added or not added next to stands of P. australis without naturally stranded wrack for one growing season, and then the added wrack was removed before the next growing season. Removing naturally stranded wrack resulted in a substantial increase in the density of shoots of P. australis compared to areas with wrack. Similarly, areas where wrack was added for one growing season and then removed before the emergence of shoots in the next, had more than double the density of shoots compared to areas of natural marsh turf that did not undergo this disturbance. Moreover, these shoots were significantly taller and produced more inflorescences. Therefore, wrack stranded in the marsh can suppress the vegetative growth of P. australis, but once it decomposes or is removed by the tides, P. australis spreads dramatically. Wrack appears to facilitate the spread of P. australis by smothering the underlying marsh turf, creating patches of bare space with environmental conditions favourable to the spread of P. australis. The only areas where P. australis did not increase substantially were those continually occupied by marsh turf, indicating that undisturbed marsh turf is an important buffer resisting the local spread by P. australis. Because P. australis is currently spreading and the litter of P. australis contributes to the biomass of wrack, disturbance by wrack may be an increasingly important factor facilitating the spread of P. australis and influencing the ecology of coastal marshes. (C) 2002 Elsevier Science B.V. All rights reserved.