Rafting on wrack as a mode of dispersal for plants in coastal marshes

Here, I examine the potential for dead plant material, or wrack, to act as a vector of dispersal for plants among coastal marshes along the Palmer River in New England, USA. Three hundred mats of wrack floating along a 2.5 km section of river spanning tidal fresh, brackish and salt marshes were sampled for propagules of plants from May 1997 to October 1998. Thirty species of plants and 2 species of algae were associated with the wrack, and 24 of the plant species were viable under greenhouse conditions. Common and abundant species of tidal fresh, brackish and salt marshes were found in the wrack, as well as introduced and invasive species. Wrack contained plants suited to different modes of dispersal, including water, wind and adherence to animals. Measurements of the movement of individual mats of wrack along a 200 m section of the river indicated that wrack travelled at speeds ranging from 1.1 to 2.6 km h−1, which translates to potential dispersal distances ranging from 6.5 to 15.9 km for one ebb or flood tide of 6 h duration. Tagged mats of wrack tracked for one ebb or flood tide travelled at least 2.5 km along the river. These results suggest that wrack may be an important agent of plant dispersal among coastal marshes in estuaries.