Does woody species establishment alter herbaceous community composition of freshwater floating marshes?

Floating freshwater marsh communities (flotant) in the Mississippi Deltaic Plain are composed of vegetation rooted in an organic mat that detaches from the underlying mineral substrate and shifts vertically as water levels below rise and drop. Unlike attached marshes dominated by herbaceous species, floating marsh mats are free from the stress of inundation, enabling establishment of woody species. Dynamics of these flotant communities are largely unstudied, and it is unknown whether invasion by woody species alters their structure and composition. To study the potential effects of woody species invasion on herbaceous community characteristics, we compared open herbaceous marsh, sparse scrub-shrub, and dense scrub-shrub thickets at Jean Lafitte National Historical Park and Preserve in coastal Louisiana. We found that species richness and composition differed significantly among the three marsh types. Herbaceous communities lacking shrub canopies had the lowest richness and were dominated by emergent species typical of freshwater marsh. Richness and composition of sparse scrub-shrub thickets were intermediate between open marsh and dense thickets. The latter had the greatest species richness with assemblages more typical of forest understories, as well as aquatic species that occurred where holes in the floating mat formed. Morella cerifera (wax myrtle), an actinorhizal shrub, was the dominant woody species and formed the shrub stratum in sparse and dense thickets; the exotic Triadica sebifera created a low-stature overstory. Bryophytes colonized the bases and lower stems of both species. We found 35 additional vascular plant species in the thickets, including two other exotics, Alternanthera philoxeroides and Salvinia molesta. Establishment of woody species in flotant marsh adds structural complexity and appears to drive compositional changes in the herbaceous community toward a combination of woodland and aquatic assemblages. The longevity of the woody phase in flotant marsh and the long-term ecological consequences of widespread Triadica sebifera invasion are unknown.