Plant zonation in irregularly flooded salt marshes: relative importance of stress tolerance and biological interactions

1 Most studies of salt marsh plant zonation have been at middle to high latitudes of the northern hemisphere, in euhaline or periodically hypersaline marshes with regular tides. In this study, we examined plant zonation in an irregularly flooded marsh in southern Brazil. Pore water characteristics were compared in four vegetation zones across a marsh elevation gradient. Reciprocal transplants between vegetation zones and removal experiments were performed to examine species interactions in low and mid marshes. 2 There was no distinctive gradient of physical stress across the elevation of irregularly flooded low and mid marshes. Moreover, the three dominant plants, Spartina alterniflora, Spartina densiflora and Scirpus maritimus, were able to grow across the entire elevation gradient, i.e. within zones normally occupied by the other species. The only exception was Spartina alterniflora, which was strongly limited by selective herbivory by the crab Chasmagnathus granulata in the Scirpus maritimus zone. 3 Although intra- and interspecific competition reduced growth of all three species, no competitive hierarchy was found in any vegetation zone. 4 These results suggest that, as in tidal marshes of the north hemisphere, competition is important in structuring salt marsh plant communities. In contrast, however, plant zonation in irregularly flooded marshes cannot be explained by displacement of competitive subordinates to physically stressful habitats. The roles of founder effects and selective herbivory in such marshes therefore merits further investigation.