Responses of two fringing salt marsh plant species along a wave climate gradient

Salt marshes are increasingly valued for their role in coastal defense. In particular, marsh plants slow the progression of waves, thereby decreasing wave heights, orbital velocities and associated energy. Practical application of these effects has driven substantial research estimating the effects of plants on waves. However, the effects of waves on plants remain understudied, especially regarding plant responses along a wave climate gradient. To begin to understand these responses, we collected above- and belowground plant data and wave data from 60 sites across Mobile Bay, USA, and tributaries and evaluated plant responses along the range of assessed wave climate conditions. Plant responses among the dominant species, Juncus roemerianus and Spartina alterniflora, varied along the wave climate gradient. However, the basal diameter of shoots in both species declined linearly with increasing wave climate. While wave climate had no observable effect on other S. alterniflora parameters, the declining diameter of J. roemerianus shoots along the same gradient was commensurate with a decline in the percentage of live canopy shoots aboveground and an increase in root and rhizome biomass in the active rooting zone belowground. In contrast to previous studies, other responses including the height, biomass and density of aboveground shoots in both species were not related to wave climate. More broadly, these results demonstrate that plant features important for wave attenuation such as shoot diameter can change in response to varying wave conditions. These feedbacks should be incorporated to improve coastal modeling and successes of coastal conservation, restoration and enhancement projects.