Vegetation's importance in regulating surface elevation in a coastal salt marsh facing elevated rates of sea level rise

Rising sea levels threaten the sustainability of coastal wetlands around the globe, thus understanding how increased inundation alters the elevation change mechanisms in these systems is increasingly important. Typically, the ability of coastal marshes to maintain their position in the intertidal zone depends on the accumulation of both organic and inorganic materials, so one, if not both, of these processes must increase to keep pace with rising seas, assuming all else constant. To determine the importance of vegetation in these processes, we measured elevation change and surface accretion over a 4-year period in recently subsided, unvegetated marshes, resulting from drought-induced marsh dieback, in paired planted and unplanted plots. We compared soil and vegetation responses in these plots with paired reference plots that had neither experienced dieback nor subsidence. All treatments (unvegetated, planted, and reference) were replicated six times. The recently subsided areas were 6-10 cm lower in elevation than the reference marshes at the beginning of the study; thus, mean water levels were 6-10 cm higher in these areas vs. the reference sites. Surface accretion rates were lowest in the unplanted plots at 2.3 mm/yr, but increased in the presence of vegetation to 16.4 mm/yr in the reference marsh and 26.1 mm/yr in the planted plots. The rates of elevation change were also bolstered by the presence of vegetation. The unplanted areas decreased in elevation by 9.4 mm/yr; whereas the planted areas increased in elevation by 13.3 mm/yr, and the reference marshes increased by 3.5 mm/yr. These results highlight the importance of vegetation in the accretionary processes that maintain marsh surface elevation within the intertidal zone, and provide evidence that coastal wetlands may be able to keep pace with a rising sea in certain situations.