Sediment and plant dynamics in saltmarshes pioneer zone: Puccinellia maritima as a key species?

Low saltmarshes are subjected to variability between sites and tidal cycles in terms of erosive forces by current and waves, the frequency and duration of flooding and soil salinity. The establishment of vegetation in pioneer zones is directly related to sedimentary dynamics but few data are available concerning the effects of plants on sediment dynamics. In the Mont Saint Michel Bay (France), the low saltmarshes, including pioneer zones, are characterized by a micro-topography composed of hummocks with vegetation dominated by Puccinellia maritima, mudflats with a low sparse vegetation of Spartina anglica, Salicornia fragilis and Puccinellia maritima and a few erosion zones. The aim of this study was to (1) investigate the sediment deposition and soil elevation patterns, between tidal cycles and between sites; (2) look for a relationship between the development and dynamics of the microtopography and the different plant species; and (3) evaluate whether Puccinellia maritima plays any role in enhancing sediment deposition and therefore plant succession in these lower marshes. The three study sites were situated in a system where accretion prevailed and soil substrate was essentially fine sand. The amount of sediment deposited varied between tidal cycles as well as between sites. The soil level changes, measured by a Sediment Erosion Bar, underlined the importance of Puccinellia maritima in stabilising sediment deposition. When the soil was covered with Puccinellia, the increase in soil level is almost doubled compared with bare sediment when the site was relatively sheltered. In more disturbed sites, such as site 1, the presence of Puccinellia was critical for the increase in soil level as it was eroded on bare soil. The presence of Puccinellia appeared to be strongly linked with the installation of hummocks and the micro-topography. In 2–3 years, Puccinellia cover increased and species typical of higher levels of the saltmarsh became established on the higher and more stable parts of the micro-topography (the hummocks ), which led to an increase in the succession rate.