Key points

Salt marshes and mangroves grow seawards and upwards by sediment accretion resulting from sediment binding by surface algae and roots of pioneer plants; tides transport sediment, nutrients and oxygen to marsh plants twice a day or less, depending on the elevation above MLW; marsh vegetation traps suspended sediment and further raises the marsh; soil salinity stressors increase in the high marsh where there is less regular influence of tidal flow; waterlogging, low oxygen and sediment mobility are the main stressors in low marshes and mudflats; elevational microhabitats have different floras and faunas according to their physiological tolerances of salinity and soil oxygen, resulting in a succession of plant communities; plant adaptations are both structural (e.g. salt glands, creeping roots with air passages, or platform roots with ‘lungs’) and internal (C3, C4 and CAM metabolism) to optimize photosynthesis when alternately submerged and dry; coastal wetlands are thus very productive carbon storage systems; pollen of the different plants marking the marsh zones provides an archive of changes in marsh zonation, salinity and climate over time; pollen of exotic species is used to trace changes in sediment accretion associated with anthropogenic impacts.