Salt Marshes and Mangroves: Tidal Saline Wetlands Dominated by Vascular Plants

Abstract

Coastal wetlands encompass an ecosystem continuum between subtidal depths (to which light penetrates to support photosynthesis) and the landward edge of sea level influence. A broader definition of coastal wetlands includes, in addition to salt marshes and mangroves, a wide diversity of environments, from tidal flats, to non-tidal wetlands at the landward edge, whose hydrology is still influenced by sea level. In humid climates, freshwater seepage and high groundwater levels fed by excess rainfall provide the waterlogged conditions necessary for the establishment and growth of peat-forming wetlands. Freshwater swamps, Pocosins, non-tidal marshes, and bogs develop in shore zones under humid climates, beyond the limits of tidal influences. For these wetlands, water from terrestrial sources (groundwater seeps and often perched water tables) cause the inundation, but sea level modulates their hydrology and impedes drainage. In the shore zone of arid climates, under a combination of high evaporation and low freshwater inputs, soils develop extremelyhigh salinities. Extensive barren salt flats or halophytic steppes characterize arid coastal lowlands instead of freshwater swamps and bogs. These flat salt-crusted wetlands in arid shore zones are never flooded by tides but sea level keeps a shallow water table near the surface. This chapter focuses on the two ecosystems dominated by vascular plants along the intertidal zone. Tidal salt marshes dominated by herbaceous or low shrubby halophytic vegetation occur on protected shorelines from the Arctic to the subtropics. Mangrove communities dominated by halophytic treesreplace salt marshes in tropical regions worldwide, although they may even extend into subtropicalregions where ocean currents ameliorate temperatures. While salt marshes still occur in the tropics, they cover a smaller area than mangroves. Despite their different appearance, salt marshes and mangroves are remarkably similar in terms of their ecological functions and relevance