Increases in global mean temperature will have substantial impacts on the world's salt marshes. Sea levels rise when high-elevation glaciers and polar ice caps melt and when seawater warms and expands. The impacts of more rapidly rising sea level depend on rates of sedimentation and uplift. If sediment accretion is equal to sea-level rise, the salt marsh remains in place, but when sea-level rise exceeds sediment accretion, the salt marsh moves inland - unless bluffs or development limits salt marsh migration. As sea level rises relative to the land, salt marsh communities will experience increased inundation, such that plant and animal species should shift upslope. However, not all species will be able to disperse or migrate as rapidly as tidal conditions change. In a few cases, for example, Scandinavia, the coast is still rebounding from the pressure of former glaciers, and land is rising faster than sea level. Salt marsh is then lost at the upper end and slowly gained near the water.
Globally, mean sea level has risen 10-25 cm during the last century. Current models predict an additional 5.6-30 cm rise in sea level by 2040. In areas of rapid shifts in sedimentation or high erosion due to wind and waves, salt marshes are destabilized and threatened with compositional changes and/or loss of marsh area. Salt marshes are also threatened by subsidence; if the land settles faster than sediment or roots and rhizomes can accumulate, vegetated areas convert to open water. USA's largest area of salt marsh loss is along the Louisiana coastal plain, where subsidence, decreased sedimentation, canal dredging, levee construction, and other human disturbances eliminate more than 4300 ha yr-1.
Coastal watersheds that experience increased stormi-ness as a result of climate change will discharge water, sediments, nutrients, and contaminants more erratically than at present, with resulting impacts on salt marshes downstream.
Soil salinity might also rise with higher temperatures, increased evaporation, and increased evapotranspiration. With more rainfall and freshwater flooding, however, soil salinity might decrease. The net effect of warming on salt marsh soil salinity is difficult to predict. Increased storminess could translate into more or stronger dune washover events during high tides, and stronger ocean swells would transport seawater further inland. The toxic effect of salt on upland vegetation, coupled with persistent salt in the soil, would favor halophytes over glycophytes in an increasingly broader wetland-upland transition areas (Figure 2). This prediction is most likely for areas of low annual rainfall, such as Mediterranean-type climates.
Climate change is likely to affect species differently, potentially altering competitive relationships. Photosynthesis, transpiration, nutrient cycling, phenology, and decomposition are influenced by temperature. Salt marshes with a mixture of C3 and C4 plants might shift toward C4 plants as mean temperature climbs; however, elevated CO2 might favor C3 species. In subtropical regions, a warming trend and sea-level rise would likely allow mangroves to move northward and displace salt marshes.
Impacts of climate change to plants and animals are difficult to estimate. European ecologists, however, have detailed information on bird use of salt marshes and can predict shifts in invertebrate foods and shorebirds given various scenarios of sea-level rise.
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