Salt marsh plants: biological overview and vulnerability to climate change.

Abstract Salt marshes are vegetative ecosystems that occupy intertidal zones in estuaries or behind coastal barrier islands where there is some degree of protection from strong wave energy. Physiochemical properties of these marshes are stressful for most angiosperms, as only a few species can tolerate the anoxic, chemically reduced, high-saline soils typical of these ecosystems. Despite the dynamic properties that make salt marshes inhospitable to most plants, they maintain some of the highest levels of biological productivity observed in nature. The vast majority of salt marsh halophytes do not require environmental salts to grow or reproduce, and only a few species are restricted to salinities greater than 0.5%. For salt marsh plants, tolerance to high salinities may involve multiple physiological strategies including ion compartmentalization, synthesis of compatible solutes, changes in membrane and/or cell wall properties, and/or salt exudation by way of salt glands or bladders. Even though salt marsh halophytes are well adapted to highly dynamic and, often, stressful environmental conditions, it is generally recognized that climate change will result in a global net loss of these ecologically and economically important ecosystems. Sea-level rise may decrease overall plant diversity by selecting species that are more tolerant to sustained flooding, or through the loss of mid- and high-marsh species that are less competitive to changing conditions. In areas where soil accretion fails to keep pace with rising waters and/or where landward migration is impeded, rising sea levels are likely to promote the conversion of marshes into unvegetated open water systems. Soil and water hypersalinity may also foster salt marsh die-offs in areas that are expected to experience seasonal declines in precipitation. Nevertheless, because of the natural complexity of these systems, the degree of salt marsh loss remains uncertain. Localized differences in climate, geology, hydrology and topography, along with biological and anthropogenic interactions, are likely to determine which marshes withstand the challenges of climate change and which marshes will be lost.