Coastal wetlands are unique ecosystems defined by the interface where land meets the sea, heavily influenced by the daily rhythm of tides. This constant inundation and exposure creates harsh conditions that few terrestrial plants can survive. The vegetation found here has developed remarkable biological mechanisms to overcome the twin stresses of salt and waterlogged soil. This specialized flora ranges from non-woody grasses in temperate zones to dense, woody forests in the tropics.
The Unique Coastal Environment and Plant Adaptations
Coastal wetland plants must contend with two major environmental stressors that would prove lethal to most other flora. The first is high salinity, which draws water out of plant cells, making it difficult for plants to absorb water and nutrients from the soil. The second challenge is the anaerobic, or oxygen-lacking, condition of the waterlogged soil, which suffocates root systems.
To overcome these obstacles, wetland plants employ specialized strategies to manage salt and oxygen intake. Some species are salt excluders, possessing root membranes that actively filter out over 90% of the salt from the seawater. Others are salt excreters, which absorb saltwater but then push the excess salt out through specialized glands on their leaves. To cope with oxygen-poor mud, many wetland plants grow specialized structures like pneumatophores, which are upward-growing roots that capture oxygen from the air.
Herbaceous Plants of Temperate Salt Marshes
Temperate coastal wetlands, often called salt marshes, are dominated by non-woody, herbaceous plants, primarily grasses and rushes that arrange themselves in distinct bands based on elevation and tidal exposure. The lowest elevation zone, the low marsh, is flooded daily by the tides and is almost exclusively populated by smooth cordgrass (Spartina alterniflora). This species is highly tolerant of prolonged inundation and high salinity, using glands on its leaf blades to excrete excess salt. It also develops networked root systems, which provide an anchor against wave energy and help stabilize tidal creek banks.
Moving slightly higher is the high marsh, which is flooded less frequently, usually only during the highest tides of the month. Conditions here are less physically stressful, allowing for greater species diversity and more intense competition. Common plants include salt hay (Spartina patens), a finer grass that forms dense, matted stands, and black needle rush (Juncus roemerianus), a stiff, dark-green rush.
The high marsh also includes salt panne areas, which are shallow depressions where water collects and evaporates, leading to extremely high soil salinity. Only the most salt-tolerant species, such as glasswort (Salicornia species), survive in these hypersaline spots. Glasswort handles the high salt concentration by accumulating ions in its fleshy, succulent stems, effectively diluting the salt content within its tissues.
Woody Vegetation of Tropical and Subtropical Coasts
In warmer, tropical, and subtropical latitudes, the woody growth form dominates the coastal environment, forming dense mangrove forests. These trees are categorized into three main types based on their adaptations and placement within the intertidal zone. Red mangroves (Rhizophora species) occupy the seaward edge, dealing with the deepest water and most frequent inundation. They are recognized by their arching prop roots, which elevate the tree and provide structural support in soft sediment.
Red mangroves are primarily salt excluders, using a root filtration system that blocks the majority of salt entry. Any salt that bypasses this filter is sequestered in older leaves, which the tree then sheds in a process known as “sacrificial leaf drop.” Further inland, black mangroves (Avicennia species) thrive in softer mud by employing thousands of pencil-like pneumatophores that project vertically from the soil to capture oxygen. These mangroves are salt excreters, pushing excess salt through glands on their leaves, often leaving visible salt crystals.
The third type, white mangroves (Laguncularia species), are typically found at the highest elevation, closer to the upland border, where they are flooded least often. Like black mangroves, they possess salt glands to excrete excess salt, but they lack the prominent prop roots or dense pneumatophores of their counterparts. The different root structures and salt management strategies allow these three mangrove types to colonize the entire intertidal gradient.
Submerged and Floating Aquatic Vegetation
Distinct from the emergent plants of the marshes and mangrove forests are the plants that live primarily beneath the water surface or float upon it. Submerged Aquatic Vegetation (SAV), commonly known as seagrasses, forms extensive underwater meadows in protected, shallow coastal waters and estuaries. These flowering plants live entirely underwater, relying on clarity for photosynthesis and possessing specialized root systems to anchor in the sandy or muddy bottom.
Seagrasses like eelgrass (Zostera marina) and turtle grass (Thalassia species) are highly productive, forming a dense canopy that slows water currents and traps fine sediment. This action stabilizes the seabed and improves water quality, which in turn supports a vast array of marine life. In less saline or sheltered areas of the coastal wetland, Floating Aquatic Vegetation (FAV) may also be present, often consisting of small, surface-dwelling plants like duckweed or dense mats of filamentous algae. Unlike seagrasses, these plants generally do not require root anchorage in the soil and are found where the water is calm and the salinity is low enough to prevent salt stress.