Wetlands and mangroves are defined primarily by water presence and the biological communities that can tolerate it. Understanding their physical setting, soils, and vegetation helps explain where they form and how they differ from dryland ecosystems.

Wetlands: what they are

Wetlands are identified by hydrology (water), hydric soils (water-saturated soils), and hydrophytic vegetation (plants adapted to wet conditions).

Wetlands are not always visibly “wet” year-round; many are seasonally inundated (flooded) or have water at/near the surface only during parts of the year.

Key characteristics used to recognise wetlands

• Hydrology: standing water, periodic flooding, or a consistently high water table

• Soils: low-oxygen (often anaerobic) conditions that slow typical decomposition and change soil chemistry

• Plants: dominance of water-tolerant species (e.g., sedges, reeds, cattails, bald cypress in some regions)

Common wetland categories (by setting)

• Freshwater wetlands: found inland; water is low in dissolved salts

• Coastal wetlands: influenced by tides and brackish or salty water

• Riverine/floodplain wetlands: associated with streams and rivers; shaped by flooding frequency

• Bogs vs. marshes vs. swamps (general distinctions):

• Marsh: dominated by non-woody plants (grasses/sedges)

• Swamp: dominated by woody plants (trees/shrubs)

• Bog: typically acidic and nutrient-poor, often peat-forming (local conditions vary)

Mangroves: what they are

Mangroves are a specific kind of wetland found along tropical and subtropical shorelines where saltwater influence is strong.

Mangroves occur in the intertidal zone, meaning they are regularly flooded and exposed by tides.

Red mangroves (Rhizophora mangle) form arching prop roots that anchor trees in unstable, waterlogged sediments while elevating parts of the root system into better-oxygenated conditions. This visible root architecture helps explain why mangroves thrive where tidal flooding creates persistently low-oxygen soils. Source

Their defining feature is the dominance of salt-tolerant woody plants, rather than the grasses or sedges typical of many marshes.

Where mangroves form

• Low-energy coastlines (protected bays, lagoons, estuaries) where fine sediments can accumulate

• Warm climates with minimal frost (temperature limits strongly influence global distribution)

• Areas with brackish water mixing (freshwater + seawater), especially near river mouths

Mangrove adaptations (how they function as plants in saltwater mud)

• Salt management:

  • Some species exclude salt at the roots

  • Others excrete salt through specialised leaf glands

• Root adaptations for low oxygen:

  • Aerial roots (e.g., pneumatophores) increase gas exchange in waterlogged, oxygen-poor sediments

• Stability in soft sediments:

  • Prop or stilt roots help anchor trees and resist tidal movement

Mangrove zonation (patterns across the shoreline)

Mangrove communities often show zonation—different species dominate at different distances from the shore—driven by:

This figure illustrates mangrove zonation across the intertidal gradient, with species occupying different positions relative to typical high/low tides and shoreline elevation. It visualizes how changing flood frequency and salinity conditions can sort communities from seaward fringe mangroves to more landward stands. Source

• flooding duration and tidal height

• salinity gradients

• sediment type and drainage

Wetlands vs. mangroves: quick contrasts

• Water type: wetlands can be freshwater, brackish, or salty; mangroves are coastal and typically saline/brackish

• Dominant vegetation: wetlands vary widely; mangroves are dominated by salt-tolerant trees

• Hydrologic driver: many wetlands are driven by rainfall/flooding/groundwater; mangroves are strongly shaped by tides