Wetland ecosystems are extremely valuable and increasingly fragile. Home to a wealth of moisture-loving plants and animals, they are crucial as sources of freshwater, brackish water, and oxygen. These transitional zones, which are rarely ever totally dry or wholly underwater, help purify water via the adaptations of their many unique plants. This is why they are often perceived as the ecological “kidneys” of the planet.
Found in the area between terrestrial landscapes and aquatic features, modern-day wetlands are economically important too. Many of the world’s largest wetland systems have designated zones for recreation, are habitats of protein-rich fish, and are protected as flood-control systems. Usually hydrated by groundwater or by nearby lakes and rivers, they are often referred to as swamps, marshes, and bogs.
Seasonal trends often lead to changes in the saturation levels of wetlands. As a result, many of their inhabitants must either migrate to areas with more moisture or develop a tolerance for both wet and dry conditions. Wetland vegetation or “hydrophytes”, which include trees, grasses, mosses, and other specialized plants, are equipped with adaptations for dynamic moisture, oxygen, salinity, and shade conditions.
Wetland Waters & Substrates
To fully comprehend the need for wetland plant adaptations, it’s helpful to review the general features of these vital ecosystems. The many microhabitats of wetlands can support the needs of aquatic, semi-aquatic, or terrestrial plants. These usually situate their stands in areas that provide their basic requirements. For example, semi-aquatic perennials are more likely to occur as emergent or edge plants along the borders of saturated zones.
Differences in climate, soil, altitude, topography, and distance to coastal areas can affect the general properties of wetland systems. These may be categorized as coastal or inland, with further subcategorizations such as tidal mudflats, mangrove swamps, riparian wetlands, or cypress swamps. Some types may experience tides and seasonal floods, whereas others may be more stable.
The speed of water flow and its changing depth directly influences plant composition. Salinity levels, water chemistry, and turbidity levels would likewise influence plant settlement and survival rates. Hydric, wetland soils are usually high in organic matter and may have anaerobic layers, necessitating the need for root adaptations to low oxygen conditions.
Special Adaptations of Common Wetland Plants
1) Aerenchyma (air pockets) in roots
Aerenchyma is a type of plant tissue that contains large, intercellular spaces. These gaps, or air pockets, support the flow of oxygen in submerged root systems. They are especially important during floods or in waterlogged environments, which are often associated with anoxic conditions in packed substrates. Typically situated in the root cortex, aerenchyma may serve as the main pathway of oxygen transport. Depending on the plant species, it may be a natural feature of their anatomy or its development may be triggered by chemical reactions in response to prolonged floods.
The broadleaf cattail (Typha latifolia) is especially well-adapted to hydric soils with low oxygen concentrations because of its root aerenchyma. The presence of this specialized tissue enhances its competitive nature in wetlands, particularly those with less aggressive grasses. As a result, it has become an invasive plant in many marsh ecosystems of the US.
2) Adventitious roots
Adventitious roots are specialized root structures found above the soil surface or emerging through the waterline. These may arise from the subterranean root system (developing in an upward manner), from the base of the shoots, or from the nodes of stems. Often found in flood-tolerant wetland plants, these roots support nutrient uptake, facilitate gas transport, and aid in water absorption.
The benefits of producing adventitious roots, which may be costly for the plant in terms of carbohydrate allocation, are most pronounced in wetlands with prolonged flooding events. In cypress trees, the roots may perform additional functions, such as the provision of structural support. The seedlings of bald cypress trees (Taxodium distichum) may begin to develop these roots after just 25 days of exposure to wet conditions.
3) Shallow root systems
Shallow root systems are a common morphological adaptation in obligate wetland plants. Compared to deep structures with an enlarged taproot, these fibrous roots are more likely to provide stability during flooding events. They tend to be extensive and fine, latching onto as large a surface area as possible to keep the shoot upright. As they are found within the upper layers of soil, shallow roots also tend to be exposed to more oxygen.
4) Rhizosphere oxygenation
Some types of wetland plants are able to mediate the release of oxygen from the tips of their root systems. Oxygen transport from the shoots and into the rhizosphere, which is the zone of chemical and biological activity at the soil-root interface, aids in aerating waterlogged soils and their ecologically important microbial communities. It also helps meet the oxygen demand of root systems in both wetland and non-wetland species.
Radial oxygen from subterranean roots can significantly influence a wetland’s capacity to purify waters and substrates. The microbial communities supported by this adaptation play key roles in their micro-environments, aiding in the breakdown of decomposing matter, the removal of metals and potentially toxic pollutants, and the filtration of excess nutrients.
5) Shoot elongation
Some wetland plants respond to increases in water level by rapidly elongating their shoots. This adaptive mode of avoiding complete submersion ensures that the plants’ leaves or upper nodes remain in contact with an oxygenated atmosphere. This is crucial for the diffusion of oxygen into aerial plant tissues, thereby sustaining the normal rate of photosynthesis.
In deep-water rice, shoot elongation appears to be triggered by the accumulation of a phytohormone called ethylene. When a sufficient concentration of ethylene is present, a cascade of cellular reactions occurs to facilitate the elongation of shoot tissues. The shoots of some species can elongate at an impressive rate of 20 – 25 cm (8 – 10 inches) per day!
6) Waxy surfaces
Many wetland plants naturally repel water by producing a waxy coating. Often referred to as a cuticle or cuticular layer, it may be found on the upper or lower surface of submerged and floating leaves. On floating hydrophytes, this layer is typically found on the upper surface, where it prevents the stomata from being clogged by water. Made of cutin and suberin, it can also help keep the leaves cool by reflecting the sun’s rays.
Apart from protecting the stomata of floating leaves, a waxy surface may also aid in reducing the loss of sap and moisture. By minimizing osmotic rates, plant fluids can be kept in the foliage, where they are necessary for nutrient transport. The absence of a cuticle can thus lead to shrinkage as plant moisture is lost to the aquatic environment.
7) Air pockets in floating leaves and shoots
Air pockets are common features of shoots and leaves in submerged and floating plants. Much like the aerenchyma of roots, these may serve as pathways of oxygen transport. The stored air can help keep leaves buoyant or afloat. In submerged plants, they can help the shoots maintain an upright orientation, ensuring that they grow toward the water’s surface and are able to move in the direction of a current.
Occasionally, air pockets may serve a more sinister purpose in highly-evolved wetland plants. In the common bladderwort (Utricularia macrorhiza), for example, they act as bladders that may contract and release or take in more air once they come into contact with foreign stimuli. This carnivorous plant feeds on invertebrates that are “sucked in” and trapped by its bladders.
8) Accumulation of chemicals in the root system
Wetland plants often have to adapt to anaerobic conditions in the substrate. These force them to shift to alternative pathways of nutrient cycles in order to convert certain nutrients into useful compounds. As a result, some potentially toxic chemicals may accumulate in the anoxic roots.
To combat the build-up of these chemicals, the root systems of some highly productive wetland plants are able to store compounds in their non-toxic forms. Some plants may also lower their metabolic rate to reduce the production of chemicals. As soon as a dry spell occurs, the chemicals may be released into surrounding substrates.
9) Capacity to trap water
The presence of considerably large colonies of peat moss creates a bog. These photosynthetic mosses are able to store rainwater. Their thick carpets accumulate large amounts of moisture and are able to provide hydrogen ions for their surrounding nutrient-poor soils. Sphagnum mosses tend to dominate moist substrates around ponds, swamps, and rain-exposed cliffs. These create their own habitats, which are known for being some of the best natural media for seed germination.
10) Seed dormancy
Many of the world’s most fragile seasonal wetlands have alternating wet and dry conditions. These are found in inland areas where rainfall events can be unpredictable and have varied trends from year to year. The waterbodies that form in these wetlands are often classified as vernal pools. These have an incredibly unique set of plants, some of which can survive through floods and droughts. Their capacity to re-colonize areas that have once dried out completely is largely due to seed dormancy. This is why, as if by magic, many of Africa’s seasonal wetlands can instantly be filled with productive greens after a few generous rain showers.
The seed bank of a wetland system is one of its most crucial components. This consists of an aggregation of ungerminated seeds in numbers that are large enough to replace entire colonies of their mature counterparts. These dormant seeds, protected by their tough hulls, lie in wait for the right combination of environmental cues. By remaining dormant through dry periods, they are able to save their nutritional resources for when plant survival rates are highest.
11) Carnivory
Many of the world’s most wondrous carnivorous plants thrive best in wetland environments. Said to be an adaptation in response to the low-nutrient and oxygen-poor profile of waterlogged substrates, carnivory provides plants with the opportunity to rely on insects and aquatic invertebrates as a source of important nutrients. Outside of waterlogged soils, many carnivorous plants are able to obtain a fair amount of their needs through their roots.
In studies that have looked into the relationship of carnivorous plants with bogs, results show that their occurrence is significantly correlated to the moisture levels of microhabitats. They are more likely to be present around the wettest substrates. This goes to show that carnivory may definitely give them a competitive advantage in wetlands, whereas it may be more disadvantageous (in terms of energy allocation) in dry, nutrient-rich soils.
Adaptations of Plants in Tidal Wetlands
Tidal wetlands, which may pose even harsher pressures on plants due to their salinity levels and wave action, are inhospitable to many inland aquatic plants. Though they may possess adaptations for fluctuating water levels, these plants are seldom able to survive without adaptations for minimizing salt concentrations in their tissues. Mangroves, some of the most highly-evolved coastal plants, have multiple ways of tolerating high salinities.
Apart from having incredibly sturdy adventitious root systems, which prop them up and keep them anchored in energetic waters, mangroves are able to either concentrate salts in their leaves or exclude them via root filtration. The red mangrove (Rhizophora mangle) is a prime example of a salt excluder, whereas white and black mangroves (Laguncularia racemosa and Avicennia germinans) excrete salts by dropping their leaves or extruding them through the leaf surface.
Coastal wetland plants also have a variety of reproductive adaptations. Unlike many terrestrial plants, which drop their seeds as soon as their casings have developed, mangroves remain attached to their propagules until they have germinated. This form of viviparity increases the chances of seedling survival. It facilitates their successful dispersal and settlement along extensive lengths of coastline.
