Salt & Soaking Adaptations
Mangroves collectively possess a unique combination of morphological and physiological attributes for living in the tidal environment. Mangrove soils are regularly water-logged and loaded with salt. High tides bring marine aquatic and estuarine conditions, while low tides expose mud and roots to aridity, heat and desiccation. Few other natural habitats are subject to such dramatic changes in abiotic variables - particularly regards the combined influences of tides, rainfall, runoff, waves, currents, climate and sea level change.
All these factors have notable and profound influences on the distribution and characteristics of Australia’s mangroves. Adaptations required for survival in this environment are shared in part by plants from at least three other habitat types, namely: deserts, rainforests and freshwater swamps.
Living with Salt
Mangroves, like desert plants, have special adaptations for growing in low moisture and high salt conditions. In estuarine and coastal environments, salinity levels of interstitial and flooding waters are often widely variable being affected by tidal fluctuations, seasonal rainfall and river flows.
Over time, a combination of strategies have shaped the diversity and character of the plants. One strategy for coping with high concentrations of salt is special salt-excreting glands on leaves. Another is the exclusion of salt from entering roots with water uptake. In general, each species has mechanisms to regulate levels of salt in sap. This is further assisted by ridding salt with old leaves, bark and wood.
Left - salt, encrusted on leaves of Aegiceras corniculatum, is expelled from salt glands in the leaves.
- Fallen leaves, wood loss and shedding foliage are ways to rid plants of salt.
- The deciduous habit of one species, Xylocarpus moluccensis, may be an extreme strategy for this species to expel salt.
Support in Soggy Soils
Mangroves have shallow root penetration and breathing roots because their soils are usually saturated and airless. Mangroves also have broad support structures, such as buttresses and sturdy prop roots, because the soils are often soft and unconsolidated. The features of shallow depth and support structures are well-known in tropical rainforests. Like freshwater swamp trees, mangroves also cope with water-saturated soils that limit gaseous exchange, by using special breathing roots. Some have shallow, sub-surface cable roots with many vertical, finger-like breathing roots, called pneumatophores. Other groups have no obvious physical adaptations except for numerous small air-breathing lenticels on their lower stems.
Right - flooding waters from tides and seasonal rains are a common feature of mangrove habitats.
- Pustular lenticels on the bark of Bruguiera gymnorhiza help the plant breath and exchange gases where roots grow in anaerobic sediments.
- The Buttresses of Heritiera littoralis provide support for stem and foliage in soft and soggy mud.