Mangroves are what we call the collection of salt tolerant plants that are found along coastal areas and up rivers in the tropics and subtropics. There are 34 species of mangroves in Queensland with a total of only 69 species worldwide. Boardwalks have been installed at several locations in the Wet Tropics area and this makes a wander through the mangrove forest enjoyable and easy - but don't forget your insect repellent!
These salt tolerant plants have adapted for where they live. Each high tide, the sea floods their roots and trunks. The mangroves have a number of different systems to handle all this salt:
their roots can prevent its absorption by filtering it out
they can concentrate the salt in older leaves which fall off, taking their accumulated salt with them
some species have salt glands which actually excrete the salt onto the surface of the leaves where it is washed away by the rain.
Some species of mangroves are more salt tolerant than others and, because of this, there are distinct zones in a mangrove forest where the boundaries between species can easily be seen. The less salt tolerant trees are actually on the seaward side as they will be frequently washed by seawater. Salt pans are uncommon in the Wet Tropics but can be found in extremely saline situations. Salt meadows of salt tolerant grasses and fleshy herbs occur in small patches among mangroves on higher and drier areas inundated only by king tides. Small depressions form salt scalds after the sea water evaporates. Salt pans, salt meadows and salt scalds are more extensive in the drier northern and southern extremities of the Wet Tropics.
Mangroves actually enhance their own environment, in a way. The root systems are designed to trap silt - the more silt builds up, the more mangroves can grow, and trap more silt and make more muddy areas for more mangroves. But mangroves have had to adapt to all this mud. In terrestrial plants, the soil gets soaked from rain and then dries out, allowing air to reach the roots. This doesn't happen with mangroves as there is little to no oxygen available in the heavy mud, so these plants have adapted their roots to be able to get oxygen without extracting it from the mud. Their roots grow up out of the mud so that oxygen is accessed straight from the air. Many of the root types are distinctive to the species of mangrove so the plant's genus can be identified sometimes by the root type alone.
Some common root types (the roots that grow up out of the mud are called pneumatophores) are:
Reproduction
Mangrove reproduction has also adapted to be successful in a salt water environment. All mangroves flower but some don't produce seeds which fall off like other plants but rather 'live plants'.
The fertilised seed develops into a seedling while still attached to the flower. The seedling is merely a long, cigar shaped 'stem' (called a propagule) and this grows for up to a year on the tree before it is ready to find a place of its own to grow. When it reaches about 20 cm (8 inches), depending on the species, it drops off and is carried by the tide. These seedlings are often washed up onto tropical beaches. If the seedling gets carried into brackish water shallows (part fresh, part salt water) and is lodged into a muddy bottom, roots are quickly sent out to take hold in the soil and the stem grows upward and produces leaves.
Of course, other species of mangrove do produce seeds which drop off and float in the water until they reach a brackish water area and their seed coat breaks away, allowing the seed to start a shoot.
The mangrove forest provides shelter and food for a wide range of animals, especially invertebrates and juvenile marine species. For example:
the algae that collects on the surface of aerial roots (pneumatophores) is food for snails and crustaceans
the leaves that drop off the trees are taken into crab burrows to stimulate the growth of algae eaten by the crabs
the flooded roots provide protection for juvenile fish, especially Great Barrier Reef species and commercial fish
the snails and juvenile fish are food for wading and water birds
the lack of large, wandering animals creates a sheltered area for a myriad of spiders and flying-foxes
the mud is the perfect medium to create burrows in for crabs and snapping shrimp
the trunks and roots which are regularly submerged by tides are a good place for bivalves (such as oysters and mussels) to attach
Mangroves are a living buffer between the land and the sea. The dense silt amassed by their root systems prevents erosion from their landward side while it also minimises erosion from wave activity on the seaward side. The forest itself bears the brunt of storm activity, allowing the coast behind it to remain protected.
So the next time you see a stretch of mangroves and think that they are boring - take a closer look. There's lots of life in there!
Mangrove distribution and diversity
Duke and Larkum, 2008:
"Mangroves are a diverse group of predominantly tropical trees and shrubs growing in the upper half of the intertidal zone of coastal areas worldwide. They are well known for their morphological and physiological adaptations for life coping with salt, saturated soils and regular tidal inundation, notably with specialised attributes like: exposed breathing roots above ground, extra stem support structures, salt-excreting leaves, low water potentials and high intracellular salt concentrations to maintain favorable water relations in saline environments and viviparous water-dispersed propagules. Mangroves are often mistakenly thought of as a single entity. But, like coral reefs, mangroves are as functionally diverse and complex as the range of species, variants and morphotypes present at particular locations. Also, like coral reefs, they provide essential structure and habitat for a host of marine and intertidal species, including residents including residents among their dense forests and complex roots, and as visitors with each flooding tide (Box16.1, Fig. 16.2).
Mangroves are also analogous in function to tropical rainforests, providing comparable canopy habitat for birds, mammals and insects. This overlap is reinforced by ancestral links between these plant habitats. But, despite the shared features, mangroves include specialist attributes and dedicated resident biota found no where else. Examples include specialist mangrove forms of the robin, mistletoebird, mistletoes, grapsidcrabs, molluscs, herbivorous insects, and numerous floral visitors."
