Coastal mud flats throughout the tropics carry a vegetation of mangrove swamp, which is particularly luxuriant in the wet tropics, such as Cairns itself. These evergreen trees and shrubs share similar habitat preferences, and a similar appearance to the untrained eye. This report will explain my findings in this biological topic, and describe the adaptations the mangroves have developed in order to survive the harsh location of estuaries and wetlands; the typical layout of mangrove communities through an original transect; representative specimen samples and descriptions; and also the typical state of the water and thus the environment through regular water samples.
The estuaries, salt marshes, swamps and wetlands mangroves live in represent hostile environments to mainland vegetation and have forced upon mangrove evolution a few peculiar adaptations such as tolerance of salty water, the development of pneumatophores (“breathing roots”), and a marked tendency to germinate their seeds on the tree. These features have consequently enabled mangroves to dominate the muddy coasts, leaving few competitors and only a small percentage of non-mangroves surviving in the harsh conditions, – most of the non-mangroves, as you will find, being part of the myrtle family.
Because of salt water and oxygen-deficient soil, many mangrove trees have roots growing above the ground. The names of these roots are: prop roots, buttress roots, pneumatophores or knee roots, and surface roots. These project above the mud and have small openings (lenticels) through which air penetrates and oxygen is absorbed through the spongy tissue to the roots beneath.
Estuaries and swamps impose very high water salinity. In fact salinity percentages can stand on average 3%’s – as high as seawater! For this purpose, mangroves have acquired three ways to survive with high and variable quantities of salt in their environment. Some trees absorb the salt then secrete it through a gland at the indented apex of the midrib. The Lumnitzera Racemosa is only an example. Other mangroves absorb the salt, then direct it to old leaves or bark that are soon to be lost from the plant, while others actually exclude salt entirely from their tissues by filtering all input in the roots. The sap of these types of species is therefore almost of the same salinity as common mainland plants. Examples of these organisms are the Ceriops Tagal and the Rhizophora Stylosa or Red Mangrove, part of the very common Rhizophoraceae family.
Another particular adaptation of many mangroves is seed and fruit dispersal by water. The swamp plants have waterborne seeds, which are buoyant being enclosed in air-containing fruits, like all species of Rhizophoraceae. For some, the seeds are dispersed by water, making the buoyancy of the fruit crucial. They lie horizontal in seawater, and on reaching brackish water, turn vertically – roots down and buds up. The fruit then lodges itself in the mud along the trip, and quickly produce additional roots, and begin to grow as a shoot. But in other cases, the seed produces roots while still attached to the parent tree. This long embryonic root emerges from the seed and grows rapidly downward, still hanging from the parent. This ensures that when the seed falls, the root is in the correct position to be driven into the mud, and send up new shoots. This germination method is called viviparity, or more accurately, viviparous seedling.
As observed in the table opposing, salinity records are extremely high, ranging from 3.3% to 3.65%. This causes the mangroves to have trouble regulating water/salt input, although they are better adapted to the conditions than non-mangroves and mainland plants. The average pH level is 6.25, which remains fairly stable. Average salt density is 1.023 grams/cm3, and is just as representative as the salinity percentages.
The most common mangrove trees in the estuaries visited in personal and school time trips were as following, in decreasing order of estimated frequency:
- Rhizophora stylosa
- Grey Mangrove (Avicennia marina)
- Orange Mangrove (Bruguiera gymnorrhiza)
- Lumnitzera racemosa
- Milky Mangrove (Excoecaria agallocha)
- Myrtle Mangrove (Osbornia octodonta)
Amongst others, these species will be presented and analysed on the following pages, giving specific labelling, descriptions, and adaptation discussion for each specimen.
- Leaf description: The leaves are alternate, simple, oval, and shorter than 10 cm; they have a shiny surface and a pointed tip. Some can have slightly toothed edges.
- Stem description: round, brown, flexible; with brown, rough bark.
- Sap description: Poisonous white sticky latex is exuded from broken surfaces.
- Root type: surface root.
- Flower type and description: Male inflorescence hangs narrow and furry, while the female inflorescence is green, beady, 1-4 cm long.
- Fruit description: three-lobed fleshy capsules, green to brown.
- Seedling type: viviparous seedling.
This species of mangrove has developed horizontal rows of corky-brown air pores on the trunk, which are called lenticels. These aid oxygen input by allowing air to enter the plant tissues on a higher part of tree, that has less risks of clogging by soil or oils, than the roots. Indeed, the root system used by this mangrove is the surface root, which only allows oxygen to penetrate at low tide, when the roots are revealed to the air, if they are even emerged from the mud.
On a reproduction point of view, the seeds are live and enclosed in three-lobed shells that contain air, and are therefore buoyant. The shell is then carried away with the incoming tide and under the factors of water salinity and temperature, will deteriorate more or less quickly, until the buds anchor onto an ideal shore.
This particular mangrove also secretes a blinding poison from any injured surfaces to protect itself from external aggressors. This sap is irritating to the skin, and some derivations of the genus can even cause blisters. However, this defence mechanism is only effective against aggressors once they have learnt a lesson.
- Leaf description: The leaves are a waxy yellow-green, shorter than 8 cm, opposite, simple, oval; they have a pointed apex, and occur in clusters at the end of the branch.
- Stem: average.
- Root type: buttress root, and knee roots.
- Flower description: small green to brown buds with five green petals.
- Fruit description: small viviparous fruit enclosed in small, brown, beady capsules.
- Seedling type: viviparous seedling.
Just like the Bruguiera and the Rhizophora, this species of mangrove uses a different type of viviparous germination from the Excoecaria agallocha.
Embryonic roots emerge from the seeds and grow downward. When the seeds fall, they are driven into the mud, and send up new shoots.
To prevent burning, the leaves are often orientated straight up in the air to avoid strong midday sunlight.
- Leaf description: 2-3cm long, opposite, simple.
- Stem: red tinge to the base of the leaves, grey and stringy bark.
- Root type: surface roots.
- Flower description: roundish white flowers with a red nucleus, usually 1cm wide.
- Fruit description: a bit larger than the flower, similar shape.
- Seedling type: non-viviparous.
Being both of the Myrtaceae family the Myrtle mangrove and the eucalyptus tree share the very same fresh smell when the leaves are crushed.
The seeds of the Myrtle mangrove are not viviparous, so the species cannot spread to a vast surface area using the water-dispersed viviparous propagules as mentioned earlier.
Interestingly, however, the crushed leaves can be rubbed on the skin as an insect repellent!
- Leaf description: Exceptionally, the leaves of this mangrove are fronds, and are known to reach up to 2m long! The smaller leaflets, though, are up to 15cm long. Leaves are alternate.
- Stem description: yellowish-green, supple.
- Root type: ‘normal’ fibrous root.
- Flower type and description: Ferns do not produce flowers, but have spores on the underside of the leaf instead.
- Fruit description: Ferns do not produce fruit either.
The Mangrove fern is the only fern that inhabits the mangal forest floor. In fact, all other ferns found in the mangroves are epiphytes (see definition chapter).
Characteristically of ferns, this species does not have any flowers or fruit, but spores instead.
- Leaf description: simple, opposite, usually 10cm long or more, the
undersides are of a lighter green dotted with brown speckles. They are gathered in clusters at the end of the branch.
- Bark description: rough, brown, and red when scraped (thus the common name Red Mangrove)
- Root type: prop roots or ‘stilt roots’ (thus the name Spider Mangrove)
- Flower type and description: small white, four-lobed flowers that are pollinated by wind or insects.
- Fruit description: viviparous propagules, generally measuring 30cm or up to 40cm.
- Seedling type: viviparous seedling.
The Rhizophora stylosa has also developed lenticels on the trunk, just like the Excoecaria agallocha. This helps to allow air into the plant when the prop roots are submerged at high tide or in the anaerobic mud.
The Red mangrove’s reproductive mechanism is viviparous, which represents a strong advantage compared to the majority of mangroves presented in this report.
- Leaf description: simple, opposite, green, epileptic leaves of roughly 10cm, with a silver underside, and salt secreting glands.
- Bark description: dull, grey-white bark (thus the common name Grey mangrove and White mangrove)
- Root type: pneumatophores.
- Flower type and description: tiny golden, four-petalled flowers.
- Fruit description: The fruits measure from 1 to 2cm in length, are green, and faintly furry.
·Seedling type: non-viviparous.
Pneumatophores are air vessels; a type of mangrove root that grow above the substratum or mud, and perpendicular to the main root underground. They transport oxygen, which is vital, into the tissues of the plant. But pneumatophores are just as susceptible to clogging as other pores in the other types of roots when the tide is high, or when parasites feed into the pores of the root.
- Leaf description: simple, opposite, large leaves (10 to 20cm), with a slightly curled side and a pointed tip, occurring in clumps at the end of branches.
- Bark description: The Bark is of a very dark brown, with a coarse, rough texture.
- Root type: knee roots.
- Flower type and description: 8 to 16-lobed red flowers, approximately 4cm long and 3cm diameter.
- Fruit description: green, wide viviparous propagules, average 15cm long.
- Seedling type: viviparous seedling.
The viviparity of this species is surely an advantage, and obviously a way to adjust to the harsh environment the travelling seeds would otherwise perish in. Indeed, the propagules are nourished on the tree, accumulating the carbohydrates and other compounds required for later independent growth. The pre-germinated seeds benefit from the achievement of a structural complexity that will help it acclimate to extreme physical conditions that might otherwise prevent normal seed germination. Any other mangrove that uses this reproductive mechanism benefits from the same advantages.
Knee roots are simply another type of root channels that emerge from the substratum to absorb oxygen through lenticels.
- Leaf description: simple and alternate. Small (up to 7cm), fleshy, spatulate. Indentation at the apex of the leaf, where a salt-secreting gland is visible.
- Bark description: grey, fissured.
- Root type: small knee roots.
- Flower type and description: small white flowers (~2cm).
- Fruit description: green, capsule-shaped fruit.
- Seedling type: non-viviparous.
One way the Black mangrove has adapted to the brackish water is by using a particular salt mechanism: excretion. (See pages 1-2)
The leaves are very fleshy and rigid to feel. This is because the plant stores more moisture in the leaves than ordinary mangroves.
Aerial root – roots grow from high branches downwards, sometimes without reaching the ground.
Buttress root – close, tightly curved roots dividing from the trunk.
Compound leaf – The blade is divided into two or more distinct leaflets.
Epiphyte – a plant that uses another plant (usually a tree) for support but not for water or nutrients. Some are able to trap their food and water in their leaves and roots.
Knee root – root emerging suddenly from the ground, in a triangular shape.
Lenticel – is a loosely packed mass of cells in the bark of a woody plant, visible on the surface of a stem as a powdery spot, through which gaseous exchange occurs. These are inactive during high tide.
Lichen – an organism consisting of a fungus and an algae or bacterium living in symbiotic association. Lichens can be crust-like, leafy, scaly, or shrubby in form.
(Many lichens are extremely sensitive to atmospheric pollution and have been used as pollution indicators!)
Pneumatophore – is an air vessel; a type of mangrove root that grows above the substratum or mud, and is perpendicular to the main root underground. It transports oxygen, which is vital, into the tissues of the plant.
Prop root – straight roots often well visible from the ground, dividing from the trunk to the ground
Simple leaf – not divided into leaflets.
Viviparous seedlings – are seeds that sprout while still attached to the parent tree. Viviparity and the long-lived propagules allow the mangroves to spread over wide areas. It is a way to adjust to the harsh environment the travelling seeds would otherwise perish in. Indeed, the propagules are nourished on the tree, accumulating the carbohydrates and other compounds required for later autonomous growth. These pre-germinated seeds benefit from the achievement of a structural complexity that will help it acclimate to extreme physical conditions that might otherwise prevent normal seed germination.
- Dominant species: Non-mangroves (26%)
- Dominant mangrove species: Orange mangrove (21.7%)
- Tallest species: Eucalyptus – non-mangrove (~11.5m)
- Tallest mangrove species: Orange mangrove (~10m)
- Tallest species by average heights: Rhizophora Stylosa (9m), Orange mangrove (8.8m), Black mangrove (8m)
- Highest substrate scale: 3.5 (sink mid calf / knee)
- Lowest substrate scale: 1 (firm)
- Estimated average substrate scale: ~1.5 (soft to walk on)
- Highest light scale: 5 (most open area)
- Lowest light scale: 1 (mud completely shaded)
- Estimated average light scale: 3 (dappled patches of light)
Approximate Species percentages Pie Graph
The transect is a linear cut through the mangroves directly from the shore to the woodlands. This transect was taken at Trinity Park Estuary, and can obviously not be drawn as an exemplary model for the standard zonation of mangroves. But the transect is, however, accurate and a representative sample of the mangal region it was taken in.
The most common mangrove species of this region is the Orange mangrove or the Bruguiera Gymnorrhiza, representing over 21% of the mangal population. The height of the mangroves ranged from 0.75m to 10m for the Mangrove fern Acrostichum Speciosum to the Bruguiera gymnorrhiza respectively. The average height of the mangroves was 6.4m.
Logically, the most open area within the transect was at a regrowth area (2.5m to 7.5m distance from shore) where very small viviparous seedlings were making their way through the anaerobic mud. This patch of mud was probably caused by regular incoming tides washing away young seedlings,
making it difficult to cover itself with vegetation. Also, the regular marching of visiting fishermen and excursion students surely didn’t help…
Mangrove swamps used to be regarded as smelly, muddy, mosquito-filled crocodile environments. They were avoided and many were replaced by rubbish tips or drowned in soil for new construction sites. The result is that today, mangrove swamps are lacking. They have finally been recognised as crucial to the ecosystem they represent, and are valued for primary resources in exportation.
Ironically, us excursion students were visiting the mangroves to learn about the mangal environment and the way these plants work, although we were also, as a group, contributing to damaging the site by stepping on young growths, ripping samples from the trees etc. The damage caused to the plants was trivial but symbolic. It is important that the new generation learns about the fragility and importance of mangroves throughout the world, for their many significant purposes. They are in fact vanishing in many countries, while it has been learnt over the years – and still should be taught – to value these places instead of taking them for granted.
Cite this essay
Mangrove Biodiversity Analysis Report. (2016, Jun 21). Retrieved from https://studymoose.com/mangrove-biodiversity-analysis-report-essay