Mangroves - Living at the edge of the sea

(Photos source: AFCD's website)

Acanthus ilicifolius L. (老鼠勒)

(Spiny Bears Breech)

Features: Low, viny, woody herb, with height of 1 to 2 m.  Spiny margin leaves (entire margin when young) with salt on the surface.  Spike-patterned flowers 

Adaptations: Salt glands for salt secretion

Features: Low evergreen tree or shrub with height of 1.5 to 4 m.  Leaves are alternate, spirally arranged, simple, entire and leathery, and often notched at the tip.  Flower of white, small, fragrant, and perfect are organized in terminal (sometimes axillary) umbels on reddish stalks.

Adaptations: Knees of root above the substrate surface; Prop roots arising from trunk and lower branches to broaden the base and provide stability; Salt glands for salt secretion; Viviparous reproduction with seed germinating inside the fruit without resting stage.   

Black Mangrove

Features: Pioneer species, with size varying from a small shrub of 0.5 m to a tree of up to 4 m.  Leaves are bronze green of 3 to 10 cm long and 1 to 5 cm wide, being simple and opposite, broadly elliptic to ovate, often rounded at the apex.  Cyme-patterned flowers.

Adaptations: Pneumatophores as erected aerial roots arising from Cable roots can facilitate exchange of gases between the submerged roots and the atmosphere; Salt glands for salt secretion; White tomentum at lower leaf surface for reflecting bright light and lowering plant temperature, and hence reducing water loss.   

Features: Trees grow to 6 m or more.  Leaves are large, smooth, thick and leathery, glossy green above but often pale green or reddish beneath.  Flowers are red, solitary, up to 3.5 cm long, pedicel naked; and fruits are green to purple-tinted color, up to 15 cm. 

Adaptations: Knees of root against the anaerobic conditions; Lenticels on bark for gaseous exchange; Buttress roots forming a flattened blade-like supporting structures; Storage of salt and abscission against the high salinity; Viviparous reproduction.

(Milky Mangrove)

Features: A dioecious tree of up to 10 m high with abundant white latex.  The bark is grey and becomes fissured; lenticels are prominent on younger twigs.  Leaves are green when young and turns red and yellow tinted when mature.  They are spirally arranged, and sometimes clustered towards the ends of erect shoots.  Raceme-patterned flowers.   

Adaptations: Sunken stomata, epidermal hairs and scales, thick waxy cuticle in leaf surface to reduce evaporation and water loss.

(Looking-glass tree)

Features: A substantial evergreen monoecious tree growing up to 20 m tall with a large buttress root system extending horizontally of up to 60 cm in diameter.  It is a species of the back mangroves and may occupy the forest fringe or rocky shore.  Leave sizes are variable but large (10 - 20 cm long), stiff leathery, smooth and dark green on the upper surface but silvery white beneath due to numerous overlapping tiny scales.  Brownish red to pinkish purple flowers are organized in loose panicles in axial position, being small of 3 - 4 mm in diameter.   

Adaptation: Buttress roots as a form to prop root to produce flattened blade-like supporting structures; Large, woody fruits containing one to several seeds, which have spongy outer layers that make them extra buoyancy.  

Features: The most dominant species that can be found everywhere in all mangrove stands in HK, occurring at the back of the mangrove communities or along the banks of estuarine rivers and creeks or on the banks of tidal rivers further inland.  Leaves are green, simple, opposite of 6 - 13 cm long by 2.5 - 6 cm wide.  They are oblong-elliptic to narrowly elliptic-lanceolate with a rounded apex.  Flowers are white colored inflorescence, axillary, bifurcating with 4 or more flowers.  

Adaptations: Knees of root that bend up like an arch and grow above the substrate surface and are also rich in aerenchyma tissues which fcilitate the transfer of air; Prop roots are the branched, looping aerial roots that arise from the trunk and lower branches for broadening the base and providing stability; Viviparous reproduction with seeds germinating inside the dropper fruit without resting stage, which allows rapid growth and establishment of young plants when they are finally detached from the plant and come into contact with the substrate.

(Lumnitzera)

Features: An evergreen hermaphroditic tree growing up to 6 m high, appearing as a much-branched shrub.  

Adaptations: Development of pneumatophores as looping lateral roots to facilitate exchange of air between the submerged roots and the atmosphere; Storage of excessive salt in vacuoles or older leaves which will then be shed to reduce the salt content inside the plants and abscission.