A mangrove is a tree, shrub, palm or ground fern, generally exceeding one half metre in height, that normally grows above mean sea level in the intertidal zone of marine coastal environments and estuarine margins. A mangrove is also the tidal habitat comprising such trees and shrubs.
The word ‘mangrove’ refers to the habitat in the same way as we think of ‘rainforest’ with its mixture of plant types. Sometimes the habitat is called a ‘tidal forest’ or a ‘mangrove forest’ to distinguish it from the trees that are also called mangroves.
Mangroves and others
Mangrove plants are not a single genetic entity because the plant types represented in the tidal zone are not all closely related. So, while they sometimes look the same, and have similar function, this tells us more about the environment they live in, rather than their family relationships. The plants growing in the tidal zone also require serious adaptations for their continued survival in this habitat. However, this does not preclude other plants from occasionally being found within the tidal zone. Some are grouped as ‘associates’ where they only occasionally occur in intertidal sediments and most of the time they are found elsewhere. Others do regularly also share the tidal niche, like saltmarsh plants, but these are smaller in size. A number of others, the epiphytes and plant parasites, perch in the branches and stems of mangroves. All these plants shape and define mangrove habitat.
Left - Nypa fruticans, the mangrove palm.
- A tall stand of Rhizophora and Bruguiera trees in the Daintree River, Queensland.
- Sea edge fringing small trees of Avicennia marina surrounded by numerous pneumatophores, Phillip Island, Western Port Bay, Victoria.
Biologically Rich and Diverse
Australia’s mangroves have the fourth highest species diversity of any country, after the Philippines, Indonesia and Papua New Guinea. Most species in Australia occur widely throughout the Indo-West Pacific region. However, one species, Avicennia integra, is found only in Australia. Furthermore, a number of additional mangrove species and hybrids are more common in Australia than elsewhere in the world. Such notable biodiversity features help define the special characteristics that make Australia’s mangroves unique.
Australia’s mangroves consist of 22 genera from 19 plant families and there are 41 species. This represents 57% of mangrove species in the world. The areas of most abundance occur along the wet tropical coast of northern and eastern Australia. Many of these species furthermore come from a diverse range of plant families. And, these families belong to two major plant divisions, including one from the fern family the Polypodiophyta, and the remainder from the Magnoliophyta.
The latter are known also as the angiosperms or flowering plants. Just one family is exclusively mangrove – the Avicenniaceae Miq. in Lehm (1845). This is unusual since most families include only one or two mangroves. It is of further interest that even the Rhizophoraceae, the so-called ‘true mangrove family’, has only four of its sixteen genera represented in the tidal wetland habitat. Generally, the nominal ‘mangrove families’ are more commonly known for their upland habitat representatives especially those within tropical rainforests.
Right - a common association upstream are Acanthus ilicifolius thickets beneath an open canopy of Sonneratia caseolaris.
- A magnificient primal forest of Heritiera littoralis and Bruguiera gymnorhiza, Daintree River, Queensland.
- Shrubby bushes of Pemphis acidula inhabit coral outcrops and coral beaches of the north.