Mangroves are salt-tolerant trees, generally exceeding one and a half meters in height, that typically nurture above mean sea level in the intertidal zone of marine coastal or estuarine environments of tropical and sub-tropical coastlines. Normally, mangrove species are divided into two groups.: Exclusive and non-exclusive. The exclusive species are limited to the mangrove habitats. Non-exclusive means species mainly distributed in land or water habitats.

Distribution of Mangroves

The Mangroves are commonly located on tropical and subtropical coastlines and cover an area of 137,760 square kilometres.

Mangroves are sensitive to chilling temperatures and can survive only at the mean winter sea surface temperature of at least 20⁰ Celsius. Their limits correlate with a 24⁰ Celsius isotherm.

There are two main mangrove centres in the world: the Indo-West Pacific and the Atlantic-East Pacific. Indonesia has the highest mangrove cover, Brazil comes second, and Australia comes third.

Around 73 species of these halophytes have been recognised, and 17 species are recorded as true Mangroves. Even with similar climatic conditions, the Indo-West Pacific region shows more diversity with 63 species, and the Atlantic-East Pacific had only 19 species. Only two species, Acrostichum aureum and Rhizophora samoensis, are commonly present in both regions.

Properties Of A True Mangrove

1. They exist in intertidal zones and do not occur in terrestrial environments.
2. They play a significant role in the structure of the community.
3. Mangroves have morphological specialisations like areal rots to adapt to harsh environments.
4. They will display physiological adaptations like salt exclusion, secretion, and accumulation.
5. They will show viviparous reproduction and are taxonomically isolated from their terrestrial relative, at least at the generic level.

Origin of Mangroves

Mangroves originated along the shorelines of the ancient “Tethys Sea”. After its origin, a dispersal along the Tethys seaway westward to the Atlantic. After the Atlantic, it reached the East Pacific, then East Asia and Australia. This dispersal happened during the late “Cretaceous” or “Early Eocene” around 80 million years ago.

Due to the northward movement of Africa, a physical barrier was created, which resulted in the closure of Tehys Seaway. This closure resulted in the complete termination of exchange routes. Moreover, this closure is the reason for the difference in species richness between Indo-West Pacific and Atlantic-East Pacific. Events like the Pleistocene sea level rise and plate tectonics are responsible for the current distribution pattern.

Mangrove Forest Structure

Mangrove forests exhibit a general zonation without Understory due to salinity, flooding, and low light intensity in the mangrove habitat and the zonation is influenced by various biotic and abiotic factors.

Arid mangrove forests have high stem density, low species richness, low canopy height, and low basal area. Tropical Mangrove forests are wet, humid mangroves with high canopy height, high basal area, and low stem density.

Adaption To The Extraordinary Habitat

1. The leaves can be small or large and glossy, and salt-exciting glands are the first feature that helps them survive in saline conditions. Their leaves have a special structure called hydathodes. These are areas where the epidermis is interrupted or where an enlarged stoma or epithelium develops. Hydathodes help in the guttation or release of water droplets.
2. Ariel roots are the most striking features of Mangroves. These aerial roots help in anchorage and nutrient and gas absorption in oxygen-deficit soil conditions. Most mangrove species have a shallow cable root system that spreads away from the mangrove trunk. There are five types of root adaptations. They are Pneumatophores, Knee roots, Prop roots, Buttress roots, and Aerial roots..
3. Vivipary is the most important adaption of many mangrove species. In this case, the embryo grows while still attached to the parent plant.

Vivipary

Vivipary occurs in a broad range of habitats and species, including cacti, and it is common among mangrove trees. But not all mangroves have this vivipary. And they have Cryptovivipary.

Cryptovivipary is another condition where the embryo grows to break through the seed coat but not the fruit wall before it splits open. Cryptoviviary can be seen in species like Avicennia. Once the germinated seed drops into the soil, they continue to grow in the soil. This special mechanism helps in seed germination while living in harsh conditions.

What is Mangal?

It is a tropical vegetation type with some outliers in subtropical latitudes, notably New Zealand, South Florida, South Africa, Australia and South Japan.

“J. Macnae” introduced this term to refer to tropical, coastal swamp ecosystems.