Mangroves

Mangroves are highly specialized plants able to grow in salt- or brackish water, in the form of trees or shrubs. They occur worldwide at the tropics and subtropics, mainly between 25°N and 25°S of latitude, so their distribution fits with that of coral reefs.
The word “mangrove” is used to refer to the plants, but also to the habitat they create. More properly, mangal is the correct term for mangrove forest biome.
Mangroves include about 60 species of woody plants and represent an ecological rather than a natural group: this means that they are plants from different families (so, not belonging to a unique specialized one), that have evolved similar adaptations to survive to harsh coastal conditions.
These amazing adaptations are morphological, anatomical, physiological and reproductive.
Mangroves are usually divided into three categories: “red”, “black” and “white” mangroves, from the colour of the trunk. This simplistic classification (that is not always corresponding to the real colour of the species' wood) is useful to understand the zonation of mangroves, which means how the different species distribute from the coastline towards the land.
The red mangroves are the first we encounter in the subtidal and intertidal zones, respectively the zone always covered by the sea and the one covered by it at high tide, but emerged at low tide. Red mangroves like Rhizophora have typical stilt roots (prop and drop roots), majestic branched and tangled roots that have the function to stabilize the sediment and the plant on a soft bottom, characteristic of the sandy and muddy areas in which these species are able to settle. They are able to create a more stable bottom, a “buffer zone” in which the black mangroves are able to settle, thanks to the creation of a more protected environment in which the sediment deposition is promoted.
Behind the red mangroves, we find the black ones with the typical aerial roots (“snorkel” roots or pneumatophores), that promote gas exchange. These mangroves settle in the intertidal zone, thus the roots are emerged during low tide and submerged during high tide. The sediment in which black mangroves settle is highly anoxic, due to the low energy zone in which they are, protected by waves and currents thanks to the presence of the red mangroves ahead. This causes particles' sedimentation and suspension, with the creation of really compacted bottoms, in which air pockets between soil particles hardly can form. The aerial roots are able to extract oxygen directly from the atmosphere (when emerged at low tide) or from the water (submerged at high tide), ensuring the oxygenation of the root apparatus. The pneumatophores arise vertically outside the sediment from a rhizome, a horizontal long tube root that departs from the normal root system (the absorbing and anchoring roots that also normal plants have) and that lays under the bottom.
Together with these peculiar aerial roots, black mangroves like Sonneratia also have lenticels, pores on the root bark that enhance gas exchange and are critical for root survival in a hypoxic/anoxic sediment. The smell of rotten eggs typical of mangrove areas is due to the sulfur production by anaerobic bacteria living in this sediment.
White mangroves are the most terrestrial group: they have no specialized root system, since they live in a more stable sediment, even if always wet, on land and behind the red and black mangroves. Anyway, they have the same adaptations of the other mangroves and in general of the other tropical trees: salt glands and selective leaf abscission (to excrete salt and protect from UV). White mangroves like Xylocarpus often have peculiar plank roots, horizontal roots radiating from the trunk base that grow also vertically, curve and wavy in a sinuous snake-like fashion: they have the function to both improve anchorage and help in aeration.
Another relevant adaption to avoid water loss is the presence of thick, leathery and waxy leaves.
The importance of mangroves worldwide is due to their role as 3D or habitat-forming species (ecosystem engineer): they represent keystone species responsible for sediment stabilization, protection of coastline from erosion and cyclones, sites of high primary production (O2, wood,...), filter zones of land runoff. The habitat they create generates a continuum with the reefs and the seagrass, maintaining and enhancing the biodiversity, with a constant exchange of organisms and matter. Also, they link land (forest) with the sea in the intertidal zone (saltmarsh area). They also act as a nursery for reef fishes and invertebrates and as beginning point of the organic matter decomposition cycle.
In Madagascar, the mangroves constitute a particular coastal ecoregion called “Madagascar mangroves”, a mangrove forest biome included in the WWF’s Global 200 list of most outstanding ecoregions.
It is clear how fundamental is to protect and restore these unique ecosystems from threats diffused worldwide, such as mangrove deforestation (for wood and coal production), transformation into rice farms or fish, crab and shrimp ponds.