Mangroves#1: trees between sea and land

This is another in my sporadic series on favourite trees; you can find the most recent one here, and work back if you so desire. Today's post, while certainly featuring some of my favourite trees, is a bit of an aberration in this series, in that mangroves are not defined by their taxonomy but by where they live. Indeed the word refers both to a range of trees and shrubs which live their lives within the tidal zone, and to the overall habitat thus formed. I love exploring mangrove habitats; they are rich with life, and somehow a bit mysterious, hovering between the ocean and dry land.

The ultimate origin of the word 'mangrove' is uncertain - claims include the Taino language of the Caribbean, and the Guarani from Brazil. It came to English however from Spanish manglar (which probably got it from Portuguese if the Brazilian origin is correct), and was anglicised with the tree-associated 'grove' on the end.

Mangroves near Darwin; there are at least two species in this picture, living partly submerged in salt
water for half their lives.

This lifestyle requires some serious adaptations, but remarkably over 100 species throughout the tropics and subtropics have successfully evolved them. Moreover, these belong to some 20 families, each of which has independently evolved complex strategies for coping with the harsh environments. 

World distribution of mangroves; the greatest area of remaining mangroves is in Indonesia, followed by Brazil;
after that there is disagreement, with sources some listing Malaysia, Papua New Guinea and Australia next
and other saying that Australia is third. Image courtesy Wikipedia and ChandraGiri.

In Australia there are about a million hectares of mangroves, of 46 species, along some 11,000km of coastline. Queensland has the greatest area of mangroves in the country. There are mangroves more than 38 degrees south, though only one species, Grey Mangrove Avicennia marina (Family Avicenniaceae - or Acanthaceae, or Verbenaceae!) which is found in isolated populations along the south coast, eg Westernport Bay in Melbourne and at Port Adelaide. In Darwin Harbour however, and tropical Queensland including Cairns and the Daintree River (thanks John), there are some 36 species!

Darwin city over the early morning mangroves from Bayview.

One of the 36 Darwin mangrove species, Sonneratia alba White-flowered Apple Mangrove, family Lythraceae.

And another one, this time Bruguiera gymnorrhiza  Large-Leafed Orange Mangrove, Family Rhizophoraceae.

Darwin is far from the only Australian capital to boast mangroves however; in fact only Perth and Hobart (and of course Canberra!) lack them.

Grey Mangroves along the Glebe Foreshore Walk, central Sydney.
That recent discovery both surprised and delighted us.

So what are the challenges facing mangroves in their tough environment? They result of course from being regularly - twice a day in fact - inundated with sea water, but the problem is not as simple as just being flooded, though of course that carries its hazards. In fact even when the tide is out the mud is generally saturated and anaerobic, so respiration via subterranean roots is not possible. High soil salt levels are another problem, competing with the plant for water, and threatening to fatally damage internal systems. Moreover sea saline levels are as good as it gets for mangroves, as at low tides the tropical sun evaporates water from the mud, increasing salt concentrations even further. At the same time the mud heats, creating another risk factor. There is no ground fresh water available, and how can a seed germinate in such harsh soil conditions? It's a pretty daunting list of threats.

Mangroves (again at least two species) at low tide, Fraser Island, Queensland; here the mud is covered by sand.

Of course every species that has evolved to the mangrove lifestyle has met every one of those challenges - and as previously noted, every mangrove family has had to work it out for itself.

In the photo above are hundreds of little 'snorkels' protruding from the sand, extensions of roots with lots of lenticels, porous tissue at the surface to enable direct gas exchange. There is also a lot of spongy tissue internally - aerenchyma - to assist in gas transport and storage, for the times the roots are submerged. The 'snorkels' above are called pneumatophores, and have developed in many species.

Pneumatophores, East Point Mangroves, Darwin (oh, and a Common Sandpiper Actitis hypoleucos).

Another common form of aerial roots are the prop or stilt roots found in many mangrove families.

Stilt Mangroves Rhizophora stylosa, Family Rhizophoraceae, East Point, Darwin.
(There are apparently some purists who would limit the term 'mangrove' to members of this family,
or even this genus, but I see little merit in that.)

Prop roots, Cairns. It is probable that these props also help support the tree in the soft mud.

There are two other exposed root types, of which I have only very muddy photos (ie muddier than the habitat) or none, for reasons that elude me.

Knee roots, Cairns - these emerge from the mud and then re-enter it.

Finally there are ribbon roots, like rainforest tree buttresses - next time I'm in the tropics I'll rectify that image omission!

The buried mangrove roots are highly impermeable to sodium, with a surface packed with oily, corky suberins which intercept up to 97% of salt before it enters the system. Some species also secrete salt through glands in the leaves. Leaf pores are tightly controlled to minimise water loss in dry times; at other times tropical downpours are important, especially in flushing salt from soil and leaves.

One of the most fascinating mangrove stories however relates to the dispersal of seeds. A seed dropped onto the saline mud faces immense difficulties, even apart from the obvious and initial one of floating away on the first tide. I find it remarkable that such a range of plant families have come up with the solution of having seedlings germinate on the plant, so it is a developed small plant which drops into mud or water, not a vulnerable seed which needs to expose itself to salt water to germinate. In some mangroves the seedling develops enclosed by the fruit, in others the seedling grows out of the fruit while still on the tree.

Grey Mangrove fruit, Cullendulla Nature Reserve, south coast New South Wales;
in this species the seedling develops within the fruit, which eventually drops to the ground
(or perhaps is carried off by fruit bats or larger fruit-eating birds?).

An excellent example of seedlings which develop on the tree, having grown out of the fruit, is provided by genus Rhizophora, widely known as red mangroves. Here are some examples from the species R. mangle (a curious name in itself, but based on the Spanish manglar, see second paragraph), which is common in the Galápagos.

R. mangle seedlings developing on the plant, Genovesa, Galápagos.

R. mangle seedling floating upright in the sea, Santa Cruz, Galápagos.
They are weighted to float upright, ready to root in the mud. I have read the suggestion that they
travel horizontally and are able to alter their internal balance to swing upright when ready to root,
and even reverse the process if it doesn't work out that time. It's a nice story but I'd like some confirmation.
It does seem well-established that they can survive for up to a year immersed, so can cover immense
distances, which explains the huge range of some species. This one for instance is found in the tropics and subtropics
of the west coast of Africa and both coasts of the Americas.

Mangrove seedling, Daintree River, north Queensland.
The brown patches are lenticels (see discussion of roots above) which enable gas exchange.

The inland limit of the mangroves is determined by the tides and substrate, and can be strikingly abrupt on occasions.

Mangrove-fringed lagoon in arid scrubland behind Playa Espumilla, Santiago, Galápagos.

Interface between mangroves (left) and Swamp Oak Casuarina glauca,Cullendulla Nature Reserve, south coast New South Wales.

Nor are all mangroves marine, though one might question the use of the term if neither the sea nor tides are involved. However there are freshwater mangroves which spend much of their lives with their roots submerged.

Freshwater Mangrove Barringtonia acutangula, Family Lecythidaceae, Kakadu NP, Northern Territory.

Mangroves have fared poorly at our hands, being widely cleared for coastal developments - residential in Australia, industrial in many places, such as the ongoing loss of Ecuador's shrinking mangroves to prawn farms. Indeed prawn farms are regarded as responsible for 25% of ongoing mangrove destruction, according to UN figures. Another authoritative source estimates that 20% of the world's mangroves have been lost since 1980. My friend Martin informs me that in Tanzania mangroves are under threat from felling for firewood and scaffolding - the timber of at least some is very tough, presumably, as he suggests, because they are slow-growing. I'm sure this is the case elsewhere too.

A major source of outrage to me is the use of the term 'reclaimed' to describe projects converting mangrove habitat to a marina or something similar - as if it was our land that we've taken back from nature, which had stolen it from us!

It is not all bad news though. The rate of loss is falling (except in south-east Asia) and regeneration programs are being undertaken in many places. Moreover I am delighted to see how many interpretive boardwalks are now being developed, both as recognition of peoples' interest in mangroves, and as a tool to foster understanding and respect. The rest of today's post comprises scenes of and from such mangrove walks, on three continents. And next time I want to introduce some of the wildlife of mangroves!

Mangroves along tidal river, Manglares Churute NP, southern Ecuador.
Manglares is plural of manglar, ie simply 'mangroves'.

Mangroves along boardwalk on Pulau Tiga, an island off Sabah, Malaysian Borneo.

Submerged prop roots, Bako NP, Sarawak, Malaysian Borneo.

An excellent walkway deep into the mangroves, ending with a sitting platform,
at East Point, Darwin.

Mangrove boardwalk, Coffs Harbour Botanic Gardens (which will feature in a forthcoming post).

Mangroves at high tide, Cullendulla Nature Reserve, from yet another very good boardwalk,
this one on the southern edge of the popular holiday town of Batemans Bay in southern New South Wales.

And this one, Ukerabagh Boardwalk, in the otherwise unlovely conurbation of Tweed Heads in far
northern New South Wales, is one we were directed to recently; it too is well worth visiting.

One of the very best and most extensive, with a wide range of tree species, is just off the road to Cairns airport, in north Queensland

A tangle of prop roots at low tide, Cairns mangrove boardwalk.

And finally, a boat is an obvious advantage if seeking to explore mangroves; two of our favourite bird-watching boat trips in Australia also incorporate mangroves.

Mangrove seedlings and impressive prop roots in the Daintree River, north of Cairns.

Mangroves at low tide at Karumba, on the mouth of the Norman River, on the Gulf of Carpentaria,
also in tropical Queensland.

I hope that you enjoy mangroves as much as I do, and that today's post might give you ideas. And, as mentioned, next time I'll introduce some of the animals that live in them.

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