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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Sepilok; an oasis in the oil palms #3

Over the past two postings I've introduced the lovely and forested enclave of Sepilok in eastern Sabah, in Malaysian Borneo. You can find the first one here. Today I finish by visiting the side-by-side Orangutan Rehabilitation Centre and Sun Bear Conservation Centre which perform valuable conservation and animal welfare roles, but which also contain a wealth of wildlife in the forests that encompass them.

Feeding platform in the forest; the ranger does not directly interact with the animals,
which are being encouraged to leave the rehab centre for the forest.

Every year many young Orangutans are orphaned – through accidents or through the illegal pet trade. The role of the rehabilitation centre (which has been operating for 50 years) is not only to save them from a lonely unpleasant life, but to teach them how to be wild Orangs again, in order to return them to the wild. Visitors can see the stages of this, including now-adult ‘rescue Orangs’ coming back in from the forest for an occasional free feed, sometimes bringing wild-born young. The food supplied, mostly bananas, is designed to offer emergency support, but is too bland and limited to encourage dependency. Indeed the feeding stations are intended to lead animals to the reserve, not out of it. The Nature Reserve within which the centre is located has only 4,000 of primary rainforest but it supports a healthy Orangutan population. Males, which need big territories, are released at more distant sites, as there are already wild males in the area.

Outside exercise area for young orphan orangs, as part of their rehabilitation.

Female Bornean Orangutan with baby. She was well away from the centre,
though doubtless visits the feeding platforms from time to time.
All such babies are fathered by wild males.

The name orangutan (also written as two words, or hyphenated) means ‘forest man’ in Malay; it probably originated from a south Kalimantan dialect. We now know that the Bornean Orangutan (Pongo pygmaeus) is a separate species from the Sumatran Orangutan (P. abelii), which makes the conservation of orangs on both islands even more critical. (And as of 2017 a third species, Tapanuli orangutan P. tapanuliensis, was recognised, from southern Sumatra.) They are the most arboreal of the great apes, seldom coming to the ground – though Bornean orangs, where there are no tigers, do so more than the Sumatran species. They are also the most solitary, living alone except while youngsters are still with their mother; they will however feed amiably in the same fruiting tree while food is abundant. They are essentially fruit-eaters, though they will supplement their diet with other vegetable matter on occasions. Orangutans are essential vectors of the fruit of various forest plant species.

Young free-living orangutan using the forest boardwalk railings as a route through the forest.

Males hold a large territory, within which are the territories of several females. Males don’t mature reproductively until they are at least 15 years old, though that may be delayed even further if there is a dominant resident male in the area. When they do mature it happens quickly, with the development of cheek pads and throat pouches. Females also do not begin breeding until they are 14 or 15 years old, and remarkably, tend to wait some eight years between births. Babies are entirely helpless for the first two years of their life, needing to be carried and fed. After that they begin to climb but don’t wean until they are four. 

Bornean Orangutans are listed as Endangered on the IUCN Red List (Sumatran and Tapanuli Orangutans are Critically Endangered). Loss and fragmentation of forests (by logging, clearing for agriculture, especially oil palms, mass burning and road building) are important causes of decline, along with illegal hunting, for bush meat, crop protection, traditional medicine and the pet trade.

The Sun Bear Conservation Centre is a rehabilitation and research centre, only opened to the public in 2014 though founded earlier, for young bears rescued from illegal captivity. They live in a very large fenced-off area of rainforest, where they learn to forage and interact – it can be quite uplifting to see them playing and generally being real bears. The first release, which was successful, took place in 2015; local release is not possible as they require a large foraging range – up to 15 square kilometres for a male.

Young Sun Bears playing in their forest enclosure.

Sun Bears Helarctos malayanus are small bears (a big male weighs no more than 80kg), living in rainforests of south-east Asia from Bangladesh to Sumatra and Borneo, though their range is now much fragmented. They climb well and often, seeking insects (especially termites, ants and beetle larvae), and honey, which they extract from hollows with powerful jaws and a long tongue, and a lot of fruit. Young bears stay with their mothers, suckling, for 18 months. They are threatened by deforestation and widespread illegal hunting, including for the pet trade. The Sepilok centre is an important hub of Sun Bear research.

We were told that this was the first time this young bear had climbed this big stump,
a significant accomplishment.

A covered shelter looks out over the enclosure, and continues as a raised boardwalk, providing views of other wildlife as well as bears.

A tough old male Long-tailed Macaque Macaca fascicularis, who was in no doubt as to who owned the railings!

Another much more demure occupant of the railing was also very special.

Paradise Tree Snake Chrysopelea paradisi; just after this photo was taken I was astonished when the little snake
launched itself into the air and glided elegantly to the ground. These 'flying' snakes glide on flattened
rib cages, covering up to 100 metres from high in the canopy.

Little Spiderhunter Arachnothera longirostra. This remarkably long-billed member of the sunbird family
does hunt spiders, but it is probably more reliant on nectar.

The best wildlife watching however is along the kilometres of walking tracks in the forest at the rehabilitation centre.

Charlotte's Bulbul Iole charlottae; this is a Bornean endemic, split from the more widespread
Buff-vented Bulbul I. crypta.

Raffles's Malkoa Rhinortha chlorophaea male; pity about his modesty with regard to the leaf!
The malkohas are a group of large non-parasitic tropical cuckoos.

Bushy-crested Hornbills Anorrhinus galeritus. A slightly odd hornbill, with no close relations,
highly sociable and very noisy!

Rufous Woodpecker Micropternus brachyurus, another single-species genus.
An ant-eating specialist, it is widespread in southern Asia.

Prevost's Squirrel Callosciurus prevostii; this north-eastern Borneo black form
lacks the striking white sides seen elsewhere in its range.

Plain Pygmy Squirrel Exilisciurus exilis, a Bornean endemic and one of the world's smallest squirrels,
with a combined head and body length of some 12cm. This little delight foraged through the
undergrowth just off the track for some time.

Striped Tree Skink Dasia vittata, another Borneo special.

Wagler's Pit Viper Tropidolaemus wagleri, an entirely arboreal mostly nocturnal species which is highly
variable in colour. Even when hunting however they are sit-and-wait predators of birds, mice and lizards.

Probably a butterfly of the family Nymphalidae - thanks Susan!

Borneo Birdwing Troides andromache; this magnificent big butterfly is restricted to Borneo.

If you go to Sandarkan for instance, you'll probably go to Sepilok and the rehabilitation centres; don't forget to stroll further along the tracks though, you're bound to be surprised and rewarded.

NEXT POSTING THURSDAY 26 JULY

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