The Mangrove that Lives on Land

The stilt mangrove family, which has most of the world's mangroves, has even more members in the tropical rainforest.  So what do the relatives of the archetypal stilt mangrove look like?  In Australia, there is only one non-mangrove member of the family Rhizophoraceae, and it is called the forest mangrove.

Some saplings have distinctive plates of corky bark

A mature tree on in gallery forest on the high bank of the Wenlock River

Forest mangroves (Carallia brachiata) live in a wide variety of habitats.  In dry landscapes such as around Townsville and on lower Cape York Peninsula, forest mangroves live in gallery forests on the banks of streams and rivers.  In wet areas, they grow in springs and swamps and on hill sides in small patches of forest clustered around rocky outcrops.  Around Cairns, they are one of the most common plants in areas with rainforest regeneration.

Make me wet and I grow stilt roots

Perhaps the forest mangrove is like the ancestral mangrove.  Occasionally a forest mangrove will sprout stilt roots from its trunk although most of these will taper out before reaching the ground.    It has the ability to live in heavy mud and live in swamps that have prolonged flooding.  Forest mangrove foliage is very similar to stilt mangrove foliage.  The clearest difference is the small red fruit, which are most unlike the water dispersed 'droppers' of stilt mangroves.   A great variety of birds tuck into the fruit by day and bats eat them by night, making for a noisy fruiting season.  In aboriginal communities like Yarrabah, the kids also climb the trees and raid the fruit.

Fig bird eating a berry - see also the mangrove like leaves

Yellow oriole - which is actually green

A male koel, which is a very large and noisy cuckoo that lays eggs in willy-wagtail and pee wee nests.

A much smaller but similarly coloured shining starling.

Australia has five migratory fruit eaters (koels, shining starlings, Torres Strait pigeons, channel-billed cuckoos and rose-crowned fruit dove) which is more than any other continent and forest mangroves feed them all.

Where the Fig Parrots Nest

Having found some possible nesting holes of the endangered southern Queensland race of fig parrots in the mangroves, I thought that I would look around the mangroves near Cairns for fig parrot nests.  In one afternoon, I found about twenty nests so it seems that the urban-mangrove fringe is a major nesting habitat.

My favourite fig parrot has now been committed to her nesting hole for more that three months.  As she is in a very busy area, she is fairly tolerant to people and even tall vehicles brushing the foliage of the tree she lives in.  Unfortunately, she suddenly realised that I was intently watching the nest.  Being in an urban area, I am restricted as to where I can place the camera and had to go within about 4 m of the nest.  This was too much for the little bird, which jumped out of the hole and flew away.  Fortunately she later returned to the nest but after the fright, she became very cryptic and was hard to observe.  She has relaxed a bit since but I need to stay at least 6 m away.  Fig parrots living in the woods are not used to people and is better to give them at least 20 m of clearance.  If you have a flip out screen on you superzoom camera, you might use that to watch the birds to avoid the need to stare directly at the birds.

Don't trust the humans

It seems that my favourite fig parrot is not the only urban fig parrot.  There is a giant fig tree at Barlow Park in the centre of Cairns which has fig parrots in a trimmed up tree branch, a perfect demonstration that trimming trees in a certain way can create habitat for these little fellows.  The British use a technique call fracture pruning to leave some dead wood in tree crowns for their wildlife.  We could do something similar here, however fracturing the end of a dead branch would allow the centre to rot out quickly and fig parrots reject trees with hollow centres as far as I can tell.  Years ago, I strapped a heap of reject didgeridoos to the trees in my yard but nothing wanted to live in those.  Falling didgeridoos are dangerous so I connected them to the tree with fencing wire. A strand of wire would run along the top of the didgeridoo with some loops of wire wrapped around the instrument.   The other end of the wire was threaded through some poly hose and secured around a solid live branch.  This way, the didgerdoo could not become deadly.  A few super duper cable ties and some UV stable rope might also be a quick way of ensuring that dead wood can't fall until it has crumbled.

Fig parrot nest hollows in a pruned branch
- this tree was on the mangrove edge before the park was filled.

Close-up of the holes, not all holes become nests
- perhaps the wood needs to be not to hard and not too soft

My first mangrove edge fig parrot hollow was spotted on my drive to town.  It was in a dead pillar of a tree that is right beside the road verge.  In a short period of time, I found a few dozen fig parrot holes in tall stag trees.  The favourite trees are dead northern paperbark, Melaleuca leucadendra, which found at the very  edge of mangrove swamps.  In fact, they are so close to the mangroves that the small about of climate change induced sea level rise has killed many of them.  When you live on the high tide line, 3 mm per year of sea level rise over thirty years is a big deal.   I can remember all of these trees being within a strip of saltwater couch beside the mangroves.  Now the trees are in the mangroves and the wide grassy strip is gone as the mangroves have squeezed it out.  The dead melaleucas are very tall, almost 30 m tall and stand clear of the surrounding mangroves and beach vegetation so provide an exposed clear trunk.  Melaleuca wood is also much softer and less durable than eucalypt or acacia timber and I have found no nests in the later species.

Paperbarks that were born in grassland will die in the mangroves thanks to sea level rise

The massive branches of dead giants are perfect for fig parrots

There is one other factor that warrants a mention.  I have been trying to set a camera trap to catch a striped possum.  I suspect striped possums eat fig baby fig parrots if they can catch them.  Where there is striped possum sign, there are few fig parrots nests.  I suspect that fig parrots are doing best in places where pussy cats keep the striped possums out.  Fig parrot central is a peninsular like patch of habitat that juts into a Cairns suburb.  There is a strip of bush with tall paperbarks at the edges that is separated from the houses by a thin band of mangroves.  Most of the nest trees are within 50 m cat infested suburban land.  Conversely, I also occassionally find parrot nests that appear to have been enlarged which suggests that stripped possums may be using parrot nests for dens.  More research is needed.  Southern Queensland has a huge population of possums and perhaps some dead trees should be fitted with possum guards to give their endangered fig parrots a chance.

Many of the branches eventually snap at or near fig parrot nesting holes

When is a mangrove not a mangrove?

When it is a mangrove associate. Mangrove associates are plants that grow along or just below highest astronomical tide level. The term mangrove associates is used more widely used but is sort of a non-category that fails to respectfully identify this interesting group of plants.  Others have also struggled with this issue and was seems to be lacking is a good simple name for these species.  In particular, there is no good name for species which can grow in saline mud together with exclusive mangrove species.  Supra-littoral forest is the place where these plants occur. Let me call these species opportunistic mangroves.  The term mangrove associates covers a broad range of species, but is mixed grab bag of species as it includes species from all types of transition zone, including rocky mangrove coasts, sandy levees and beach ridges and within freshwater backswamps.  Let me define opportunistic mangroves as species that will take opportunities to grow below the high tide line on mangrove soils in places that experience fully saline conditions for at least one event each year.  The opportunities to grow in the mangrove edge are common but never abundant and opportunistic mangroves are always limited to small stands or scattered individuals.

Opportunistic mangroves are salinity tolerant trees and shrubs which can grow in the supra-littoral zone, which is defined as the zone above normal high tide level. The roots of these species are covered by seawater on a few occasions every year so they have to be halophytes (salt tolerant species).  To be exposed to raw seawater, the opportunistic mangroves have to be located in places with a dry season and with a large tidal range.  They also need to be in places where fresh groundwater seepage does not fill or flush the ground, protecting the trees from exposure to salt.  Often these places are very dry during prolonged dry conditions.  A final criterion would be that the trunk of the plant is located in a place with an unambiguous mangrove understorey, usually bare ground with crab holes or bare sand that is washed over by tides.

For comparison, some rivers which have freshwater baseflow all year round have freshwater tidal zones and river bank and freshwater swamp plants often dominate these places. These places are about as close as you can get to Pandora in the movie Avitar. There are several specialised mangroves species that live in tidal freshwaters but most of the plants live there are not referred to as mangroves as they can also be found in lowland rainforests other non-saline wetland habitats.  Which of these mangrove transition zones is an incubator for new mangrove species?

A tidal freshwater system near Cooktown, complete with beasts that squat in the mud and want to kill you.

Opportunistic mangroves are not restricted to growing near tidal waterways. Some can even grow on elevated inland areas as paradoxically the same adaptations that work in swamps make plants tolerant to swings between seasonal drought and waterlogging. Indeed one of the main minor supra-littoral mangroves around Cairns, Acacia oraria is common on the western flank of the Great Dividing Range.  The label opportunistic mangrove describes a capacity to grow on tidally affected land and is a functional category. Weeds are now defined as plants growing where they are not wanted. Calling a species a weed is discouraged, as a given species can be a weed in one context and a desirable plant in another. The term opportunistic mangrove should be similar, it describes plants than can grow in tidally affected areas with heavy soils and not just plants that have to grow there.

The definition of opportunistic mangroves solves a issue for me as the separation between mangrove and non-mangrove vegetation can be indistinct in areas with very flat terrain and places with moderate to high rainfall. Some species are classed as mangroves, yet other species with similar appearances and very similar habitat preferences are not. The concept of opportunistic mangrove lets me sidestep this issue.

In the Barron River Delta, near Cairns, some hectare-sized areas with opportunistic mangroves can be found on flat expanses just below highest astronomical tide level. These places have vegetation that is intermediate between mangroves and littoral rainforest. The largest supra-littoral areas are only a few tens of metres wide but may more than one hundred of metres long as they follow the tide line.  Unlike the landward zone of the mangrove swamp which has larger than normal mangrove trees, the trees in the supra-littoral mangrove area are usually short and shrubby.  Some supra-littoral areas are isolated low rises located deep in the mangroves which have no terrestrial habitat at all.  Only seasonal rainfall stops these areas from being salt pans. 

A mangrove with a light trunk surrounded by poor littoral rainforest. 

A stilt mangrove growing around a rainforest tree and an acacia (trunk at the back).

In my naval gazing about how mangroves evolved, these places seem to be one of the potential sites of crossover from a terrestrial to intertidal existence. Last week, I examined beach ridges and found that most of the species on beach ridges shrivel when exposed to seawater. Opportunistic mangroves seem to have much greater salt tolerance. Indeed the few millimetres of sea level rise that is now occurring has already greatly increased the exposure of many of these species to regular tides. Even without sea level rise, the land surface in these places is falling fast. As leaf-litter is swept away by tides and eaten by crabs, the ground is unprotected and tropical deluges spatter the surface and carry away the loosened material. Tonnes of material are lost every year, particularly in areas with overland flow. Imagine an extensive, slowly falling ground surface covered with opportunistic mangroves, would that not be an ideal environment for mangrove evolution? Each new generation of trees would encounter an environment that is slightly more marine than before. My horticultural experience tells me that the worst growing conditions of all is salty, dry and shady, with intense root competition from surrounding trees so this may not be the case. I have seen that combination around saltwater swimming pools, under stands of coconut palms and in the landward zones of mangrove swamps. The clear understorey in most of the transitional areas suggest that horticultural combination of death often applies to this zone and this may be why healthy existing opportunistic mangroves do not leave many seedlings.

Seedlings of mangroves, ixora (I. timoriense) and mock orange (Atractocarpus fitzalanii) were common in this thirsty contested zone.

Another factor may prevent trees from passing through the supra-littoral portal into the mangrove zone. One of the current theories of evolution, is known as punctuated equilibrium. This theory notes that species stay the same for long time scales and then new species rapidly evolve. A key part of this theory is the idea that small reproductively isolated populations are required, as large populations that are spread over wide areas are subject to evolutionary pressures the pull in opposing directions. A tree might be selected for seed dormancy in one part of its range and for seeds that germinate and take root as soon as they touch the ground in another. The result of these opposing pressures is that the species as a whole does not change. With some species of opportunistic mangroves, the bulk of the population lies in the terrestrial environment and this limits their ability to adapt to mangrove life. In some cases, the mangrove fringe often has small populations of plants that are very far from their terrestrial kin (Myoporum montanum, Bauhinia binatum, and Cathormion umbellatum to name a few. Perhaps genetic research will find the legendary reproductively isolated small population that is undergoing rapid change and the mangrove fringe would be one of the first places I would look.

A paperbark and a stilt mangrove are as close as old mates, surely they experience similar conditions.

A tuckeroo (front), probably Brisbane's most common street tree grows in the same saline muddy environment as a giant sized white mangrove (back).

Further Reading:

• List of well known mangrove associates
• Description of mangrove associates and broad types of mangroves

Did Mangroves Evolve from Beach Vegetation?

Mangroves are not the only trees with floating seeds that are dispersed by sea.  Plants growing in tropical sandy foredunes also have floating seeds and fruit.  The most famous of these plants is the coconut.  Despite growing side by side with mangroves, there seems to be a strong ecological barrier that prevents plants from the foredune from evolving into mangroves.  Others have noted this barrier but there is only vague speculation on what the nature of this barrier is.  In this post, I will investigate the barrier between mangroves and beach vegetation.

Strand vegetation near Cairns with taller dark green littoral rainforest behind

I refer to vegetation dominated by plants with seeds that are dispersed by sea as strand vegetation.  Wikipedia defines strand vegetation as ‘a plant community of flowering plants that form along the shore in loose sand just above the high tide line.’  It goes on to describe the key features of this habitat.  “Many plants that grow in this area are endemic to the strand. The community has low species diversity because so few plants can tolerate the harsh conditions of high winds, battering salt spray, and extreme high temperatures in the summer. Plants must also be adapted to sandy saline soils, with extremely low nutrient loads, and low water holding capacity.”  Although this definition was crafted for North American strands, the term strand vegetation is also used in the same way from Africa to Asia.  In Australia, the term foredune vegetation is often used, however I tend to associate this term with the high energy surf beaches and not the relatively calm tropical coasts.  There are several distinct vegetation communities that occur beside the high tide line, so vegetation community names should be used with care.  For example, strand forests often occur beside littoral rainforests but littoral rainforest trees are almost all bird dispersed and littoral rainforests create and grow on soils rich in humus and depend on efficient nutrient recycling.

Nature provided a natural experiment which helped me to see the barrier between mangroves and beach vegetation.  Some recent weather events effectively raised the normal high tide level by twenty or thirty centimetres and the strand vegetation was subject inundation with sea water.   Cyclone Dylan came to visit in January 2014, however Cairns being on the northern side of the system had offshore winds and the sea was quite calm save for a low swell.  The low atmospheric pressure associated with cyclones can lift the surface of the sea by approximately one metre.   In the Cairns area, the cumulative effect of the lower air pressure and high spring tides was to raise the sea by approximately 30 cm above normal (~3.6-3.8 m AHD) and the lower parts of the foredune were inundated.

High sea levels associated with Cyclone Dylan and swells gently swashing over the foredune

Some low lying coastal parks and some freshwater swamps were also inundated.  In the parks, fig trees dropped their leaves from the shock of salt water.  Freshwater swamps usually occur in basins such as old billabongs and swales.  As the seawater has trouble draining back out of the basins, many of the freshwater swamps were badly or permanently damaged with large paperbark trees and groundcover vegetation being killed.  Many of these swamps will change into mangrove swamps with time, especially as erosion of the barrier between the swamp and sea is often reduced by these events.

A park tree (Ficus lyrata) dropped its leaves due to a short period of saltwater inundation.

Seawater gets into some freshwater swamps but can't get out and kills all the vegetation.

A freshwater swamp turning into a mangrove swamp, Redden Island.

It turns out that even though strand plants live by the sea, most of them are also sensitive to saltwater inundation.  Many species drop their leaves.  In mangroves, salt is accumulated in old leaves before they are dropped.  I am not sure that this is the case with other species.

A beach almond dropping leaves after saltwater inundation.

Beach creepers were complete defoliated.

Cyclones are not the only reason plants suddenly find themselves exposed to seawater.  Sometimes erosion removes the land from around the trees.  Other times, plants become established on low lying ground and grow for months before encountering very high tides and rough seas from trade winds that put swash across the beach.  

Beach Cabbage (Scaevola taccada) with signs of salinity stress

Crinum lily damaged by exposure to seawater

Severe wilting of Cordia subcordata seeding on a sand bar in a mangrove swamp.

However not all strand trees are impacted as severely by salinity.  Some species hardly seem to notice.  Beach casuarinas are a bit special.  They drip concentrated saltwater from their leaves and are perhaps the only sandy soil specialist to easily cope with high loads of salt.  Most other species which cope with seawater inundation can also survive on riverbanks with dense silty soils and clays so I think there is a link between soil type preference and reaction to salinity.  Beach hibiscus (H. tiliaceus) , Portia trees (Thespesia populena) and Pongamia (Milletia pinnata) keep on growing until the sea physically washes them away. In one small section of swamp, I counted more than twenty living Portia trees suspended by mangrove roots.  They can almost teach mangroves tricks about living in salt water.

Beach casuarina growing at approximately neap high tide level.

A Pongamia tree survives where other trees have perished.

A leaning Portia tree perched in the mangroves well below spring high tide level.

Portia trees continue to grow and fruit despite exposure to the sea. 

In summary, salinity has dire effects on most vegetation.  However some species seem to cope easily with high levels of salinity and have seeds dispersed by sea, yet are not mangroves.  My conclusion is that most species that are specialised for growing in clean sand are likely to have adaptations that seem to make them susceptible to salinity. Mangroves are unlikely to have evolved from strand species.  A few of the species present in strand vegetation also grow around the terrestrial margins of salt flats or adjacent to the landward zones of mangrove swamps where soils are heavy silts and clays.  These species handle seawater inundation better.  I suspect that mangroves evolved in river deltas, rather that from beach species that crossed over the high tide line.

Red-blooded Mangrove Trees

Floating objects are listed as one of the hazards of mangrove life and some mangrove trees are said to have adaptations for reducing damage from being rubbed by floating objects.  The crossed grain of Avicennia timber is said to be such an adaptation.  Avicennia timber is like plywood with each layer of timber having grain that runs in a different direction to the layer below which makes the timber very tough.   Despite having heard that floating objects pose a threat to mangroves, in my decades of experience, I had never seen much in the way of damage to trees.   Then I found a wild coastline were the trees looked like they had been on the losing side of a battle.  Ripped and bleeding trees were everywhere.  A combination of very high tides, very strong trade winds and logs from a nearby river did more damage that a recent cyclone.

Fresh wounds on spurred mangroves are bright orange

A log 'torpedo' has stripped stilt roots of bark on its way into the forest.

These trees are the victims of logs, many of which come from rainforest trees that fell into a nearby river.   The trees have been rolling around on the sea bed or on the beach gradually losing their branches and roots and becoming shaped like torpedos that can penetrate many metres into mangroves where they roll around with every passing wave and do great damage.  Their streamlined shape means that they are unlikely to be trapped and they slowly work their way through the forest attacking trees as they go.   Being a retreating coastline, some of the floating timber comes from fallen mangroves but timber from this sources tends have roots and branches and is generally smaller.  Mangrove timbers tend to get trapped under roots of a living mangrove creating awful wounds to the tree that traps it.

A red or stilt mangrove has trapped a small fallen mangrove.

Stained sand at the base of a wounded mangrove

Many of the trees were standing in a patch of red, stained from rain flowing over the fresh wound and transferring the colour onto the ground.  It reminded me that mangroves were once cut and stripped of bark with the bark being used to stain and tan leather.  Even in early Australia, mangrove bark was harvested for this purpose.  Mangrove bark is recognised as one of the plant-based tanning agents.  In Africa, India and Pakistan mangroves still supply bark but the destruction of mangroves for this purpose was outlawed as much as a century ago around much of the world.  Stilt mangroves are known as red mangroves mainly due to the colour of the stain they provide.  Spurred mangroves provide the most potent stain which is used to colour and preserve fire water made from palm sap.  In parts of South East Asia, an illegal fire water (tuba) industry is depleting mangroves.   Everything in this world has value to someone which is why conservation cannot be left to chance.  Cutting mangroves is illegal in Queensland.

Small waves on an incoming tide roll the logs around striping bark from live mangroves.

Further Reading:

• Useful Products from Mangroves and Other Coastal Plants