Mangroves in the Sky

Evolution of mangroves interests me and in this blog I have often written about the specific situations that may give rise to new mangrove species.  I am fairly convinced that mangrove soils are just as big a barrier to mangrove evolution as salinity.  The US Government actually maintains a database of salinity resistant plants with several thousand species in it so why don't these species dominate our shores?  What if plants could adapt to mangrove soils first and later learn to live in the intertidal zone where they are exposed to high levels of salinity?  Evolving in two steps rather than one might greatly reduce the entry barrier to mangrove evolution.  I know of a place where this process is possible, where mangrove soils have risen out of the sea and have become mountains of mud – Gulf Province in Papua New Guinea.

Small plateau with near vertical sides are common and landslides are frequent

At 125 m above sea-level and about 50 kilometres inland, I was looking at the stream banks and realised that the patterns in the rocks were the preserved burrows in marine invertebrates, probably yabbies and crabs and worms similar to those that are found in tropical estuaries today.  In fact the rocks are not even really rocks, they are just compressed sandy marine mud and you can break the rocks with your hands.  Trapped beneath about 2 km of this marine mud is the gas field that InterOil is developing.  It is the highest pressure gas field in the world as the soft mud is not self supporting like stone but transfers its full weight onto the gas below creating the highest lithostatic pressure of any production gas field in the world. This place really is a mountain of mangrove mud.

Testing the gas flow rate at the gas field - all the other photos were taken with a few kilometres of this place.
Photo from DrillingAhead.com

On the crest of a nearby hill, soil development was less than a metre thick then almost unaltered grey marine mud

Fossil cockles could be found in some of the stream banks.  Many types of mollusc shell could be found but I could not find any crustacean body parts.  I began talking to local tribesmen to ask if they had ever seen any crocodile or sharks teeth in the ground and at that very moment, I found a megalodon tooth in the ground where we were standing.  Megalodon sharks were largest and most dangerous shark that ever lived and they lived between 1 and 25 million years ago, so I had an upper and lower limit for age of the fossils I was finding.

This looks like fresh dredge spoil but is actually sub-fossils that are millions of years old

Broken Megalodon tooth - Gulf Province PNG

Fossil marine animal burrows in river stones

Even after millenia, the mangrove mud had not lost its characteristic appearance.  Unfortunately I did not test it for salinity or hydrogen sulfide. Only a shallow band of reddish soil had developed even in the most aerated parts of the landscape.  In North Queensland, weathering has created soil profiles up to 30 m deep, even on recent volcanic lava flows so the contrast between the soils of tropical Australia and PNG is extreme.  Near the coast, there are also coral reefs that have been uplifted and are now coastal headlands.

So the gradual uplift of PNG has created a situation where marine substrates are lifted above sea-level and the character of the soils remains essentially marine for millenia.  In theory, plants could adapt to these soils over time and then be dispersed downstream to colonise the lower catchment and possibly even mangrove areas.  So are new mangroves evolving here?  I have not had the pleasure of looking the area over.  It is said to be full of pirates armed with automatic weapons so one can't just go there.  There is at least one mangrove species which is unique to this region, which suggests that it may have evolved there.  A word of caution though, rapidly rising and falling sea level over geological time has sent many mangroves species locally extinct and some mangrove species now only occur in areas away from their birth places.

The mangrove (Camptostemon schultzii) only occurs on Cape York Peninsula and in the Gulf of Papua - could it have come from the uplifted marine deposits?

Camptostemon mangroves have giant sized knob roots.

The extreme rainfall of Gulf Province, about 9 m per year prevents the soils from drying out and is probably the reason why there is mangrove mud in the mountains.  The mud has many consequences, mountain streams flow with turbid water, trees are shallow rooted and easily fall over.  When I was in the forest, a strong trade wind blew over the forest, perhaps 40 km/hr and big trees started to fall.  We had to leave the primary rainforest with the big trees and head for secondary forest which is regenerating after a landslip as the risk from big falling trees was considerable.  In north Queensland, trade winds blow for more than half the year and the big trees are unaffected.  Palms, bamboo and large lilies do well in the soft muddy soils and eternal wetness.  Only a few plants in here have mangrove relatives.  This part of PNG is perhaps too wet to provide an ideal nursery for mangroves as tolerance to drought stress is also an important mangrove characteristic.

Flood plain forest and creek

Inside the forest - most of the big trees have plank buttress roots

Secondary forest, which is where you want to be when the big trees fall.

Creek meanders often have bamboo, gingers and palms, plants that don't fall over in soft ground.

Even in the hills are signs of marine life

A creek in undisturbed, uninhabited rainforest on a hillside is completely brown with sediment and full of fallen trees

As far as I am aware, there has been no formal research into evolution of new mangroves in response to tectonic uplift.  Maybe this is something for you to research?

The coastal plains with grasslands, forests and tidal influence may not have been properly explored.

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.