Friday, 12 December 2014

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.

rainforest landslide
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
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.

sub-fossil scallop and cockle shells
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.
Camptostemon schultzii
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 schultzii
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.

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