Sunday, 1 June 2014

Unexpected Survivors

In the mouth of Richters Creek is a stand of battered mangroves that is directly exposed to the Coral Sea. Waves more than half a metre high persistently slam into the stand whenever the trade winds blow.  For twenty years, I have watched this stand defy the sea and although it is built on soft mud it has barely retreated.  An engineer has solved this mystery, apparently the ability of structure to survive destructive waves is related to the forth power of the empty space the structure contains.  Emptiness is the key to the strength.  Modern seawalls are built in this principle but a mangrove swamp on soft mud riddled with crab holes provide its best demonstration.  The weakness of a solid mass is also demonstrated by this curious stand of mangroves.

Rhizophora swamp

This season, the stand was also tortured by two cyclones.   Whilst the mangroves stand their ground, the beach and foredune that had built up within the stand was washed away.  In the background of the photo below is sand cliff two metres high.  Roots that were buried and are now exposed but remain brown up to the former sand level where the bark turns grey.  Beach creepers that grew on the sand are draped over the roots.
Rhizophora roots.

There is even a Pongamia tree, which is a rainforest tree rather than a mangrove, that grew when the mangroves were previously filled with a sandy foredune.  By the time the previous foredune had washed away, the tree had developed long roots that ran back into the hind dune.  When I first saw this tree, the foredune was gone and the tree was suspended by only by the tangled stilt roots of the mangroves and it was perfectly healthy.  A mature rainforest tree growing suspended in empty space by mangroves!  Conduit-like roots that ran across tops of the stilt roots for as much as 30 m and kept the tree alive.  With the attack of the cyclones, the mangroves that supported this tree have fallen and the tree now lies exposed on the mud at the front of the stand, a single branch still alive.

Pongamia tree

The Pongamia tree is not the only unexpected survivor in this stand.  On the very exposed end of the stand was a tree that bears witness to an animal that I have never seen in the mangroves, the Striped Possum.  Striped possums eat beetle larvae that feed on the timber or dead and dying trees, leaving deep pits where they have chewed into the trunk to catch their prey.  In this case, the trees at the front of the stand which are succumbing to the relentless punishment of the sea.

Striped possum sign

Striped possum damage

Survivors such as these are interesting but they are also important.  Following the 2004 tsunami, the importance of mangroves as a sea defense is widely recognised.  As sea-level rises, we will need trees that can survive marine incursions as many developed areas and natural areas are not defensible.  And important native animals such as stripped possums - Australia's most intelligent marsupial - can show up in the most unexpected places. 

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