Saturday, 6 September 2014

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.

Orange wound on Ceriops tagal
Fresh wounds on spurred mangroves are bright orange
Storm damaged mangroves
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.

Rhizophora stylosa bark wound
A red or stilt mangrove has trapped a small fallen mangrove.
Red mangrove bark stain - Ceriops tagal
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.

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