Big Cyclone, poor mangrove recovery

 Fifteen years ago Category 5 Cyclone (~hurricane or typhoon) Yasi slammed into the coast just north of Cardwell on the Queensland coast in Australia.  The seven metre storm surge caused almost total mangrove death over many square kilometres.  I had not been back to this area in fifteen years.  The last time I saw the area, it was about 3 months after they cyclone, and I walked through a dead standing forest. This site once had a luxuriant Ceriops (yellow-leaved spurred mangrove) swamp.  Remains of the original trees are still standing and a new stand of mangroves is growing up below.  Fifteen years of regeneration have only filled in half the space, regrowth is only about 20% the height of the original trees and dead timber from fallen trees covers the ground.  In contrast, the beach vegetation, mainly littoral rainforest has totally recovered and it luxuriant.

Dead mangroves seen on previous visit fifteen years ago

Beach vegetation that took the seven metre deep surge with six metre waves

Getting into the swamp was a challenge as the landward fringe was filled with bushy mangroves that had retained strong dead twigs on their lower, now shaded stems.  One passed that barrier, I was in a mosaic of open ground with fallen trees and dense bushy mangroves.  The fallen timber was firm and whilst sun bleached, was not weakened by decomposition.  Absent were signs of insect attack or ship worms.  Stepping over the fallen timber would be easy for a young man, and the mud was wet and cracked but not deep or sticky.

Getting in is tough, lots of twigs and mosquitos

In the middle, there are wide spaces and bushy thickets

As far as I wanted to go, it was much the same

Only one type of crab was abundant, a small grapsid or sessarmid (Sesarma messor?) and these crabs ran from me across the surface when crabs usually run down crab holes.  It was strange behaviour that caught my eye.  Just after they cyclone a small dark red crab was everywhere, yesterday I saw just two of them.  Rakali tracks covered the ground and I think that they go down larger crab holes and attack the residents.  I don’t imagine that near complete predation of larger crabs is helping the recovery of the mangrove forest.  Tracks from feral pigs, large and small, indicated that these animals lived in the area and routinely crossed the swamps.  More than crocodiles, I fear encounters with feral pigs and even found a rubbing tree with scars from the pig’s tusks.  Wallaby and bandicoot tracks were also present.

Sesarma messor? Rather small for this species

Rakali scratches

A feral boar rubs against this tree

The one unexpected animal was a moth as large as a small bat.  I could feel the vibration from its wing beats as it flew past.  Fortunately it landed on a nearby tree and I could photograph it.

Despite a number of good years, with above average rainfall and no cyclones, the recovery of the mangroves has been very slow.  The regrowth mangroves are flowering like crazy, aware that half the space is yet to be filled.  Rainforest trees hold back on flowering to concentrate on growth as a larger tree can support many times more flowers and fruits.  It tells me something, that the mangroves are flowering and fruiting so precociously.

Heavy fruit load for such a small tree (Ceriops sp.)

Not much alive on the surface

I did not see any molluscs.  There were a few dead mangrove clams.  I also did not see bird footprints.  It seems that nothing has thrived due to the changed conditions.  The conditions twelve years ago are shown in this post https://queenslandcoast.blogspot.com/2014/06/legacy-of-cyclone-yasi.html.  These photos were taken in Girrimay National Park about 1 km north of the last car park.

Plants that Harvest Water from Air

Can trees make use of water condensed from the air?  I think so and mangroves and beach casuarinas may condense water on their leaves to help them excrete salt.  In some conditions, beach casuarinas drip during the night.  Mangroves growing near salt pans also drip.  It depends on the species, Avicennia eucalyptifolia drips volumes.  Shake a small tree and you will get wet.  Adjacent Ceriops mangroves do not drip at all.  So it is not a case of a fog condensing on everything.  If the trees that condense water on their leaves cannot do so, I think that they die from salt accumulation.  About 2000 square kilometres of mangroves in the Gulf of Carpentaria died about ten years ago, and I think that this is the main cause.  

Dead mangroves in the Gulf (photo by Norm Duke)

See a few live branches (photo by Norm Duke)

Dead mangroves in upper intertidal zone (photo Norm Duke)

The official reason for the mass mangrove death was that sea level dropped from 20-40 cm.  This can occur when wind stops piling up warm water on the coast, which allows the warm water to drain away and the sea level to fall.  Warm water floats on top of cold deep sea water, so if the layer of warm water is 100 m deep as opposed to 300 m deep, the level of the sea surface will fall as the thickness of the less dense warmer water is reduced and there is less uplift of the sea surface due to buoyancy.  A drop in sea level would certainly stress the mangroves and mangroves near the tidal limit would find themselves above high tide line and dying of thirst.  However, some observations do not make sense to me and I would like to examine an alternative hypothesis.  What if the humidity dropped in the prevailing airflow and during those conditions, it became impossible to condense moisture from the air?  Karumba is a port and will have both tidal records and humidity/temperature records and it would be possible to data mine the records and find out.  For me the observations that do not fit are that a few branches the were low down and which were washed by high tides survived the mass mortality.  Also salt excluders seemed to be fine.  

Avicennia trunks are wet from condensation

Shake the tree and it rains

Avicennia mangroves are salt excretors, whereas Ceriops are salt excluders.  Young Avicennia leaves have salt glands that excrete salt onto the undersurface of the leaf.  The glands cease to function on older leaves.  Also of note is that Avicennia will drip when nearby beach casuarina are completely dry, so dripping seems to be an active process.  During the wet season, beach casuarina would not need to drip away salt as there would be sufficient freshwater in the soil but mangroves are still flushed be seawater.  

Beach Casuarinas also drip salty water

Dry sand with drips of salty water, below the casuarinas

Karumba seems to be the centre of the mass mortality.  I went there just before the die-off and the mangrove community looked strange.  It did not have the species diversity that I expected and it looked like the mangrove swamp was new.  I GPS most of my photos and donated the ‘before’ photos to the Queensland Herbarium to assist with their investigation into the mangrove death.  An overview of what the mangroves were like is contained in this post. 

Avicennia mangrove have recently died near Townsville, in exposed coastal situations.  The death affects many larger trees and enough smaller trees survive to mask the phenomenon.  However, I have found some strange mutations which seem to increase a trees ability to lose salt and will cover this in a future post.

Source of official explanation: https://nesplandscapes.edu.au/2022/07/28/gulf-mangrove-dieback-discovery/