Saturday, 28 June 2014

Legacy of Cyclone Yasi

Cyclone Yasi was a category 5 storm that crossed the Queensland coast just north of Cardwell in February 2011, which is more than 3 years before the photos in this post were taken.  Cyclones happen nearly every year in Queensland and most have little effect on mangroves, a few large, old mangroves fall but smaller trees survive and the ecology continues uninterrupted.  The difference with Cyclone Yasi is that it had a storm surge.  At Cardwell town which is only 1.5 km south of Meunga Creek, the storm surge was about 2-3 m above highest astronomical tide level and in places was accompanied by waves that were a few metres high.

The effect of the cyclone on the mangroves surrounding Cardwell was catastrophic.  A very high proportion of mangrove died along a 20 km stretch of low lying coastline was destroyed.  Many square kilometres of mangrove forest look like the photos below which were taken on 3 June 2014.

This was previously very tall and productive mangrove forest with a canopy at about 20 m
Rafts of timber which are held of the ground by stilt roots make walking challenging.
Wear a hard hat if you go in here and don't grab the trees as they are ready to fall.
Flakes of bark cover the ground - crabs eat leaves not bark and only a few sesarmid crabs are present.
Debris forms a timber floor in parts of the forest closer to the high tide line
Thirty days before the cyclone, a satellite image of the mouth of Meunga Creek was captured.  The site was also captured in late 2013, six months before the photos above were taken.  In the post-cyclone aerial photo, the living forests are mostly non-mangrove vegetation, mainly littoral rainforest or strand vegetation. The grey areas are central areas of mangroves swamp which were hit the hardest.  Patches of mangroves survived around the fringes.

Meunga Creek Mouth days before the cyclone - Jan 26 2011
Image taken 3 years later showing large grey dead areas.  Blue numbers show photo locations*.
*Photos relating to point 1 are in a previous post, those from point 2 are shown above and the regenerating mangroves are at point 3.

In Edmund Kennedy National Park which begins on the opposite side of Meunga Creek, there was a board walk for tourists through a similar mangrove swamp (no closed to the public).

Edmund Kennedy NP Mangrove Board Walk (now closed)
There are many varieties of mangrove swamp.  Spurred mangroves occur on higher ground, which is not flooded by every tide.  These mangroves are adapted to tolerating higher levels of salinity and do not  have the stilt roots or breathing roots that most other mangroves have.  Death in these forests was almost total.

dead Ceriops tagal
Destroyed spurred mangrove (Ceriops) swamp - spurs are another name for buttress roots
Paracleistostoma wardi
View of ground - every hole has a crab and the come to the surface and look at you when you walk
Recovery is slowly taking place.  In some areas, new stands of mangroves are developing.  After three years these dense stands have reached eye level.  I do not know whether these stands are developing from seedlings that were present before the cyclone or seedlings that established afterwards.  The stands are 20-30 m wide and beyond the stands, establishment of new mangroves is scattered to widely scattered.

Regenerating mangroves
Mass death of mangroves also occurred as a result of Cyclone Tracey and was documented but not investigated.  Searching Google did not reveal any clues as to why the mangroves died.  The canopies of the mangrove trees were intact.  Cyclone Larry shattered many trees but did not kill them.  Smaller mangrove that would have been relatively protected within the forest also died.  One clue is that strand vegetation which grows on the beach between the mangroves and the sea survived.  Mangroves on the leeward edge of the foredune also survived.  To me this implicates the soil as these mangroves were exposed to a least as much wind and wave action as the central areas of mangrove swamp.
Living mangroves on lee of foredune then dead mangroves further inland
Foredune vegetation is battered but recovering - many trees survived.
 I think it is time we found out what kills the mangroves.  We should also plan to replant mangroves should a cyclone every hit Cairns, as leaving the vast swamps that surround Cairns looking like a wasteland is not an option.

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