During the early hours of the morning on 6 February 2011, the most powerful cyclone in Queensland’s recorded history struck the coast. Cyclone Yasi crossed the coast as a Category 5 storm between Cardwell and Tully Heads. Where it crossed the coast, the storm surge was said to have been 7 m high. Fortunately, the cyclone crossed the coast in an unpopulated area of swampy Girrimay National Park. Several kilometres to the south saltwater inundated the Cardwell esplanade and adjacent town up to the ceilings of low set houses.
Immediately after the cyclone, it is hard to inspect the impacted area. Low clouds and rain prevent aviation and aerial imagery. On the ground, power lines are down and often areas are closed to the public to prevent looting. It was not until the middle of April that I could go in and record the damage. My role was to assess the level of damage to trees and coastal reserves. Some of the records that I collected are presented in previous posts including Environmental Damage from Cyclone Yasi and Legacy of Cyclone Yasi.
By April, the mangroves were completely dead. Wherever there were deep green mangrove forests, there were now broad grey fringes of dead forest. Something similar happened in Darwin after Cyclone Tracey. However, neither the process of mangrove death or the gradual recovery of the mangrove ecosystems were recorded. Exactly why the mangroves die is not known to me. I suspect that the mangroves are shaken so hard by waves that the soil releases its poison gasses into the roots of the mangroves and that the trees are poisoned. Coastal trees growing on thin fingers of sand within the mangroves survived and had partially regenerated their crowns within only a few months of the cyclone. Whatever kills the mangroves is mangrove specific and extremely lethal.
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| Dead mangroves on Cardwell foreshore, April 2011 |
The best area to observe the long-term impact of the mangroves is near the boat ramp at Meunga Creek, which is about 2 km north of Cardwell town. In the ten years since Cyclone Yasi, mangrove forests that were approximately 20 m high have recovered to about 5 m tall. Log piles formed from toppled adn washed-up mangroves there lined the seaward fringe are now gone. Gaps in seaward stands where trees once stood remain open as there is very little regeneration within the remnants of these once dense stands. Sonneratia mangrove seedlings formed a band in front of the tall stilt mangroves but ten years on, they remain shrubby and wind burned.
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| Seaward mangroves in 2014 |
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| Seaward mangroves in 2018 |
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| Seaward mangroves in 2021 |
On the sandy levee which runs through the mangroves, there is now a 5 m tall stand of a type of Orange Mangroves - Brugiera parviflora. These trees can grow taller than 25 m, so they are still young. After the cyclone, it was difficult to walk here as fallen mangroves were so densely packed that they almost formed a deck that could be walked on. The maelstrom of crashing waves and grinding logs would have cleared a patch of ground on which these mangroves could regenerate. Mangrove propagules would have been washed in and could start growing.
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| Orange mangrove stand in 2014 |
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| Same stand in 2018 |
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| Inside stand in 2021 |
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| Looking up inside stand in 2021 |
In the dense stand behind the levee the mangroves were not toppled but died off in droves for reasons unknown. There was so much fallen timber that very few mangrove propagules would have been washed in. Also as very few mangrove trees survived, local production of propagules was almost non-existent. Regeneration in these back swamps has a slow start.
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| The back swamp was mostly dead in 2014 |
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| By 2018 some new mangroves had established |
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| In 2021, the mangroves are starting to close the gaps |
It has taken 10 years for mangrove regeneration to fill most of the gaps. Based on gaps created by Cyclone Winifred in 1986, which were visible from the now closed mangrove boardwalk at Cairns Airport, it will take approximately 30 years before the regenerating mangroves start to blend in with the trees that survived. It might take twice as long for the forest to recover its original stature.
