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.
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| Dead mangroves in the Gulf (photo by Norm Duke) |
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| See a few live branches (photo by Norm Duke) |
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| 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.
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| Avicennia trunks are wet from condensation |
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| 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.
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| Beach Casuarinas also drip salty water |
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| 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/
