What if there was a tree that liked salt?
Most of the so-called higher plants - land plants with trunks and branches, fruit and flowers - don’t do so well in salt water. Even elevated salinity in the soil is enough to kill many species of tree, and salinity is a major problem in inland Australia as water-tables fall in times of drought (or through excessive irrigation).
One exception to the “salt is bad” rule is the grey mangrove. This is the species found in Australia, and it’s one of the toughest trees on the planet.
Usually, Australian plants and animals are restricted into bands more or less associated with climate and rainfall. Up north, we get the remains of ancient rainforests. Down south, more temperature species and the “dry sclerophyll” eucalypt forests. Inland, hardy desert plants.
But grey mangroves are found in every state, even in Western Australia where there are huge gaps in populations. A mangrove forest at the Albrohos islands is 500km from the next group of trees at Bunbury, and 300km from one at Shark Bay.
The secret to the grey mangrove’s success is of course its tolerance for salt water. A unique adaptation allows it to take sea water in, and excrete the salt from its leaves. But the grey mangrove can also tolerate less salty conditions, such as in times of flood.
Another challenge living in estuarine conditions is the lack of oxygen in the soil. The grey mangrove deals with this via its distinctive pneumatophores, or aerial roots.
As well as acting as anchors against the tide, the pneumatophores are able to absorb oxygen directly from the air. All plants breathe oxygen through their roots, but usually underground.
The final challenge for a tree that lives in an environment that’s frequently underwater is in germinating offspring. Seeds can sink and rot. So the mangrove uses a structure called a propagule.
About the size of a walnut, the propagule floats in a natural orientation that points the eventual roots toward the ground and the stem to the sky. It allows the new tree to get enough light, while also finding nutrients.
Because of their relationship with salt water, grey mangroves are an excellent indicator of water quality, climate change, and the overall health of a coastline.
This grey mangrove propagule has anchored itself in the sand and turned itself upright.