Grey mangrove

Stand at the edge of Barker Inlet or St Kilda, just north of the city, and you are looking at a tree that has no business being this far south. Mangroves are a tropical proposition — dense, humid, equatorial forests fringing the coasts of the world’s warm seas. South Australia has exactly one of them, the grey mangrove Avicennia marina, and the forest it builds here is among the most southerly mangrove stands on the planet, a slice of the tropics making a living in cool-temperate Adelaide’s unfavourable water (mangroves-2021).

Its trick is the opposite of its neighbour’s. A little further up the flat, samphire survives the tide by drinking the brine and locking the salt away in its own swollen flesh. The mangrove refuses the deal outright. At the root it filters out most of the salt before the water is ever admitted, an act of quiet, unglamorous plumbing; what slips through anyway it sweats back out through its leaves, so that by the end of a hot day a fine crust of salt crystals can be found sitting on the leaf surface, sparkling faintly if you catch the light right. Excluding at the gate and excreting the leftovers — that is the whole trick, and it is enough to hold the plant clear of a poison that would kill almost anything else growing here.

The mud underneath presents its own problem, because tidal mud packed this tight holds almost no oxygen, and ordinary roots suffocate in it. The grey mangrove’s answer is the pencil-thin pneumatophore — hundreds of them, jutting up out of the sediment around each tree like a bed of blunt snorkels, each one breathing air down to the buried roots through pores in its tip. Walk the St Kilda boardwalk at low tide and you are walking over a forest floor built entirely on that single piece of engineering.

None of this is ornamental. The forest’s waterlogged, airless mud buries carbon and keeps it there, hectare for hectare outpacing most forests on dry land (macreadie-2017), while its channels nurse the gulf’s prawns, whiting and crabs through their first months of life. And it is not indestructible: in 2020 a stretch of the St Kilda mangroves abruptly died when hypersaline brine leaked from an adjacent salt field into the tidal flat, a plain reminder that a tree living this close to its limit has very little room to spare (epa-2020). To find a healthy grey mangrove forest at all, this far from the tropics, is to look at something improbable.