The St Kilda mangroves

Walk out from the shore at St Kilda, just north of the city, and a boardwalk carries you level with the canopy of a forest that has no business existing in a cool-temperate gulf. The grey mangrove, Avicennia marina, is South Australia’s only mangrove species, and the stands it makes here and around Barker Inlet are among the most southerly mangrove forests on the planet — a tropical way of life, quietly persisting at the cold edge of its range (Mangroves 2021). Under your feet, poking up through mud too airless for ordinary roots, are hundreds of pencil-thick pneumatophores: breathing snorkels that let the tree take in oxygen its submerged roots cannot reach.

The mangrove’s whole trick is managing salt without the samphire’s method of storing it. Where the glassworts of the flat beyond swell with brine and lock the salt into their succulent tissue, the mangrove filters most of it out at the root and sweats the remainder from its leaves — a different solution to the identical problem of living where fresh water never is. Between them, mangrove and samphire and the mud they anchor do work far beyond their looks: the waterlogged, oxygen-starved sediment buries carbon at a rate that rivals a standing forest, and channels through the flat serve as a nursery for the gulf’s prawns, whiting and crabs.

None of it is as durable as the boardwalk suggests. In 2020 a stretch of this same forest and its fringing saltmarsh abruptly died: killed by hypersaline brine that leaked from the Dry Creek salt field next door and swamped tissue built to tolerate seawater, not something saltier still (EPA SA 2020). Official mapping the following March counted roughly nine hectares of mangrove and ten of saltmarsh dead, with a further five hectares of sparse or bare ground besides (Mangroves 2021) — modest acreage, but a blunt demonstration that a forest holding on at the limit of what salt a mangrove can bear has almost no margin left for one more insult. The state’s environment regulator eventually settled the matter for a hundred thousand dollars: a small sum, most visitors to the boardwalk would agree, for nine hectares of the world’s most southerly mangrove forest.

Stand on the boardwalk today, though, and the lesson is not only loss. The forest beyond the dieback scar is still there, still breathing through its thicket of pneumatophores, still doing the improbable work of being a tropical tree in a temperate gulf. It is a good place to learn what “finely balanced” looks like — and to notice that most of this coast’s real value lies exactly in the parts that look like nothing at all.