Living in salt (halophytes)

Take an ordinary plant and water it with the sea and it will die of thirst. Salt water is a trap: the salt outside the roots is so concentrated that, instead of the plant drawing water in, water gets drawn out of the plant’s own cells, and it shrivels from the inside. Almost nothing can drink from the gulf. Two things north of Adelaide have found a way round the trap, and they solve it by opposite methods.

Samphire — a knuckly little sausage-jointed shrub the botanists file under Tecticornia — cheats by not fighting the salt at all. It drinks the brine and stores the salt away in the swollen, succulent flesh of its own stems, diluting the trouble rather than repelling it. Late in summer, as the plant keeps concentrating salt it has nowhere else to put, whole flats of it turn from blue-green to a startling crimson — a coast blushing under the strain of its own trick. It is a good trick, but not a free one: it depends on the tide staying roughly where it has always been. On the eastern gulf’s shrinking samphire flats, populations have declined sharply as rising seas drown the seaward edge faster than the plants can recolonise new ground behind it (Delta Environmental Consulting 2017).

The grey mangrove, Avicennia marina — the only mangrove species South Australia has, and among the most southerly on Earth — takes the opposite approach. Rather than swallow the salt, it filters most of it out at the root before the water ever gets in, then sweats the remainder back out through its leaves, leaving a fine crust of crystals on the leaf surface after a hot day at St Kilda or Barker Inlet. Excluding at the gate and excreting the leftovers, it holds itself just as clear of the salt that samphire absorbs and stores.

Between them, these two tricks are the reason the samphire coast exists at all, rather than bare mud. The saltmarsh and mangrove these two build hold carbon in their waterlogged soil faster than almost any forest on land (Macreadie et al. 2017), and that same community — for all its ordinariness of grey-green and mud — is now formally listed as a nationally vulnerable ecosystem (DCCEEW 2013), drained and disregarded for two centuries as wasteland by people who never learned to read what a plant that drinks the sea is doing.