The grey mangrove

By every ordinary botanical reckoning, nothing should grow on a tidal flat at all. The ground is flooded and drained twice a day; the mud is so waterlogged that it holds almost no oxygen, going black and sulphurous a few centimetres down; and the water is salt, which for most plants is simply poison, dragging moisture out of their tissues as fast as the roots take it in. It is the coastal equivalent of trying to farm ground that is on fire half the day and frozen the rest. And yet a small guild of unrelated trees has cracked the problem independently — which tells you how large the reward is for anything that manages it — and on this coast the one that dominates the tidal forest is the grey mangrove, Avicennia marina, the most widespread mangrove in Australia.

Take the salt first, because the grey mangrove fights it at both ends. Its roots are a filter, excluding most of the salt from the water before it ever enters the tree — a remarkable piece of plumbing that lets the plant drink seawater and keep most of the salt out of its sap. Whatever slips past is dealt with at the other end: glands on the leaves excrete the excess, pushing it out onto the leaf surface, so that on a hot still day the foliage can be faintly crusted with its own exuded salt. The tree is, quite literally, sweating out the sea. That double strategy — keep most of the salt out at the root, sweat the rest out through the leaf — is the heart of how the mangrove survives its impossible water (see mangrove-salt-strategies).

Then the air, or the lack of it. Because the mud is suffocatingly airless, the grey mangrove does not even try to breathe through it. Instead it sends up thousands of slender, pencil-like roots — pneumatophores — that stand vertically out of the mud all around the trunk, like a bed of nails pointing at the sky. These are snorkels. At low tide, when the flat drains, they take air directly from the atmosphere through pores in their surface and pipe it down to the drowned root system below. Wade out through a grey-mangrove forest at low water and you are walking through a forest breathing through its ankles — a whole drowned woodland respiring through a thousand upright straws.

And then the seed, which is stranger still. Mangroves do not gamble their young on the tide as a naked seed the way most trees do; the grey mangrove germinates its embryo while the fruit is still attached to the parent, the embryo sprouting inside the fruit and waiting there until the fruit falls — a habit botanists call cryptovivipary. (Its stilt-rooted relatives, the red mangroves, go further, the seedling bursting clean through the fruit and dangling from the branch as a long green spear before it drops: true vivipary.) Either way the tree sends its young into the world already growing rather than as a helpless seed, primed to root in the first patch of mud it lodges against. For a plant colonising a surface that is underwater half the time, that is a decisive head start.

Put the three tricks together and you do not simply have a tree that survives the tidal zone; you have one that builds it, holding the middle band of the estuary between the saltmarsh above and the seagrass below (see estuarine-zonation). The wall of breathing roots slows every incoming tide, dropping its load of silt, so the forest traps sediment and reaches steadily seaward, making new land out of muddy water; the same tangle absorbs the punch of storms and floods, filters the water, and shelters the juvenile fish and prawns the coast's fisheries are built on. It is easy to walk the boardwalk and see a dark, muddy, unlovely thicket. Read it instead as one of the most accomplished plants on the coast — filtering salt, sweating brine, breathing through snorkels and launching its young pre-grown — and doing, for nothing, the work of a sea wall, a nursery and a water filter at once (Leiper et al. 2022; McPhee 2017).