Concept · Ch 8
How a mangrove beats salt and mud
A mangrove faces two impossible conditions at once — salt, which is poison, and waterlogged mud, which holds no oxygen. It keeps most of the salt out at the roots and sweats the rest from its leaves, and it breathes through snorkel roots that stand up out of the airless mud.
By every normal botanical reckoning nothing should grow in the mangrove band. The water is salt, which for most plants is simply poison; the mud is so waterlogged that it holds almost no oxygen and goes black a few centimetres down; and the whole surface is drowned and drained twice a day. A tree that lives here has to beat two problems at once.
The grey mangrove sweats out the sea. Its roots strain most of the salt out before it ever enters the tree, and whatever gets through is pumped back out through glands on the leaves, so on a still day the foliage carries a faint crust of exuded salt. For the airless mud it grows snorkels — thousands of slender pencil roots that stand vertically out of the mud all around the trunk and pipe air down to the drowned roots below. Wade through a mangrove forest at low water and you are walking through a forest breathing through its ankles: a plant doing, quietly and for free, two things that ought to be impossible.
In depth
"Mangrove" names a habit, not a family: a guild of unrelated trees that have each independently solved the same set of conditions, which is itself a measure of how large the rewards are for anything that manages it. Two problems have to be beaten together.
Salt. Sea water draws moisture out of ordinary plant tissue as fast as roots can take it in, so the grey mangrove (Avicennia marina, the dominant species on this coast) attacks it at both ends. Its roots ultrafilter most of the salt out before it enters the tree, and whatever slips through is pushed back out through glands on the leaves — on a still day the foliage can be faintly crusted with exuded sea salt. Some mangroves also store water in succulent leaves or shed salt-loaded leaves; the milky mangrove on the landward fringe barely bothers with the machinery at all and keeps to the drier edge instead.
Airless mud. Waterlogged sediment holds almost no oxygen and turns black and sulphurous a few centimetres down, and roots must still breathe. The grey mangrove does not try to breathe through the mud; it sends up thousands of slender vertical pneumatophores — pencil roots standing out of the mud like a bed of nails — that take air at low tide and pipe it to the drowned roots below. The stilt-rooted red mangroves solve the same problem with arching prop roots instead.
A head start for the seedling. Because the surface is underwater half the time, mangroves do not gamble a dry seed on it. In the grey mangrove the embryo germinates inside the fruit before it drops (cryptovivipary); in Rhizophora the seedling bursts clean through the fruit and dangles as a long green spear before falling (true vivipary). Either way the young tree arrives already growing, primed to root in the first patch of mud it lodges against.
Primary sources & further reading — the doorway
- Leiper, G. et al. (2022). Mangroves to Mountains (3rd ed.). Native Plants Queensland. — The mangrove species of this coast and their salt/root adaptations (Mangroves to Mountains).
- Davie, P. (1998). Wild Guide to Moreton Bay. Queensland Museum. — Regional mangrove natural history — pneumatophores, salt glands, vivipary in context.