The grey mangrove

Walk out onto any tidal flat on this coast at low water and the tree standing in it will almost certainly be the same one: the grey mangrove, Avicennia marina, the most widespread mangrove in Australia and the owner of the middle band, estuary after estuary from the Noosa to the Pumicestone Passage. It is not much to look at, a dull grey-green thicket rooted in black mud, and by any ordinary reckoning it should not be there at all, since its ground floods and drains twice a day and the water it stands in is salt. How a mangrove beats salt and mud is worked out elsewhere. This one does it well enough to turn up almost everywhere, which is why it is the mangrove you meet. Learn one mangrove and it should be this one; the rest of the estuary reads more easily once you can place its dominant tree.

The salt and the airless mud are what let the grey mangrove live here. What makes it worth a second look is what it does to here once it has arrived. It builds the estuary it stands in.

Its thicket of trunks and breathing roots does one simple thing with enormous consequences. It slows the water. Every incoming tide has to filter through a dense tangle, and slow water cannot hold its load of suspended silt, so the grey mangrove traps sediment, tide after tide and year after year, lifting and extending the very ground it is rooted in and creeping out into the channel. Left to itself, a grey-mangrove stand grows seaward, making new land out of muddy water. Survey a river mouth against an old aerial photograph and you can often watch the green edge march bayward across the decades, a forest advancing on open water at a pace slow enough to miss and fast enough to map. Ecologists call organisms that remodel their own surroundings ecosystem engineers, and few living things earn the name more literally.

That engineering pays out to everyone downstream. The same wall of roots that catches mud also takes the punch out of storm waves and flood surges, a sea defence that builds itself and never sends an invoice. It strains sediment and pollutants from the water before they reach the seagrass beds and the reef. And the flooded maze beneath it shelters the young of half the animals the estuary feeds, though the full account of that nursery belongs to the tidal system as a whole (see estuary). Strip the fringe away and the coast loses a self-repairing edge, and the bill for holding the line lands on us instead, paid out in rock walls and trucked-in sand.

So the grey mangrove holds the middle band of the estuary (see estuarine zonation). From a boardwalk it reads as a dark, unlovely thicket. It is also laying down coastline, a tide at a time, manufacturing the ground the whole tidal country stands on.