Wetland zonation (the swamp reads like a tide-gauge)

Wade into a freshwater swamp from the dry side and, long before your boots find water, the plants tell you it is coming. You cross out of scribbly gum and heath, and ahead stands a wall of pale trunks; behind it a sward of blue-grey stems; and beyond that, open water with lily-pads laid flat like coins. You have not measured a thing, yet you already know, to within a season, how wet the ground is at every step. A wetland sorts its plants into bands. You might reach for height, or for the richness of the soil; the sorter is time, the number of days a year each patch of ground spends underwater. Ecologists call it the hydroperiod, and it rules here the way salt rules the estuary next door.

Read the bands in order and you read the hydroperiod off the shore like a gauge. Out in the water that stays all year lie the waterlilies: the native blue waterlily, Nymphaea violacea, rooting in the dark bottom and floating its round pads and pale-violet stars on the surface. The wettest band of all. Where the water shallows toward the bank, the lilies give way to sedge, and on the poor sand country the master sedge is the grey twig-rush, Lepironia articulata. It looks unpromising, a dense stand of round, leafless, blue-grey stems a couple of metres tall, but it is quietly ingenious. The leaf has been dispensed with almost entirely, the work handed to those smooth stems, partitioned inside like bamboo and light enough to float, so the twig-rush marches out into deep water and builds the swamp’s green walls almost single-handed. Finer twig-rushes (Baumea) grow among it, and the saw-sedges (Gahnia), whose leaf-edges are exactly as sharp as the name warns.

Behind the sedge, on ground wet but no longer permanently drowned, stands the paperbark swamp: Melaleuca quinquenervia, pale-trunked and dim, a tree built to keep its feet wet where an ordinary tree would suffocate. It keeps company by the same rule. Swamp mahogany (Eucalyptus robusta) holds the slightly firmer margins, and where the floodplain runs down toward the estuary and the fresh water turns brackish, swamp oak (Casuarina glauca) takes over on the saline mud. Then, on the first ground that dries between floods, the wallum heath and scribbly gum close in, and the wetland shades off into ordinary land. Channel to dry land, lily to sedge to paperbark to heath, each community tuned to a slightly shorter drink than the one before it.

One thing swaps out along the way, and it repays a second look. Out on the hungry wallum sand, sedge rules the shallows. Move to a richer river floodplain, the Maroochy, the Mooloolah, where the soil is heavy, nutrient-holding mud, and tall reeds take the sedges’ place: the common reed (Phragmites australis) that lines wet ditches the world over, and cumbungi (Typha orientalis), with its brown sausage of a flower-head. Same band, same job, the same point on the hydroperiod, but a different plant, and the difference is written in the soil beneath. The bands answer to water; which species fills each band answers to the ground.

All of this rests on something out of sight. On the great sand masses of Cooloola and the Noosa back-country, the swamps do not dry out in the dry season the way you would expect a shallow sheet of water to. They are fed from below, by the freshwater table stored in the sand itself, the same buried sponge that fills the dune lakes and keeps the wallum alive between rains (Dyring et al. 2025). That groundwater carries the wettest bands through the drought, which is why the zonation is more fragile than it looks. Lower the water table, by pumping it, draining it, or letting a warming climate draw it down, and the reading unravels from the wettest band inward. The open water goes first, the lily and the deep-water sedge follow, and the gauge resets to a drier number than the country used to keep.