Rare grassland at home in southern NSW

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Birds nest in it.
Fish feed under it.
Frogs find refuge among its spiky stems.
But what is ‘it’?

In the Barmah-Millewa Forest you’ll find a distinctive grassy wetland known locally as the Moira grass plains.

This aquatic grass – also known as spiny mud grass – dominates vast open wetlands in the Moira precinct of the NSW Murray Valley National Park.

With its spiky rush-like leaf, Moira grass thrives in warm to hot conditions, lying dormant in the soil before bursting into life when water arrives.

But not just any water will do. Timing is everything. Water depth is another. And the length of inundation is just as important.

That’s where the work of environmental water managers comes to the fore.

A total of 15,000 megalitres of environmental water has been delivered to the Moira grass plains at Reed Beds Swamp over spring and early summer.

The NSW Office of Environment and Heritage is managing the latest delivery of water to allow Moira grass to grow before setting seed over summer.

Senior Environmental Water Manager Paul Childs said Moira grass performed several vital functions on the floodplain.

“Historically, the Moira grass plains were a highly valued food resource for local Aboriginal people,” Mr Childs said.

“This is evident by the large number of scar trees, oven mounds and middens located in the immediate area.

“Moira grass provides nesting and foraging habitat for an array of waterbirds including species protected by bilateral agreements between Australia, China and Japan.

“Grebes and whiskered terns build their nests on floating platforms created by the Moira grass.

“The grasses provide a hunting ground for the birds that feast on frogs, insects and other small invertebrates in the wetland.

“Floating Moira grass platforms offer a shady sanctuary for native fish including golden perch and the critically endangered silver perch.

“They feed on the abundance of insects and small invertebrates whose numbers have exploded with the arrival of water.

“Their young also make use of the slower moving water to feed in the warm, nutrient-rich wetlands.

“Beneath the surface of the water, the tangle of Moira grass stems slows and filters the water as it moves through the wetland and back to the river.

“When first flooded, the Moira grass plains release large amounts of carbon that energise the river system and strengthen the food web – from tiny freshwater plankton and middle-order consumers up to top-order predators such as native fish and waterbirds.

“The roots of the Moira grass plant also add stability to the soil during inundation.

“And, as the water recedes, the matted vegetation settles to the floor of the wetland and draws on soil moisture for a period of time before drying and providing a thick compost layer which further slows evaporation from the wetland.

“It’s an ingenious system, but one that requires careful management to ensure its survival,” Mr Childs said.

The extent of Moira grass wetlands in the Barmah-Millewa forest is declining.

The plant has very specific water needs in order to fulfil its life cycle to the point of flowering and setting seed.

“Since river regulation, reductions in the frequency, depth and length of inundation mean that Moira grass does not have the same opportunities it once did to complete its life cycle,” Mr Childs said.

“These changed conditions have allowed river red gum and giant rush to encroach on the Moira grass plain.

“It’s estimated, from aerial imagery, that the extent of this particular wetland has retreated some 90 per cent since the 1970s.

“That’s why active management of the site and strategic delivery of water is essential.

“Especially when you consider the multitude of benefits this plant provides for the wetland and river system including habitat, food, water filtration, carbon supply and soil stabilisation,” he said.

Environmental water for this event has been allocated from NSW Adaptive Environmental Water (NSW AEW) holdings, NSW Additional Environmental Water Allowance (AEA), The Living Murray (TLM) and Commonwealth Environmental Water Holder.

The event has been complemented by additional rainfall inflows, operational transfers to meet downstream consumptive demands and a rainfall rejection event which generated high flow pulses that have been strategically directed into the Barmah-Millewa Forest. Combined, these flows have demonstrated the highly efficient use of environmental water.

Fast facts:

  • Moira grass is listed as a “critical wetland type” and wetland of international importance under the Ramsar convention.
  • Moira grass can grow more than 20mm per day under ideal conditions.
  • Before it reaches the surface, Moira grass is thought to oxygenate the water.
  • The Moira grass plains were a highly valued food source for local Aboriginal people.
  • Early settlers used the Moira grass plains as a source of supplementary feed for stock.
  • While found in small numbers at several locations, the Moira grass plains of the Millewa (NSW) and Barmah (Victoria) forests are rare because of their dominance in the wetland.
  • The drying phase of the Moira grass life cycle serves to insulate the wetland – as water recedes, the grassy mats settle on the floodplain floor, helping to slow evaporation from the wetland.
  • Moira grass is also known as spiny mud grass (Pseudoraphis spinescens).
  • Water managers in Victoria are also managing environmental flows for the benefit of Moira grass plains in Barmah National Park.

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