A burning issue – grass invasion and seed dynamics across Central Australia

It’s July in Central Australia. The ground is moist with morning dew and the skerricks of rain from last month’s cold front. By mid-morning the sun is heavy and the sand is warm, despite the night’s frost. Hakea blossoms drip with nectar, and the air heaves with  babbler birds squabbling and butcherbird babies gurgling. A falcon feasts on a freshly-fledged chat in the crook of a bloodwood.Simpsons Gap heaves with life.

I stand up, stretch my shoulders, and gaze across a sea of dead, yellow grass to a sandstone ridge behind a line of river redgums. The sea is dotted with the blackened fingers of dead wattles. This bed of straw is deep, thick, crinkly and brittle, and the canes snap beneath your feet and against your shins as you wade through it … buffel grass country

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Buffel grass infestation across the alluvial plain at Simpsons Gap, west of Alice Springs

Buffel grass (Cenchrus ciliaris) is an invasive plant that was introduced to vast areas of inland Australia to ‘improve’ the grazing potential of the landscape for ranging cattle. It’s a perfect storm of a species: rapid growth, ability to persist in extremely dry, hot and dusty deserts, ability to resprout post-fire, a vast and deep root system with which to compete with native plants for what limited nutrients and moisture exists in these ancient, arid soils. Now that the cows are gone and the pastures lay abandoned, buffel grass infestations have burgeoned across the river flats and alluvial plains and rocky slopes of the ranges.

The blanket of hay and dead canes heralds the end of a flush of buffel grass growth, followed by an extended period of little to no rain… a perfect fuel for a hot, expansive and severe fire. Buffel grasslands burn often, and when they do it is hot and fierce and fast. If you are a lone pine or Hakea or Atalaya, standing amidst a sea of buffel grass, you are burnt often and most likely die. The holes left by the dead shrubs and trees are almost instantaneously monopolized and filled by buffel grass seedlings. The more holes that are filled and the more space that buffel grass covers, the hotter and faster and fiercer and more voraciously consuming the fires become …  a positive feedback of invasion-begets-fire-begets-invasion-begets-fire that drives the demise of the few natives that can persist in this harsh, infested landscapes.

We know that invaded areas contain far fewer native plants than once grew on these river flats. We know that the fires are fiercer and hotter and the native trees are burning to death amidst a sea of buffel grass. We know that these dead trees are not replaced by seedlings. Why? Why don’t the adult plants get replaced by seedlings post-fire? Is it because buffel grass inhibits the germination of the native seeds from the soil? Do the seeds germinate but rapidly die when faced with indomitable competition from buffel grass?

Another unexplored explanation: buffel grass-invaded areas may be limited in the abundance and diversity of seeds in the soil. The adult trees, floundering in a sea of buffel grass, may simply not produce enough fruit to replenish the reservoir of seeds in the soil (or they do but the seeds cannot get to the soil), such that when the trees and shrubs die there are few opportunities for seedlings to germinate and replace them. Many of the animals that plants ‘use’ to disperse their seeds long distances may visit invaded areas less frequently (or not at all?) than they once did.

Does buffel grass invasion impact on the soil seed bank?

On this warm winter’s day you can see a head lolling around in the grass. Kaisha Edwards is nestled beneath a bloodwood, covered in tree sap and red, powdery soil. Meat-ant sisters scurry about her boots, foraging for corpses of grasshoppers and moths, and the fallen, rotting berries of Encylaena shrubs. Her shoulders heave up and down as she digs another trench. 479 to go. The soil is dug out, strewn across a sieve and shaken to separate the seeds and rocks and leave and twigs from the sand and silt.

Kaisha and I have come to Alice Springs to hunt for seeds. Along with the leader of our buffel grass research Dr Christine Schlesinger at CDU, we seek to discover whether invaded areas contain the same set of seeds in the soil as non-invaded areas. This work may crucially enable us to determine the pathway by which  buffel grass causes long term declines in native plant populations. Does buffel grass cause a decline in the density and identity of seeds in the soil? If not, the decline in populations is probably linked to suppressed seed germination of seedling mortality, not disrupted dispersal of seeds to the soil and their storage below-ground.

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Kaisha plotting out her next set of soil cores

Back in the lab, Kaisha and I (and some wonderful volunteers) will arduously and carefully extract and identify all the seeds from each core. We are already observing an apparent trend towards reduced seed abundance in buffel grass-invaded areas, but we won’t hold out breath just yet.

We thank CDU for their generous hospitality during our recent research expedition, and Christine Schlesinger for her enduring support of our collaborative work on buffel grass. I also wish to warmly thank the Hermon Slade Foundation for supporting our work on understanding the mechanisms by which buffel invasion invasion disrupts recruitment dynamics of native woodlands in the Red Centre.

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