Fire and spear grass: a case for wet-season burning in Kakadu

Results from CSIRO’S Kapalga fire experiments raise some intriguing questions, writes Greg Miles, a ranger from Kakadu Park with more than 20 years’ experience. He says apparent contradictions can be explained when seen from a perspective of a landscape under invasion by fire-loving grasses.

Role of spear grass in the system | Spear grass on the march | Wet-season burning | Kentia palms protected | Conclusion | Kapalga Fire Experiment |

Wet-season burning in Kakadu Photo: Greg Miles

In March 2000, a workshop was held summarising the vast amount of work on fire research undertaken at CSIRO’s Kapalga Research Station (see box on this page). Some valuable detail was revealed that placed more pieces into an otherwise sparsely filled jigsaw. However, many of the speakers detailed ongoing ecological processes in the Kapalga experiment that are apparently inexplicable: the eucalypt E. miniata should be either extinct or vary rare—instead it is neither.

The results showed that young and old eucalypts are most at risk of being killed by hot fires; E. miniata reproduces only by seed and most seedlings are killed by fire be it early or late dry season. Frequent fire has a negative affect on seed production. There was also approximately a 5 per cent loss of nitrogen each year from the biota under an annual fire regime used at Kapalga.

When examined together, and knowing that Kapalga has a history of hot and frequent fires, this information tell us that E. miniata (at least) should be either extinct or rare in the study area. This coupled with the finding of nutrient loss tells us that the Kapalga woodlands should rapidly be converting to grasslands or at least host only crippled and stunted woody vegetation. Why is this not so? To my mind, this is a great mystery.

To resolve this and other mysteries posed by the experiment I propose to look at the results from another perspective, in which Kapalga is seen as part of a landscape being transformed by spear grass invasion.

Role of spear grass in the system

From an ecological point of view, local native spear grasses ( Sorghum spp.) could be considered indigenous weeds in the Top End.

One only need observe their distribution and density in relation to other grass species to get an inkling of this. The grass occupies an extraordinarily wide range of habitats from high, lateritic woodlands, seasonally inundated flats through to sandstone sand-flats and scree slopes. Its hyper-abundance gives the appearance of a plant in disharmony with local floristics.

Fire in the woodlands of Kakadu is largely about spear grass. It constitutes around 40 to 50 per cent of the annual fuel load and the majority of the free-standing fuel, so without spear grass, fires would be much less intense. It is not uncommon to see lowland forest standing in spear grass suffer 100 per cent scorch as early as late April.

Leaf litter is the second major contributor to the annual fuel budget. Leaf litter fires (without spear grass) tend to have a low scorch height and intensity, although in extreme weather conditions, a heavy accumulation of litter can produce very hot fires ¹ . Despite its major influence on annual fire, little is known about spear grass in terms of changes in its distribution and density over time.

The role of native sorghum in Top End fire regimes needs more examination.

Spear grass on the march

Some authors have alluded to an expansion in the distribution of spear grass over the past 70 to 90 years. They attribute this to disruption of traditional burning caused by the depopulation of the region by the 1890s and that without a resident and stable Aboriginal population, traditional burning largely came to an end. In its place were many decades of hot, late-season fires which were unchecked by traditional early burning ² . There is evidence to suggest that this same scenario is now occurring in central Arnhem Land and the Kimberley ³ .

Anecdotal evidence from resident old timers also suggests that spear grass was less common 40 years or so ago than it is today ⁴ .

This apparent invasion of spear grass could be having profound and disturbing consequences. It may be promoting more ferocious and frequent fires that have been implicated in the thinning of the woodlands canopy (a finding reinforced by the Kapalga study). At the same time, this new fire regime may be promoting yet more spear grass.

In many parts of Australia, trees will invade grasslands if fire, especially traditional fire, is not maintained ⁵ . For most of the 20th century, however, the opposite was happening in the Kakadu region. That is, because of hot and frequent woodland fires, the tree-scape thinned and monsoon forests shrunk, apparently in favour of habitats dominated by spear grass.

I believe that in many parts of the Top End not subject to robust fire-management regimes, this negative scenario has significantly worsened since the mid-1980s. At this time the buffalo population suddenly and spectacularly crashed as a result of the National Brucellosis and Tuberculosis Eradication Campaign. Spear grass, which was probably thinned due to trampling by buffalo in the wet season, was apparently released from the constraints of bovine activity and increased in density throughout the region.

Thus, it is my view that from the mid 1980s, dry-season fires became hotter and possibly more frequent with a resulting impact on fire-sensitive ecological comm­unities (such as lowland monsoon forests). However, this increase in fire frequency and, more part­icularly, temp­erature, has been subtle and not recognised by the casual observer.

Wet-season burning

The problem is clear enough and most people agree that fire in Top End spear grass landscapes occurs too frequently. A fire regime that burns the same areas one year out of two or three, curtails recruitment of many plant species and, on balance, has harmful ecological consequences. While a rise in spear grass density and distribution seems to be a factor here, it is generally agreed that this situation is also due to the high frequency of fires lit by land managers, and by careless tourists or arsonists.

What is difficult is finding agreement upon a management regime that will reduce this problem. It is clear to most Top End land managers that fire is an annual inevitability and they must plan for it. Unfortunately this backs managers into the ‘I will burn it before someone else does’ mentality. Right now this is, sadly, probably the best policy, even though it forces managers to burn a lot of country every year. As a result, some places get burnt every year. To turn this negative cycle around only one method seems to hold any promise: wet-season burning (WSB).

For the uninitiated, wet-season burning is used during December or January (in Kakadu) to clear a given area of spear grass. A fire at this time will kill newly germinated grass seedlings growing among the stalks of last season’s parent grass. As sorghum is an annual, almost all seed in the soil will germinate in the early wet season. Following the fire, there will be little or no seed remaining and the affected area will be largely free of spear grass (and therefore fire) until the grass population re-asserts itself. This may take three to seven years. In the interim, any fire will effectively be a leaf litter fire.

WSB involves risk. To be effective, spear grass must be retained over a full dry season in order to supply enough fuel to carry fire in December/January. The risk occurs during the late dry season when areas, which have been saved for WSB, are ignited prematurely by lightning or (more commonly) arsonists.

Young Kentia Palms at the West Alligator River in Kakadu are protected from fire by a wet-season burn Photo: Greg Miles

Kentia palms protected

An excellent case study of WSB in Kakadu was carried out near the West Alligator River crossing on the Arnhem Highway adjacent to Kapalga. Here, Top End endemic Gronophyllum ramsayi palms were roasted every year by hot spear grass fires. In 1993 Kakadu rangers subjected the area to a wet-season burn—a well-timed fire that rendered annual spear grass almost extinct locally. Without any further fire suppression the area remained untouched by wildfire until July 2000. Precisely what is called for—and all from one strategic wet-season burn.

One study on the effect of WSB on woodland floristics concluded that any benefit from WSB would be relatively short lived ⁶ . But it has taken many years for rangers to refine WSB methods to achieve the best result. It is probable that the one-hectare area used by the floristics study was not large enough to achieve results such as those described for the Gronophyllum palms.

WSB is part of an intensively applied and sophist­icated prescribed burning regime in Kakadu that is starting to reverse the impacts of spear grass. Evidence drawn from aerial photography and fire-monitoring plots, maintained by Kakadu rangers under the guidance of Dr Jeremy Russell-Smith, point to a cooling of fires and subsequent improvement in the general structure of the woodlands in many areas of the Park.

Conclusion

To return to the mystery of the surviving eucalypts, I would suggest it is because the various fire regimes documented at Kapalga (this would probably apply to much of the centre of the Top End) were relatively recent and atypical. In other words, the Kapalga fire scenario, which may be exterminating the eucalypts, is simply not old enough for us to witness its inevitable and ultimate consequence. The decline in eucalypt recruitment and loss of nitrogen found at Kapalga was a product of relatively recent fires fuelled by spear grass. Arguably, this situation did not exist 200 years ago.

It is proposed here that the careful and sustained use of WSB in the woodlands across the Top End will provide the best means available to reduce both the intensity and frequency of fires. I am unaware of any other practical method available which can be used on a landscape scale.

However, if wet-season burning is to be widely adopted it must be better understood. To date we know little about its broader ecological impacts. It is paradoxical that the main—if not only—ecological tool available to significantly manipulate fire on a landscape scale has attracted so little scientific attention. Such attention is needed to answer important, and as yet poorly understood, questions about wet-season burning.