Nature's bulldozer: tree dieback in the savannas

Large areas of dead trees will have confronted anyone who has driven around Queensland’s Charters Towers area in recent years. Drought, pure and simple, or overgrazing? Rod Fensham’s research into tree dieback in a TS-CRC project has found that drought can be the primary cause of dieback; and that the last century shows a pattern of drought and dieback. By Rod Fensham

Tree dieback: these denuded landscapes in the savannas could be a natural phenomenon reacting to drought. Rod Fensham’s study shows average rate of dieback was 29 per cent of tree basal area Photo: Rod Fensham

Introduction | Basalt wall study | Broadscale survey | Why so patchy? | Historical record | Natural phenomena | Nature's bulldozer | References |

Introduction

The possible cause of tree dieback sparked a lively debate last year in the North Queensland’s local press with opinions divided between overgrazing and drought (North Queensland Register 15/5/99, 27/5/99, 3/6/99 and the Townsville Bulletin 10/5/99, 18/5/99). One side of the debate assumed overgrazing caused tree dieback, and while some research supports this theory, there was a general consensus amongst landholder correspondents that the real cause was drought.

Basalt wall study

The first time I became aware of the significance of this dieback event was during a visit to the Great Basalt Wall and surrounding country. This site provides a unique opportunity to study savanna woodlands never previously grazed by cattle. The lava flow is relatively young in the grand scheme of things and thought to be about 12,000 years old. As the lava flowed down the valley it braided in places leaving pockets of the underlying land surface surrounded by a wall of fresh basalt rock.

Animals such as the common wallaroo have no trouble crossing the wall and find their way into the pockets. Cattle however, cannot gain access to some pockets. Thus the Great Basalt Wall provides the opportunity for a natural experiment where we can compare areas that were never grazed except by native animals with directly compatible areas grazed by cattle.

In 1995 there was substantial tree death in the pockets that had never been exposed to hoofed beasts. There was also no appreciable difference between any of the areas whether cattle had grazed them or not. Since this study1 was completed tree death has increased in one of the ungrazed pockets to 69 per cent of the basal area of the trees. The results cannot be taken to mean that grazing has no impact on tree dieback, but clearly indicate that substantial tree dieback can occur regardless of cattle. We established a permanent monitoring site in this pocket to record post-drought recovery2.

Broad-scale survey

More recently we completed a broadscale survey of the tree dieback in north Queensland3. Our study covered 55,000 square kilometres in north Queensland and randomly sampled the area. This revealed that the average rate of dieback was a staggering 29 per cent of tree basal area.

Some trees were particularly susceptible with ironbarks (Eucalyptus crebra, E. xanthoclada, E. whitei, E. melanophloia) in general more susceptible than bloodwoods (e.g. Corymbia clarksoniana, C. erythrophloia), while the cabbage gum (C. dallachyana) was relatively immune. In general dieback did not preferentially affect small or large trees.

Why so patchy?

The question which arises is, why was dieback so widespread over this area of north Queensland and far less intense and patchy over other regions of Queensland? An analysis of the rainfall record is illuminating. The 1990s drought in north Queensland was the most intense drought on record for this part of Queensland, but not for elsewhere.

The really intense droughts for most other regions occurred at the turn of the last century, during the 1930s and the 1960s. This all tends to suggest that drought per se is the primary cause of this dramatic dieback event in north Queensland.

Historical record

It was tempting to trawl through the available records to try and find evidence of other dieback events during the droughts earlier in the 20th century.

The most comprehensive source of information was The Queenslander newspaper of the early 1900s. This now defunct newspaper had a strong regional focus and published detailed correspondence from the pastoral frontier. Sure enough the media and other reports contained numerous evidence of severe dieback events immediately after drought events. A sample from The Queenslander in 1902 was typical but particularly interesting because of its speculation as to alternative causes:

‘Ever since the black soil downs of the far West have been occupied the question as to what cause brought about the large areas of dead timber to be seen in the mulga, gidya, and boree scrubs of the interior has been a matter of controversy amongst bushmen. Some attributed it to the action of fire, others to a cold snap, possibly accompanied by snow; again others to a plague of caterpillars, which ate all the leaves and so destroyed the trees: but seeing that timber is now dying in all districts of western Queensland, it would seem not unreasonable to conclude that drought was the cause of thousands of square miles of country in the “Never Never” being denuded of scrub.’

Natural phenomena

I have recently discovered another report of drought-induced dieback in an area where stock grazing cannot be implicated. It is Beard’s account of the consequences of the early 1960s drought in one of Australia’s most inaccessible regions, the Gibson Desert4. He records a dramatic impact on the desert ecosystems:

‘. . . it was observed that most of the vegetation had been severely affected by drought during some recent period with widespread death and dieback.’

Clearly dieback can be a natural phenomenon. What is more uncertain is the influence of grazing and or fire on our tree stocks. The flipside of dieback is recovery. With reasonably clear evidence that drought can cause dieback it would seem reasonable to hypothesise that the widespread phenomenon of vegetation thickening may primarily be accounted for as part of the natural cycle of regeneration.

Nature’s bulldozer

This has implications for carbon credits on pastoral lands because existing rules for greenhouse accounting require carbon fluxes to be human induced. It is also a clear rebuke for those that argue that land clearing is justified on the grounds that there are more trees than ever before. When the time is right climate can be nature’s bulldozer! The remainder of our Tropical Savannas CRC project, Assessing structural change in tropical woodlands-Queensland and Northern Territory pilot areas, will concern itself with trying to correlate changes in vegetation structure as determined from the aerial photography record over the last 50 years with both climatic and land-use patterns.

References

1. Fensham, R.J. (1998) The influence of cattle grazing on tree mortality after drought in savanna woodland in north Queensland. Australian Journal of Ecology 23: 405-407.

2. Bowman, D.M.J.S., Wilson, B.A. and Fensham, R.J. (1999) Relative drought tolerance of evergreen-rainforest and evergreen-savanna species in a long unburnt Eucalyptus savanna, north Queensland, Australia. Proceedings of the Royal Society of Queensland 108: 27-31.

3. Fensham, R.J., & Holman, J.E. (1999), ‘Temporal and spatial patterns in drought related tree dieback in Australian savanna’, Journal of Applied Ecology vol. 36, pp. 1035–1050.

4. Beard, J.S. Drought effects in the Gibson Desert. Journal Royal Society WA 1968, vol. 54, pp. 39–50