2000
Australia was forged in fire during millions of years of lightning strikes and volcanic eruptions. During the last one hundred and thirty thousand years Aboriginal Australians used fire to hone established reactions. Some plants came to depend on fire, some accepted it then restored themselves: some learnt to shrink away from it.
Eucalypts give the best example of the differing intensity of the fires that still shape Australia. They encourage fire with a high, volatile oil content in the leaves, with streamers of inflammable bark, with a massive drop of dead leaves so low in minerals that they burn exceptionally well. But a forest never risks more than 5 per cent of its total mass, its soul is solidly enclosed in living wood. Grass yields everything that it displays above ground.
Most species of eucalypt are prepared to meet every possibility When a fire jumps a damp valley it can scorch the tops of trees on the way without setting fire to the understorey. For this eventuality the eucalypt has naked buds at the base of the leaf stalks that break open in a few days. If one of these attractive new shoots gets eaten, microscopic accessory buds grow to replace it. If a hot, high fire through the understorey burns the whole crown, epicormic buds, protected by the thick, heat-resistant bark, sprout in hundreds. Within a month trunk and main branches bristle with disordered growth. The tree looks fuzzy but it is functioning again; it is connected to the air. After nine months three or four of these shoots take over. They lift up like real branches and the unsuccessful shoots die and drop off.
When a particularly intense fire burns down a whole tree, it grows quickly from a lignotuber buried safely under the ground. Earth is an excellent insulator. The hottest fire scorches no more than the top three centimetres. Once again, the growth is undisciplined. Within three weeks the tree can have a hundred prospective trunks sixty centimetres high from which it will select one or two. These lignotubers are storehouses in exactly the right proportions of every mineral and chemical in the soil. They vary in size from that of a football to distorted masses weighing a couple of tonnes in some of the mallees.
The two great timber trees of Western Australia, Jarrah (Eucalyptus marginata) and Karri (E. diversicolor) show marked difference in their attitude to fire. Jarrah has great ability to recover from the hottest fires. Karri resists light ground fires, but top fire kills it.
The imposing Mountain Ash (E. regnans) in Victoria is extremely fire sensitive yet it requires a hot fire to free its seeds from a hard capsule and a bed of rich ash to give the seedlings a good start. How many millions of years did it take for forests of these trees to decide that they would thrive on three fires each thousand years? Frequency is critical. Any fire through a young Mountain Ash forest before it seeds at about twenty-five years is likely to wipe the forest out completely. It will have to have to begin again as a scrub, a thicket of mixed shrubs.
As a new forest of Ash grows the understorey changes. At first shrubs like the untidy, cream-flowered Common Cassinia (Cassinia aculeata) dominate. This dies after twenty-five to thirty years, then small trees like Olearia argophylla, one of the Daisy Bushes with cream flowers, take over for seventy-odd years, after which the damp darkness favours moss and ferns with scattered grasses and shrubs here and there.
The sturdy Blue Mountains Ash (Eucalyptus oreades), with smooth white trunks up to two metres in diameter, is also fire sensitive but it takes care of its future in a different way. It had to, the Blue Mountains are subject to frequent fires. It suppresses the growth of its saplings and they put their energy into flowers instead of wood. They become sexually mature at ten years and sprinkle the forest floor lavishly with seed that the old trees can no longer produce. When fire or wind or timbergetter gives one of these trees room, it grows quickly.
In The South West Book published by the Australian Conservation Foundation in 1979,W. D. Jackson gave a fascinating account of how fire determines the composition of Tasmanian forests. It has absolute control of what grows. A fire every ten to twenty years produces a buttongrass plain with belts of scrubby eucalypts. When the period extends up to one hundred years eucalypts dominate a dry understorey of Olearia, Acacia (especially the cream-flowered dealbata), Pomaderris, one of the several shrubs known as Dogwood with big heads of yellow flowers, and some rainforest shrubs in damp patches. One hundred to four hundred years between fires produces mixed forest; the thinning eucalypts are the tallest at ninety metres but many rainforest trees to forty metres have spread under them. In wetter places rainforest and tea-tree, Leptospermum, have taken over. After four hundred years all the eucalypts die; they rot and feed a rainforest dominated by Atlantic Beech, Celery Top Pine and Leatherwood (Eucryphia lucida) that produces such distinctive honey. These trees maintain a damp atmosphere which discourages fire.
Short regimes always favour grass. For years during the 1950s and 1960s a section of the Coffs Harbour–Orara road on the north coast of New South Wales ran along a central contour of a mountain growing a eucalyptus forest. The uphill side of the road which burnt every few years from motorists’ cigarette butts or faulty mufflers carried a thick grass understorey. The downhill side where fire was reluctant to run grew rainforest.
The several species of resinous Triodia known as Spinifex in the Centre can produce tremendous fires. In 1975–76 one million square kilometres burnt around Alice Springs. That is 13 per cent of Australia, equal to the combined areas of New South Wales and Victoria . . .
Smoke aids the germination of many Australian plants. When the seeds of ninety-four species of West Australian plants proved unwilling to germinate, botanists subjected them to smoke from burning native vegetation. Forty-five germinated readily. These seeds germinated when soaked with water that smoke had bubbled through. In the field, smoke drifted over an area caused a plentiful germination in the next rain. The action of the smoke is not known; ammonia might play a part.
Australia: A Biography, University of Queensland Press, Brisbane, 2000