Winners and losers from fire in northern Australia’s tropical wet dry savanna.
It was early in the season for thunderstorms but the thunder rolled as the lightning cracked and hit the dry grass lighting a small spot fire. The annual sorghum grass in the savanna country in northern parts of Australia had only just turned brown because of the earliness in the dry season but was just brown enough to burn steadily. A Gouldian finch and her flock whirled and swooped in the wind of the storm and came into land on another patch of grass, hungrily looking for seeds. The Gouldian finches pushed hard through the grass to feed on the grass seeds lying on the ground. Then the finches sensed the fire and smoke and they took off as one, into the wind and away from the fire to a patch of grass that had been burnt months before with not much left to burn now. The finch and her flock were safe from the fire and she had a sense of satisfaction because instinctively she knew that they would be back the next day, when the smoke had nearly gone, to eat the seeds left on the ground with no grass to get in the way. The small fire had made the grass seed accessible to the birds without burning the seed and made the Gouldian finch and her flock the winners that day.
Meanwhile, not so far away a golden-shouldered parrot sat on her nest which she had built in her favourite spot on top of a termite mound. She liked it there because she could see all around and if predators came would warn them off. But over the years the Melaleuca trees had grown up next to the termite mound which enabled the butcher birds to perch very close to the parrot’s nest and she wasn’t able to keep the butcher birds away and they stole her baby birds when they hatched. There hadn’t been a hot, high flame fire for 20 years and the Melaleucas had been able to grow. The low, cool fire lit by the lightning nearby was kept small because it hadn’t been that hot and the grass was still partly moist from the humidity and from not curing completely yet. Many small fires and no hot fires which could burn the woody Melaleuca, had resulted in the golden-shouldered parrot being susceptible to attack from the butcher birds perched nearby, and being the loser because she wasn’t able to defend her chicks who would have been the next generation.
The low fire crackled as it spread through the sandstone rock outcrop country, it made a dramatically rugged and beautiful scene, the many gullies and rock high lands, dotted with low Acacias and clumps of grass, and a broken line of low, red and yellow flames creeping across the landscape. The small Acacia’s are killed by fire but set plentiful seed which remains in the soil ready to germinate after the fire. Perennial Spinifex grass burns readily as its leaves are rich in resin and are held upright which creates ventilation for the flames, it can also be killed by fire but it sets seed quickly, which is left protected in the soil and ready to grow the next generation.
The Acacias grow amongst the Spinifex but the two have different adaptions to fire. They can grow together because the frequency of fire is low as a result of the bare rocks providing a break in the fuel and sheltering many plants. Fires can invade by burning material being blown in and direct lightning strikes but the frequency is still low. Acacias require up to 10 years between fires in order for the new plants to grow to maturity and flower and set seed taking possibly 5 years and then more years of seed set to build up an adequate seed bank in the soil. Acacias are the winners here, even though they have been killed because of the fresh stand of newly germinated seedlings, but can easily become the losers if another fire comes too soon. Too frequent fires, generally human lit, can result in local extinction of fire sensitive species like the Acacias in the sandstone country.
Another fire sensitive species is Callitris intratropica Cypress Pine which is killed by hot, high flame height fires, but benefits from frequent cooler, low flame height fires which reduce the annual fuel load of dry grass and fallen leaves, therefore reducing the chance of a large, hot killing fire.
Fire management, the timing, frequency, intensity and size of fires is a complex undertaking.
Traditional Aboriginal fire patterning, that is, fine-scale mosaic burning, leaving many areas burnt and unburnt, provides the temporal and spatial burning variation requirements to benefit the most species, making them the winners.