Introduction
In the last 45 million years as Australia moved northward on its continental plate it began to change from a predominantly wet, forested country to one wracked by alternating dry and wet periods of the El Nino. As a result, both plants and animals have had to adapt to these changes in unique ways.
Currently (2003) in Australia the average area affected by fire is an astounding 30 million hectares a year, 90% of it in the tropical north of Queensland, Western Australia, and Northern Territory. Humans initiate almost all these fires, with only a small part being wild fires. In all an estimated 150 million tonnes of carbon are burned compared to 85 million tonnes of fossil fuels. Fire as practised in Australia is a major contributor to global warming.
Fire has some direct impacts on the soil. Heating temperatures of 300 – 425 degrees Celsius can cause chemical transformations in the top few centimetres of soil and at lower temperatures sterilisation of the soil including death of leaf litter flora and fauna occurs. Loss of nitrogen also occurs (up to 1.7 million tonnes per year in northern Australia) and this is important considering the naturally low levels of nitrogen in Australian soils. Smoke and soot travel great distances affecting climate. The ash created by the burning process is alkaline chemically when dissolved in water and can enter creeks and the subsoil affecting plant growth and the health of waterways.
Effects of frequent fires
Frequent fires can have many effects that are mostly negative. They can kill many plants before they reach reproductive age thus not allowing them to produce seed. If fires are repeated often enough the plant will become locally extinct.
Trees use their reserves of starch to grow new leaves, bark and branches so if burning occurs frequently they will eventually run out of reserves and die. Areas that are regularly burnt exhibit smaller and fewer trees. Trees often appear less healthy and may begin to look mallee-like.
Seeds require rain soon after a fire has occurred in order to germinate or they will soon die. Frequent burning without following rain will result in a loss of plants and possible desertification.
Effects of fire on heathland
Heathland is a specialised community within Australia. It is a low, dense community found on poor soils deficient in nitrates and phosphorus. Hence a lot of plants, particularly Acacias, and Casuarinas fix nitrogen on their roots. The leaves of heath plants are generally highly flammable and fire quickly burns through a heathland. Like other plant communities a heathland can suffer if burnt too often but requires a regular period of burning to maintain its diversity.
It was found in one study of heathland in South Australia 36 plant species were identified after a fire in 0.1 hectares of heathland; after 25 years this had dropped to 20 species and after 50 years the number had dropped to 10. Thus it is important for regular but well-spaced (in time) burning to occur.
Plant adaptations to fire
Plants unlike animals cannot escape so must be able to survive incineration or regenerate from a fireproofed seed bank. Plants use a number of strategies to survive fire.
Some trees and shrubs have epicormic buds buried deep within the trunk In normal conditions the crown of leaves on the tree produces hormones which inhibit the epicormic buds from sprouting. Once a fire has removed all of the leaves the hormonal influence is gone and the epicormic buds soon sprout covering the tree in a new flush of leaves and branches. In a severe fire even the epicormic buds on the smaller branches may be killed and the plant can only recover from the epicormic buds present on the main trunk. Within a few years the plant will have attained the same growth status it had before the fire. Examples include many Eucalyptus species, eg. E. haemastoma, E. Sieberi, some Angophoras eg. A. costata, A. hispida , grass trees (xanthorrhoea) and some Banksia species eg. B. serrata.
Coppicing is where a large root-like underground structure called a lignotuber is covered by buds which sprout after a fire has removed all of the leaves. This is a very effective strategy because the lignotuber is covered by earth, which is an excellent insulator against fire. This is a common practice of mallees and snow gums (Eucalyptus pauciflora).
Some plants have seed banks on the plants where they store their seeds within hard fruits to protect the seeds then open a few days after the fire has passed through. The seeds fall on a deep layer of ash, which supplies them with all the nutrients they need to germinate. There is also fewer plants to compete with the seedlings and they also get plenty of light as well. Plants that use this strategy include Banksia ericifolia, Hakea species and the she-oaks (Allocasuarina and Casuarina species).
Many plants create seed banks in the soil, where seeds have fallen, become buried and may stay dormant for many years. The heat of a fire destroys the hard protective coat allowing the seed inside to germinate. Examples include wattles (Acacia species) and many plants belonging to the pea family (Fabaceae).
Animal responses to fire
FIRE SURVIVAL. Unlike plants, animals can move to escape fire. Animals escape fire by taking refuge in a burrow either of their own making or one made by another animal. Reptiles, wombats, rats, and many invertebrates do this. Other animals take refuge in tree hollows, which are safe providing the fire isn’t too intense. Logs and rocks provide shelter for many frogs, reptiles, small mammals and invertebrates. Birds may escape the fire by flying over it or outrunning it. Leaf litter animals are generally destroyed.
POST-FIRE SURVIVAL. Predator pressure: Once foliage has been destroyed, small mammals like bush rats and antechinuses are exposed to predatory birds like owls and ground predators like foxes and cats. Consequently their population takes a sharp dive and may take up to 6 years after the fire to regain normal population levels. Tree hollow destruction: Over 40% of Australian mammals, a large number of bird species eg. Parrots and kookaburras, frogs and invertebrates rely on tree hollows for nesting and shelter. Regular burning will promote hollow formation in some plant communities but discourage it in others like the mountain ash forests in Victoria. Grazing animal survival: Animals like possums, koalas and ground-dwellers like wallabies and kangaroos suffer starvation if the fire is widespread. They will focus on new shoots appearing and destroy many plants before they can recover. Only in fire mosaics, where burnt areas are interspersed with unburnt areas will the animals survive and allow the bush to recover with minimal interference.