Tuesday, February 10, 2009

Fire in Australian forests: The Phoenix from the ashes

The bushfires of the last week on the east coast, particularly those in Victoria, have been tragic and devastating. Loss of human life and property, as well as death and injury among the fauna of the burnt areas are tragic and immediately confronting.The landscape itself seems completely dead in much of the area affected by the fires.

But the native vegetation we have come to love in this country not only responds well to fires, but in the case of many plant species, relies on fire for survival. Although the trees look black and dead now, and the understorey is completely gone in some places, it will not take long before signs of life begin to emerge.

Underneath the bark of many large trees, Eucalypts in particular, but other species also, are dormant buds. These epicormic buds are not actively growing for most of the life of a tree, but in times of stress, for example, when the tree is defoliated by animals or fire, the epicormic buds spring to life, and grow leaves and small branches to provide energy to the roots of the tree. This is very important, as when tree roots die, they cannot support the canopy. This epicormic growth will allow for the surviving branches to re-grow all their foliage and re-establish themselves much faster than new seedlings are able.

Some trees, especially among the Eucalypts, also have lignotubers below the ground, the soil providing even further protection from heat of passing fires. These trees will sprout new major limbs and trunks from ground level, and multi-stemmed trees in the forest are often the result of such a fire response.

Of course not all trees will survive, but even the dead stags provide shelter for animals, from wood feeding insects, to larger creatures such as birds and mammals, who may also feed on the insects. These animals take some time to return to burnt out areas, and their numbers will be slow at first to increase, but neighbouring populations and survivors will eventually recolonise the fire stricken areas.

At the soil level, the ash bed after a fire provides a fertile nursery for seed germination, of understorey shrubs, herbs and grasses, as well as new trees. The so called "soil seed bank" is an accumulation of seeds dropped every year by most plants, which have not been given the appropriate conditions for germination. Warmth and moisture in just the right quantities will germinate most seeds, and the clearing of the overshadowing plant canopy enhances this. This includes thousands of Eucalypt species which will eventually fill in the gaps in the canopy left by the trees that do not survive.

But even more reliant on fire, are the Wattles. Most Acacia species have a very hard seed coat. While occasionally passing through an animals gut will break it down enough to allow it to germinate, more effective is the application of heat. A bushfire will weaken or crack the dormant wattle seeds in the soil, and allow them to begin growing. The importance of wattles as a "pioneer species" can not be underestimated. The roots of all wattles are covered in nodules, which provide habitat for Rhizobium bacteria. These bacteria suck nitrogen out of the air and fix it in a form that plants can use for their nutrition. After a fire, nitrogen levels may be significantly depleted, and the wattles help restore levels to a productive state. They also provide shelter from sun and evaporation that allow smaller species to re-establish beneath them, before dying in a relatively short period of usually 20-30 years.

Banksias also rely on the heat from fires to crack open their seed cases, famously characterised by May Gibbs as Banksia Men. The seeds are held tightly in woody follicles and only released when heat is applied, most naturally by fires. The flowers of these trees provide a nectar source for birds and insects, and even some tiny marsupials. Allocasuarina species, sometimes called native pines, retain their seeds cases, or cones, on the tree, often until fire events cause them to fall, where they open and deposit seeds on the ground for germination. These trees also fix nitrogen in the soil, and are another important pioneer species after fires.

While it may look at present like a desolate and barren landscape, invisible to our eyes, processes are already underway to restore the bush to its familiar beauty. A beauty that actually relies on this periodic ordeal. Rather than point fingers at forest management practices as scapegoats for these occurrences, it would be far more useful to spend time and energy minimising their impact on the people who live with the dangers of the Australian forest environment, and protecting human life from these essential, and inevitable fires.

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