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.

Just the facts #4

Recently I was asked

Why are trees tall?

The simple answer to the question is

Because other trees are tall

but I think this deserves some qualification.

Most people are familiar with the theory of evolution by natural selection, in which the individuals most suited to their environment successfully produce more offspring than less suited individuals. The concept is most commonly reduced to a truism "Survival of the fittest", and is just as commonly misunderstood.

Many people have the idea that fitness means the strongest, fastest or largest individuals will be favoured in selection. This is obviously not the case, as there are far more species smaller than a tennis ball than larger. In fact, there are vastly more species of life on earth smaller than the eye can register than those we humans can view. So clearly bigger is not necessarily better when it comes to natural selection. Except when it is.

The fiercest competition for survival is not between species, for example the rabbit and the fox, or the antelope and the lion, but within species. Because each individual of a particular species is competing directly with every other member of their population for all resources: food, shelter, territory, mating partners, etc; the competition is far more direct and consequential than the occasional dodging of a potential predator.

This brings us back to the tall trees. In certain forest systems, trees tend to be taller, and with fewer low branches than where trees are spaced further apart by natural means (for example water availability) . The trees which germinate and grow tall the fastest are favoured, and contribute to the failure of their immediate competitors by blocking essential sunlight to slower growing trees of the same species.

This fast growth is influenced by environmental factors, such as rainfall, and physiological factors, such as the natural etiolation (or elongation, a process separate to actual growth) of stems in shade. But it is fundamentally coded for in the genes of the tree species. So, even when grown outside the forest, forest trees retain some of the height of their counterparts in natural environments. The genetic information of the species is known as the genotype,, and this, in combination with environmental factors, goes on to produce the final shape of the individual, referred to as the phenotype.

Growing species outside their natural environment does not always result in similar looking specimens, however. A famous example is the Lesser Flamingo (Phoenicopterus minor), the common, pink wading bird from the Rift Valley in Africa. In its natural environment, the bird feeds on a kind of algae or cyanobacteria which are metabolised by the birds, and give them their distinctive colour. removal of the natural food source of the bird results in their feathers fading to white over time. The genotype of the bird allows for their pink plumage, but the phenotype is influenced directly by environmental factors.