It’s an ill wind...

Press, 1 May 1981, Page 11

It’s an ill wind...

Gardening

by

M Lusty

The film “The Dangerous Summer” is now showing at a local cinema; it deals with bush fires in Australia, and focuses on one in particular. Although this film is a secondary feature, it' is well produced, especially when one considers the very difficult circumstances under which much of it was made. I feel that some further insight into the effects and consequences of such fires as applied to the natural vegetation was desirable, and I hope that this column may be of some assistance.

Bush fires are a regular occurrence in the Australian bush, particularly in summer. They occur in almost all regions to a greater or lesser extent, depending on the composition of the vegetation and prevailing weather conditions at the time.

Large fires can sometimes burn over many thousands of hectares devastating all in their paths. They are no modern phenomena resulting from the expansion of civilisation with its advanced technology, but occurred long before European : settlement, and even before the arrival of the Aboriginals centuries earlier.

Accidental, or deliberate, fires are now responsible for an incredible number of outbreaks, but many fires are started from natural causes.

Any major outbreak of fire may destroy not only crops and property, as it is fanned by changing winds or even

jumps many kilometres by means of wind blown bits of burning or smouldering material of some kind or another. It also razes natural vegetation of forest, grassland and scrub.

Fire is a natural hazard which the country has to contend with, just as with the extremes of temperature and rainfall. Over aeons of time plants have adapted themselves to exist with it. In fact, they have come to rely on the periodic fire to maintain the continuation of generations of their kind. The most obvious effect of fire is to remove competition by the destruction of trees, shrubs, and grasses. After a fire all that may remain is a bare expanse of ground with possibly a few charred skeletons of trees.

Following the next rains there occurs a natural transformation in the scene of scorched terrain. Early plants to appear are annuals which now have maximum light to grow and flower, seed and reproduce until once again competition from more permanent subjects becomes too great. Herbaceous plants, which survived as subterranean storage organs, grow away to produce their flowrs and continue to appear until they, too, succumb to competition from still stronger plants; terrestial orchids spring up in quantity in an area recently cleared of vegetation by a bush fire. The dominant trees in the Australian landscape are species of Eucalyptus. At the base of' their trunks is a meristimatic region known

as the ligno tuber, a storage organ for regeneration. Most noticeable as swellings near the cotyledons in seedlings, they merge together with age and soon become indistinguishable in developing trunks. From charred stumps, masses of dormant buds are stimulated into juvenile growth resulting in a large number of strong growing shoots. Eventually one of these may dominate to grow away and form a trunk whilst the remainder usually abort. Although this type of regrowth is most noticeable with Eucalyptus it occurs in other genera within the family Myrtaceae and also in some other families of Australian plants. Within a period of ten years an area of forest razed by fire will once again be covered by trees.

As well as mature members of Myrtaceae being able to grow away again, completely new plants appear in the area as seedlings and their appearance is largely as a result of fire.

Species of many genera within this • family have woody capsules which remain unchanged on the parent plant sometimes for many years. It takes considerable heat for then) to open up and this is where’the bush fire plays its vital role. As the flames engulf the vegetation the seed capsules are slowly forced open by the heat if mature enough or they drop to the ground to lie in the ashes where they gradually open so that the seed is shed in the vicinity; following rain a mass of seedlings springs up. Apart from Eucalyptus other members of the Myrtaceae family which have woody seed capsules include Callistemon, Calothamnus, Leptospermum and Melaleuca. Australian members of the Protaeceae are equally dependent on fire to release their seeds. -They are Banksia, Dryandra and ■Hakea.

The plants comprising the family Leguminosae is second only to Myrtaceae as the largest Australian plant group with woody members. Fire is important to them also in ensuring seedlings appear but for an entirely different reason from that previously mentioned. Seed coats of almost all Australian legumes are very hard and so seed may lie on the ground for years unable to absorb water which is required to set off germination. Seeds of this type need to have their seed coat damaged in some way before water can enter. Under suitable conditions bacteria and fungi can play a part in gradually weakening the testa but this is , a slow process which takes a long time.

A light charring resulting from a bush burn can cause sufficient damage to the seed coat to allow water to enter, and so legume seedlings appear in quantity as the aftermath of a fire.

There are a number of species of Acacia which become forest trees and grow in company with Eucalypts but they are shorter lived and less able to withstand competition. Mature forests tend to be of one or two dominant species, but these are not always Eucalypts.

Where rainfall is too low to support forest the resulting under storey is much richer in many different kinds of woody plants which are able to survive and thrive through less overhead competition and more light. Recolonisation of a fireravaged terrain is surprisingly quick. One £roup of plants gradually replaces another as the former fall away due to competition. It may seem unbelievable that regeneration of the area is possible when looking at the apparent devastation wrought by a bush fire immediately or ■ soon after it had died out dr been quelled. It really has to be seen to be realised.