Fire and Plant Responses

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The evolutionary adaptation of eucalypts to fire

Most eucalypt species have adaptive traits ensuring their survival-even after very intense fires. Moreover, following the recovery of tree crowns, eucalypts may grow, for a number of years, more vigorously than they did before the fire. There are a number of hypotheses which may account for the adaptation of eucalypts to fire in these and other ways.

1. It is often said that eucalypts evolved in a fire environment; that is, such characteristics as lignotubers, epicormic shoots, and thick bark are taken to be direct adaptive responses to fire. This concept suggests an evolutionary interdependence between eucalypts and forest fires and presupposes that eucalypts may have evolved in the direction of large fuel loads and high flammability as a mechanism of attracting fire to itself, and through the response of regeneration to fire, ensuring its continuing existence. 
2. Alternatively, eucalypts could have evolved in a largely fire-free environment as a nomad which is a species able to exploit disturbed and other specialized niches in rainforests, such as rock walls, riverbanks, earth slides and lava flows. The characteristics of the present-day eucalypts which enable them to regenerate and grow rapidly after fire may have developed in this way (i.e. they may not necessarily have been acquired through long adaptation to fire regimes in the distant past).
3. A third hypothesis is that adaptation of eucalypts to fire may be seen primarily as a by-product of its evolutionary adaptation to declining soil fertility and a drying climate. The outstanding capacity of eucalypts to respond to a number of environmental stresses could reflect, in turn, prior adaptation of its progenitor to more specialized and disturbed niches within the Gondwanan rainforests.

While it is possible that all three pathways played some role in their adaptation to fire, it remains difficult to accept that some of their more significant growth attributes represent a direct evolutionary response to fire. Both the lignotuber and the epicormic shoot have biological and ecological significance in no way connected with fire. The lignotuberous habit has been seen as a cardinal attribute contributing to drought tolerance and occupancy of infertile sites. New shoots which arise from reserve buds within the tree crown, and along the bole and branches of the tree after fire (epicormic shoots), can also be a response to other agencies which defoliate or weaken the tree, including insects and drought. Moreover, and perhaps most significantly, it is the epicormic shoot that maintains the eucalypt crown throughout the prolonged mature and overmature growth stages (see Growth Habits of Eucalypts), and contributes to the dynamic use of a limited nutrient pool within the tree.

If, as seems likely, some of these growth attributes reflect environmental adaptation to infertile soils, then low levels of foliar and litter nutrients, and high flammability, might be linked in some way. Litter with low nutrient concentrations will affect flammability indirectly through its slow rate of decomposition and, hence, rapid accumulation of flame energy. It has also been suggested that there could be a more direct linkage between nutrient concentrations and flammability of biological materials. Sclerophyll-type tissue is seen as an evolutionary response to declining nutrients in the soil in an aging landscape when eucalypts were evolving. It also tends to have greater calorific value than plants growing in better environments. Flammability and adaptation to soils with low nutrient levels and other environmental stresses may be linked in this way, rather than in the sense that a high concentration of nutrients in the leaf will determine flammability directly by acting as a sort of flame retardant.

The response of individual plants to fire

Eucalypt forests will respond to fire in different ways. At one end of the spectrum forest trees might be totally defoliated and the shrub stratum burned back to ground level. Yet within a few years the tree crowns and understorey shrubs may have recovered their pre-fire status. At the other end of the spectrum, both trees and shrubs may be killed by fire, so that eventual recovery of the forest ecosystem will be possible only through regeneration by new seedlings. Between these two extremes, recovery may come from a mixture of processes involving vegetative reproduction and new seedling regeneration, the contribution of each depending on the characteristics of the forest, its stage of development and the intensity of the fire. Thus, there is no single pathway of recovery in eucalypt forests. In order to predict the effect of fire on a community, it will be necessary to appreciate the survival and recovery strategies for each of the species making up the community.

Survival strategies

Woody plant species can be classified by their response to fire into one of two broad classes:

1. the non-sprouters: those in the reproductive phase which will not recover through sprouts (epicormic shoots developing from reserve buds) following 100% leaf scorch, but depend entirely on seed and seedling regeneration; and
2. the sprouters: those which will recover from 100% defoliation through stimulation of reserve buds below the ground (subterranean regenerative buds) or above the ground (aerial regenerative buds).

Table 1 provides a useful classification on the response mechanisms of plants commonly found in forests to fire.

Adapted from Gill, A.M. (1981). Adaptive responses of Australian vascular plant species to  fire.  In Fire and the Australian Biota, eds Gill, A.M., Groves, R.H. and Noble, I.R. 

Release of seed stored on the plant

There are many Australian forest species with woody fruits which release seed in response to fire, although with the possible exception of some Banksia species, fire may not be an absolute prerequisite for seed release. Accelerated dehiscence of fruits at time of fire has been noted for members of the families Myrtaceae (including the eucalypts), Casuarinaceae, Proteaceae and Cupressaceae.

There are a number of forest taxa in which dehiscence of the fruit and release of seed will occur only after prolonged desiccation or death of the supporting branch, or direct contact of the fruit with flame. The woody follicles of some Hakea (Proteaceae) species open upon desiccation of the parent branch, and this normally occurs upon the death of the plant. Alternatively, dry, woody fruits of Banksia ornata may not open unless they have been in direct contact with flames. Other Banksia species showing fire-dependent dehiscence are B. ericifolia, B. serratifolia and B. asplenifolia on the coastal heathlands of New South Wales.

The seed of the eucalypt is normally released from its woody capsule at the end of a seasonally dry period (e.g. late summer to early autumn in southern Australia). Depending on weather conditions, the release of seed may be sporadic over an extended period, or a large part of the seed crop may be east in a relatively short period. A fire which scorches the crown of a eucalypt but does not burn the capsules may trigger a near total release of seed from a mature capsule crop soon after the fire, sometimes leading to `wheatfield' regeneration.

Seed stored in soil

One of the notable responses of some plant communities to fire is the way in which seed that has accumulated in soil, often over very long periods, will germinate, sometimes in remarkable quantities. This applies mainly to small trees and shrubs with hard-coated seeds. It does not apply to eucalypt seed which will remain viable only a short time in soil, probably no more than 6-12 months. Germination of soil-stored seed may also follow mechanical disturbance of soil, for example, tractor-working during logging, although the germination response may be more sporadic and stocking density much less spectacular.

Species with hard seed coats show a lack of imbibition, swelling and softening of the seed when exposed to water. The seed may germinate quickly only when the seed coat is softened, cracked or removed. High temperatures following fire may lead to cracking of the seed coat, and this may also occur as a response to high soil temperatures during summer and daily temperature fluctuations. Softening of seed may also be induced in nature by scarification in stream beds or passage through the gut of animals, particularly birds.

Hard seededness is a common property among leguminous plants (Fabiaceae, sub-families Faboideae and Mimosoideae), but is also found in a wide variety of plants including species of Anacardiaceae, Asteraceae, Malvaceae, Poaceae and Proteaceae. Occasional fires may make a positive contribution to the forest ecosystem through the germination of leguminous and other species with soil-improving potential. That contribution is enhanced through the highly durable nature of the seed of many of these species, and their long-term survival in soil.

Where an overmature, mixed eucalypt-rainforest community is felled and the debris burned, massive Acacia regrowth may develop very rapidly from soil-stored seed. This seed may have accumulated in the soil following an earlier fire, which had established the community in the first place. Seed of woody species other than Acacia may not survive in the soil more than 100 years. Seed of some non-woody species may also remain viable after being stored in soil for a long time; for example, grass, herb, rush and sedge seeds may germinate in soil from a Nothofagus forest, even though these taxa were not present at the time of soil sampling.

The environmental conditions needed to release seed from the hard seeded condition will vary with species. Within the sub-tropical forest, Dodonaea, Acacia and Kennedia species all require heat to stimulate germination, although the response will vary with the soil temperature, the duration of high temperature, and the depth at which the seed is buried. This also means that for each species there is a critical temperature and depth at which seed may be killed. For example, Acacia seeds are more readily destroyed in the soil by high temperatures than are Dodonaea and Kennedia seeds. Thus, it may be possible to manipulate the development of fireweeds, as regards species and abundance, by paying attention to the way slash is distributed, the intensity of the burn, and the judicious use of tractors in preparing for a slash-disposal burn.

Plants with reserve buds below the ground

Buried buds may be present in vertical stems or rhizomatous roots and are characteristic of dicotyledons and coniferous trees and shrubs. Plants with reserve buds protected by soil will normally survive fires which destroy the aerial parts of the plant. The amount of heat partitioned to the soil from fire is low, being of the order of  5% - 10% of the total released by the fire. In addition, the soil is a very effective insulator so that high temperatures are confined to shallow depths of soil.

Where the aerial parts of the plant are destroyed by fire, regeneration may take place from a single buried vertical stem; that is, the plant cannot spread in this way. Where regeneration is from roots or horizontal rhizomes, the chances of spread and multiplication of the plant are enhanced, at least immediately after a fire. Plants with root buds include Acacia dealbata, and with rhizomes, Leucopogon suaveolens and a few eucalypts from northern Australia.

Many species of the Australian flora will recover from severe fire by the response of buds in the stem below or just at ground level. Eucalypts with their lignotuberous habit belongs to this group. The actual lignotuber is most apparent at the seedling stage and young sapling stage and, usually, will be incorporated in the stem as it develops. In the case of the multi-stemmed mallees, the mature stage is characterized by a massive buried lignotuber.

The lignotuberous habit has been described as a response to a range of environmental stresses, of which fire is but one. Ecologically it is most significant on dryer or otherwise environmentally harsh sites. Here it may take many years, or a succession of favourable seasons, for a newly established lignotuberous seedling to reach that stage where it is capable of growing vigorously through sapling and pole stages. Thus the presence of a more or less permanent lignotuber pool may be vital to the recovery of woodland or lower quality forest following a major perturbation. Alternatively, the non-lignotuberous eucalypts are mainly those which are restricted to sites with good moisture relationships (E. regnans - alpine ash, E. delegatensis - mountain ash, E. fastigata - brown barrel, E. grandis - flooded gum, E. pilularis - blackbutt), or in the case of the southern provenances of E. camaldulensis (river red gum), a species able to thrust a vigorous tap root through saturated soil following flooding.

Plants with reserve buds in the stem

Where a plant has reserve buds along the bole and branches, it may respond very rapidly to the loss of its green crown by producing a massive number of new shoots, as in the case of eucalypts.  If reserve buds are not present in a plant, it will probably be killed by a fire of sufficient intensity to scorch the complete crown.

Trees and shrubs with reserve stem buds will depend upon the insulating properties of bark for any resistance they may have to fire. Measures of thermal diffusivity or insulating capacity suggest that all eucalypt barks have generally good insulating properties, although there may be some differences between them. Stringybark and gumbark species of the same thickness generally have similar insulating characteristics, a feature not  readily appreciated. The resistance to fire will be influenced not only by the thickness of the bark at the time of the fire, but also by the amount of bark lost during or soon after the fire. An appreciable amount of fibrous bark may be lost where the outer bark is burned during a fire. Alternatively, a gumbark species may lose thickness after a fire as a result of the abscission of the outer layer of the bark, the extent of the loss being a function of the duration and temperature of the fire. 

Young eucalypt stems survive damaging fire in two main ways. by epicormic shoots developing along the stem and branches, and by shoots developing from epicormic buds protected from lethal temperatures by soil surrounding the base of the stem. 

The flowering of plants following fire

Some plant species are stimulated into flowering after a fire.  Many of them are monocotyledons belonging to the families Xanthorrhaceae and Cyperaceae.  The response may represent a direct selection for increased seed production after fire, but it could also be related to a number of environmental pressures.

Source: Modified and supplemented by J.A. Duggin, from Florence R.G. (1996). Ecology and Silviculture of Eucalypt Forests. 

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