Australia: Living in a Land of Fire

Over most of Australia (excepting perhaps only the rainforest pockets and the alpine systems) fire is a natural, even essential, part of the ecology, and has been so for at least 20 million years since the land began to dry out and the narrative of receding rainforests and expanding dry forests, woodlands and grasslands began to unroll. During the current Quaternary Ice Age (ie the past 2.6 million years) the typical 'modern' sclerophyll - ie hard-leaved vegetation - began to appear and fire became more dominant in the landscape.

Aftermath of a managment burn, Townsville Common, Queensland.

The presumption (still occasionally asserted) that fire in Australia is a human construct, or that fire ecology was human-sculpted, needs to be put into that context. If you were to drive across Australia from Perth to the east coast (some 3,300km), tracing the history of fire in the continent with kilometres as years, you would reach the start of human influence just 8km from the east coast. Even if we consider just the current ice age, the time of greatest fire influence, we would be within just 60km of the end of the journey before people entered the equation.

This is not to say that humans did not manage the landscape via fire, and influence it to varying degrees.

Managed burning of spinifex grassland, central Australia.

People used fire, in a very directed and sophisticated manner, to tilt the ecological balance in favour of large grazing herbivores, as well keeping portions of the country open for ease of usage and almost certainly even wildfire control. (It continues to be suggested that we should try to rediscover and employ indigenous fire management in national parks, but their management now is for an entirely different purpose from that of our management predecessors.) Over much of the country the knowledge has been lost, wholly or partly, but in some remote traditional areas it is still applied.

'Hunting burns', Great Sandy Desert, Western Australia, above and below.
People, (mostly women, I understand) of the Kwirrikurra community, burn small areas to expose
and flush out small animals, and to encourage a patchwork of habitats.

Before I leave this interesting and important part of the discussion, I must at least acknowledge the impact a recent book, The Biggest Estate on Earth by Australian National University historian Bill Gammage, has had on the debate. Meticulously using a wealth of historical accounts, he concludes not only that people managed the land, but did so at a continent-wide scale to the extent that no 'wilderness' remained by the time Europeans arrived. It is inevitably controversial, but has sparked a very valuable and ongoing debate. Bill's a former neighbour and ongoing friend, who I greatly admire, but I don't agree with all his conclusions, a situation which I see as perfectly normal and healthy. For a couple of randomly-selected and well-argued opposing reviews of his book, see here and here.

However, today I want to focus more on how plants have adapted here to surviving in this Land of Fire. Firstly however I should be more precise than I've been so far. Organisms and landscapes don't adapt to a fire, but to a fire regime, which is defined by intensity of 'normal' burns, their frequency and the season they usually occur. This is a much more complex evolutionary challenge. However there is no doubt that Australian ecosystems respond, and quickly, to being burnt, no matter how devastating the immediate aftermath may appear.

Burnt Mulga (Acacia aneura) woodland, central Australia.

Burnt Mulga woodland starting its recovery, north of Alice Springs.
Importantly, this was in the midst of an intense drought.

In January 2003, fires of a scale and intensity locally unprecedented in European times, burnt vast areas of the New South Wales and Australian Capital Territory mountain forest and adjacent lowlands - including over 500 homes in Canberra, the nation's capital. (I wrote in some detail about it here if you're interested, on the tenth anniversary of the fires; even now I don't find it easy to reread and I'm not going to revisit what I wrote then.) My photos of the immediate aftermath are few and muddily pre-digital, but the later shots of the recovery show both the immensity of the original event, and the wonder of the recovery.

Hypoxis hygrometrica flowering in an otherwise entirely blackened landscape.
I can truthfully say that in trips into the mountains in the days following the fires I didn't see a
single green tree. (Again, see the link above for more detail.)

Ten years later. The view west into the Goodradigbee Valley from Mount Franklin.
The original trunks of the Snow Gums are still dead, but they are vigorously regrowing from the base.

OK, so what actually happens to make the landscape recover? There are a couple of basic strategies at work, and both can be seen in this photo, taken in Morton National Park, south of Sydney, a couple of months after the fire in January 2013.

Both the seed cases retained on the trees, and the ground-sprouting, are part of the process.

For this purpose we can divide fire-adapted plants into 'sprouters' and 'seeders'.  The former survive (though all above-ground parts may be destroyed) by reshooting after defoliation; this epicormic growth is a ubiquitous characteristic of an Australian landscape.. Seeders are killed, but regenerate from seed. In both cases, only mature plants are expected to be able to respond in this way.

The buds from which the shoots grow are there under the bark all the time, striving to burst forth but held sternly in check by the hormonal influence of the crown; these buds may be under the bark of branches and trunks, or under the bark of roots. When defoliation occurs though, (which may also be  through insect attack or drought, as well as fire), the buds are released to provide the necessary energy-fixing photosynthesis until the crown recovers. At this point the hormonal block is re-imposed and the shoots die back.

Epicormic growth, Morton NP, above and below.

 

New epicormic growth on Red Stringybark Eucalyptus macrorhyncha,
Aranda Ridge, Canberra

 As suggested in these photos, epicormic growth is mostly a eucalypt trick, but not exclusively. One of the requirements of this strategy is very thick or heat-resistant bark. While outside temperatures may be 400 degrees C, millimetres away beneath the bark the temperature may be only 40 degrees C.

Epicormic growth on Broad-leafed Geebung Persoonia levis, above, and
Leptospermum sp., below, Morton NP.

After the 2003 fires I was surprised to see widespread epicormic growth on River Oaks, Casuariana cunninghamiana. More recently my friend Martin reported such growth on acacias after a fast-moving grass fire destroyed properties east of Canberra; some of his photos here. 

On the other hand underground buds are possessed by a range of plant species. Soil is an excellent insulator, and most – 95% in fact – of a fire’s heat rises, so underground is a good place to bury your fire insurance. Eucalypts again have developed this to a fine art, with most of the known eucalypt species having ‘lignotubers’ – a massive woody subterranean structure bearing repressed buds that function in much the same way as the epicormic ones. For subterranean root buds of shrubs in particular, the inhibiting factor seems to be the existence of the stem. When it dies, root suckering commences.

Eucalypts sprouting from underground lignotubers; these four photos were taken in Morton NP.

Lomandra formosa.

A sedge (sorry not to be more precise!).

Lomandra sp. (a lily relation, family Asparagaceae).

Which brings us to the seeders. Most flowering plant species have ‘populations’ of seeds as well as of living plants. There are two essential places to store these seedbanks; ie on the plant, or in the soil. In general obligate seeders which store seed in the ground are shorter-lived than those whose aerial seed banks die when the plant does. In the former case the seeds don’t ‘need’ their parents; in the latter they do. In addition to the ‘ash bed’ effect, providing nutrients for the seedlings and the relative absence of competition (especially from overshadowing mature plants), it has now been shown that burnt soil is ‘sterilised’ of harmful bacteria and fungi which inhibit seedling growth. Additionally, work pioneered in Western Australia has shown that chemicals in smoke can stimulate germination.

Seedlings in burnt soil, Morton NP.

Some species retain most of their seeds on the plant, in the fruit. Such seeds are themselves heat-sensitive, but are protected by well-insulated seed cases. Such species include Casuarina, Hakea, Banksia and some native conifers. This mechanism – of only dropping seed after a fire – ensures that spring seedlings are not destroyed by summer fires without having reproduced. However they must wait a few days before opening, so that the seed is not dropped and cooked.

Banksia cones, opened to drop a single seed from each capsule, Morton NP.

Casuarina cones, likewise having shed their seeds, Mullewa, Western Australia.

Petrophile sp., Morton NP.

Perhaps counter-intuitively, seeders tend to dominate the rarely burnt canopy of wet sclerophyll forests. They do not wait for a fire to seed though; in the case of the ash eucalypts (named not for fire association, but for a European tree with apparently similar timber), normally they do so annually but hold the accumulating seed on the tree for some years after flowering. At any moment there are several years’ crop of seeds on the tree. When the capsules’ pedicels die or are killed, as by fire, the seed literally rains down, saturating the capacity of seed-foraging ants to carry them all away. Normally we would not expect any of this seed to successfully produce a sapling that emerges from the understorey; it is only when the understorey has been cleared by fire that the seedlings can prosper. This is why an Alpine or Mountain Ash forest, unlike most other forests, eucalypt and otherwise, tends to comprise even-aged stands of trees.

Even-aged stand of Alpine Ash E. delegatensis, Namadgi National Park.

Most plants however constantly release seed as they produce it. For these species the seedbank is in the soil and the seed population may long outlive the generation of plants which produced it. Many such seeds – and this includes many wattles and peas – are unlikely to germinate without a fire. That is, they actually require a fire’s heat to initiate germination. It is only when such seeds are cracked by the fire’s heat that water can be absorbed and germination begins. 

Senna sp. post-fire seedling, Watarrka (Kings Canyon) NP, central Australia.

The corollary is that lower intensity fires leave the seed bank of at least some species to await the optimal conditions provided by the aftermath of an intense fire. Some such seeds are very long-lived; Acacia dealbata seeds are extraordinarily persistent and it is suggested that they might wait in the soil for centuries for a fire.

The seedbank of other ‘seeder’ species does not survive fire; either seeds are kept on the plant or on or close to the surface of the ground without a fire-proof safe. In these cases seeds must be introduced to the area from elsewhere after a fire. Examples are mistletoes (carried in by birds) and some daisies (blown from elsewhere).

Some species are triggered to flower by the fire effects, such as leaf loss, the ash layer, or the increase in light. Apart from these ideal conditions, the mass flowering and seeding may increase the chance of pollination, and of escape from seed predators. Such flowering is known as pyrogenic.

Goodenia bellidifolia flowering post-fire, Morton NP.

Some 16 Australian orchid species virtually never flower without the stimulus of a hot wildfire the previous summer. Many others are stimulated into a mass flowering in the spring after a summer wildfire. Drosera spp. (sundews) do likewise. 

The Undertaker Orchid Pyrorchis nigricans (the 'blackish fire-orchid'), never flowers except after a summer fire,
here in the Brisbane Ranges, Victoria.
The strange common name, and the species name, arose because the pressed specimens used turned black!

In the first summer after the 2003 fires in the Brindabellas, for tens of kilometres along the ridge of the range the ground was carpeted with Prickly Starwort Stellari pungens flowers; this is a common enough herb that one can usually find scattered through the understorey, but such a display had not been seen in the ranges before or since. Clearly untold millions of seeds had been waiting in the soil for an intense fire to release their potential.

In the summer following the 2003 fires, tens of kilometres of mountain roads were white with
starwort flowers, as far as one could see in both directions.

After a fire, the regrowing grass trees, Xanthorrhoea spp., produce a spectacular flower spike, bearing up to 10,000 seeds. Timing is important though. Xanthorrhoea will flower the winter after a summer fire, but if the fire comes later, the flowering may be delayed until the second winter.

Xanthorrhoea flowering spikes, Brisbane Ranges NP, Victoria.

Needless to say, this has been a very abridged and simplified version of a complex and intricate story, but it's one that everyone living here should be aware of - and of course elsewhere in the world there are equally wonderful stories of fire adaptation. I hope it's been of some interest.

And as ever, I welcome your response.

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