Many ecosystems, diverse fire
So far in Fire Ecology 101, I've focused on individual plants and species. However fire also operates at the level of plant communities, vegetation types, ecosystems.
Ecosystems in Australia cover a very wide range. This is hardly surprising as our continent stretches from the monsoon tropics to cool temperate Tasmania, from deserts to mountains.
Variety in ecosystems is matched by variety in fire regimes.
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Many factors affect what type of vegetation grows where. Climate influences include temperature, rainfall, when rain falls during the year (seasonality) and how much rainfall varies between years. The first three photos on this page show vegetation growing in places receiving differing amounts of rain.
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Right (above on small screen): Mean annual rainfall approx. 1000 mm. Tall forest near Margaret River, WA. These smooth-barked Karri trees can grow to over 60 m. Most fires in Karri forest are understorey fires that do not burn the tree canopy, as these trees hold their branches well above the plants below them. When fire does get into the Karri canopy, trees recover through resprouting from their trunk and branches (epicormic resprouting).
Left (above on small screen): Mean annual rainfall approx. 600 mm. Mallee scrub on Kangaroo Island, SA. Mallees are eucalypts that have multiple stems arising from ground level; on KI they grow to about 8 m. Lack of separation between understorey shrubs and eucalypt branches mean fires are more likely to 'crown' here than in Karri forests. Regenerating stems arise from woody swellings (lignotubers) at the base of each plant, rather than from epicormic shoots on stems.
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Landscape characteristics also influence what grows where. Factors include altitude, slope, aspect and position (eg hilltop or valley), geology and soil type. Soils have many dimensions – depth, particle size (sand, silt, clay), nutrient levels.
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Variation in climate and landscape factors inevitably delivers a complex mosaic of vegetation types across the continent.
Right (above on small screen): Mean annual rainfall approx. 300 mm. Spinifex grasslands at Barrow Creek, NT. Note gaps between hummocks. This landscape is only prone to wildfire when rain produces ephemeral grass fuel to fill the gaps.
Photo by John Coppi, CSIRO Science Image.
Many of the factors that affect what type of vegetation grows where, also influence fire regimes. Here's some examples:
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Rainfall seasonality affects when fuel is dry enough to burn. Around Brisbane, the bushfire season peaks in September after the dry winter, and is usually over by summer when rain starts to fall and humidity rises. In Victoria, SA and Tasmania, where rain falls mostly in winter and summers are dry, bushfires occur much later in the season. Both the Ash Wednesday (1983) and Black Saturday (2009) fires in Victoria happened in February.
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When mean annual rainfall is above about 600 mm, wildfire follows drought, as we know all too well. However in the semi-arid and arid parts of Australia, wildfire follows rain. This is because the vegetation is less dense, ensuring fuel is not usually continuous. Rain brings up gap-filling, short-lived grasses, which dry out over subsequent months, providing continuous fuel all set to burn.​
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In hilly country, north-facing slopes dry out before southern slopes, and slopes dry out before gullies. Big wildfires happen when gullies dry out to the point where the landscape becomes continuous fuel ready to burn.
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In rocky places, soil is thin, vegetation tends to be low, and fire may be patchy. See photos below.
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Heath next to the ocean on Kangaroo Island, SA, almost 4 years after fire. The old dead stems are bent sideways - wind blowing off the ocean has pushed plants over, and contributed to keeping plants low. It Iooks like plants in this area formed a fairly continuous mat of vegetation / fuel before the fire, so almost everything got burnt.
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Heath and stunted trees on Bald Rock in northern NSW. Here, islands of vegetation are separated by expanses of bare granite rock, making fuel discontinuous. Thus even quite intense fire in the surrounding forest will burn only some of these miniature rock gardens.
Within climate and landscape limits, fire regimes can change vegetation, as we will see in upcoming Fire Ecology 101 topics. And vegetation, in turn, influences fire.
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All this means that even before Aboriginal people arrived in Australia, fire regimes across the continent were far from uniform. Note that fire has been driving evolution on this continent for a very, very long time – at least 65 million years.
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On top of these natural forces, we have at least 65 thousand years of Aboriginal stewardship. Over that great span of time, Indigenous Australians found ways to live in their fire-forged land. Indigenous custodians tailored their burns for their particular country, using different fire for different purposes and places.
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So fire regimes across the country are definitely not all the same: they weren’t through geological time, nor through the millennia of Aboriginal management, and they aren’t, and shouldn’t be, today. Some ecosystems are not fire-prone, nor in need of fire – these include rainforests, saltbush-dominated communities, mangroves and saltmarshes. And among fire-prone vegetation communities, fire regimes compatible with conservation vary widely. To highlight two extreme examples: tropical savannas in Arnhem Land, NT, burn about once every 2-4 years. Whereas the tall Mountain Ash forests of Victoria and Tasmania, which grow in high, wet country, are dependent on occasional intense wildfires, which ideally would be many decades, or even centuries, apart.
​Top left (top photo on small screen): Early dry season indigenous burning in savanna, north-east Arnhem Land, NT, keeps country healthy (photo by Cate Storey).
Bottom left (middle photo on small screen): Mountain Ash (Eucalyptus regnans) forests rely on occasional intense wildfire for regeneration (photo by David Clode, Unsplash).
Above (bottom photo on small screen): Saltmarsh and mangroves at Nudgee Beach, Brisbane, are neither fire-prone nor fire-reliant.
I won’t try to cover the whole wide range of Aussie vegetation, on this website. Currently I’m planning to address two broad categories of fire-prone ecosystems: grassy, and shrubby. I’ll focus on temperate and subtropical regions, and on the eastern side of the continent, though other places may get a mention.
I’m going to start with grassy ecosystems. Why? Because they’re in dire need of TLC, often go unrecognised, are misunderstood or even maligned, and are – or were – an iconic part of the Australian landscape. Plus they’re beautiful, in an understated, Aussie sort of way.
Contrasting fire regimes