Trees grow short in the mallee. Little rain and poor soils stunt their growth. Over thousands of square kilometers, in semi-arid Victoria, South Australia and New South Wales, many mallee eucalypts reach just 6 to 7 meters high. The short trees burn well, especially when growing above Spinifex and, after wet years, above flammable Spear-grasses. In 1974, fires burnt over 650,000 ha of mallee in south-west NSW.
Large fires kill most of the mallee tree trunks. The trees survive by resprouting from the lignotuber (or ‘mallee stump’), and the new coppice stems grow up to form the iconic, many-trunked trees. Like an axe with new handles, each tree (or more precisely, each lignotuber) is old, but the trunks and branches are younger, and date from the most recent fire.
About ten years ago, a team led by Mike Clarke and Andrew Bennett (from La Trobe and Deakin University) turned the coppicing habit of mallee trees to their advantage. They wanted to work out the fire history of the mallee, to discover how often different areas burnt, and to find out the age of every stand of trees. The boundaries of older fires are poorly known, so they headed to the field to gather new evidence.
A formula for fire
First the team first visited lots of sites where they knew the date of the most recent fire. There they measured the diameter of thousands of post-fire coppice trunks. Because they knew when each area burnt, they knew how old the trunks were. As you’d imagine, trunks in older patches were bigger than those in recently burnt areas.
Back in the lab they ran the numbers. They developed a formula that predicted trunk ages and fire dates based on the diameter measurements. ‘Awesome’ they cried. Armed with their nifty formula, they visited areas where fire dates were unknown, and measured thousands more coppicing trunks.
Back to the lab for more number crunching. They typed in the new measurements, sipped their chai lattes, and gazed at the stats package as it spat out the estimated stand ages and fire dates for all of the unknown areas. Bingo.
Their method isn’t perfect; the formula under-estimates the real age of old fires and old trees to some extent. Nevertheless, the team created a practical way to work out when every patch of tree mallee last burnt and the approximate age of every stand of mallee eucalypts – across thousands of square kilometers.
Counting the scars
Their simple technique helped re-write the fire history of the mallee. Before their field study, the team mapped recent fires from the fire scars seen on satellite images. Satellite mapping gives accurate information on all fires after 1972 – the year of the first Landsat image – but nothing before then. Many older fires are known, but their boundaries are uncertain or imprecise.
The team’s map of Landsat fire scars showed that 40% of tree mallee burnt between 1972 and 2007. The bulk of the mallee – a sizeable 60% – sits in the category ‘old mallee’, burnt more than 35 years ago. ‘Old mallee’ means ‘older than Landsat’, nothing more. Nevertheless, in the absence of better information, the satellite fire map created the impression that much of the mallee was ecologically old and perhaps in need of another burn. In Mike Clarke’s words, based on the information available to them:
the managers of one reserve in New South Wales were really keen to carry out a landscape mosaic burn to reduce what they perceived as an over-supply of old-growth Mallee … that is [mallee] greater than 35 years of age.
The world before Landsat
How old is this pre-Landsat mallee? Were the ‘old’ stands last burnt 40, 80 or over 100 years ago? To answer this question, the team divided the big chunk of old mallee into age classes using the fire dates they calculated from their tree measurements.
They found that the pre-Landsat mallee contained a huge diversity of age classes, ranging from 35 to over 160 years old. Remember that their formula under-estimated the real age of old stands. This means that the 160 year old stands could be 200 years old or more.
Importantly, they discovered that really old stands of mallee, over 100 years old, are really rare. Most pre-Landsat mallee is Gen-X and Baby Boomer mallee, 35 to 70 years old. Only 5% of mallee sites were over a hundred years old. There is no surplus of ancient mallee.
Dead stems do talk
But wait. There’s more. Burnt mallee trees have live and dead trunks. The live trunks came up after the most recent fire. The dead trunks came up after an earlier fire and were killed by the most recent fire. By measuring the live trunks, the team worked out when each stand was last burnt, as described above.
Dead stems tell the stories of earlier fires. By measuring lots of dead trunks, the team could estimate the date of the second last fire. The dead stems disclosed the time between the two most recent fires (the ‘inter-fire interval’) and gave a glimpse into the ‘invisible mosaic’, the hidden pattern of past fire regimes.
A new fire ecology blossoms
If we don’t know the age of a stand of trees, a patch of bush, we don’t know much at all. We can’t predict how quickly (or slowly) habitats will change, hollows will form, litter and fuel will accumulate. We can’t say how or when the fires we light will alter ecosystems in the future. Accurate fire histories underpin future ecologies.
Mike Clarke and Andrew Bennett’s enthusiastic team unlocked the mysteries of ageing in the mallee. In future blogs I’ll describe more of their results. Their many papers have been published in the best ecological journals. But they all stem from one simple field technique. Everything sprouts from a burnt mallee stem. From little things, big things indeed do grow.
Avitabile SC, Callister KE, Kelly LT, Haslem A, Fraser L, Nimmo DG, Watson SJ, Kenny SA, Taylor RS, Spence-Bailey LM, Bennett AF & Clarke MF (2013) Systematic fire mapping is critical for fire ecology, planning and management: A case study in the semi-arid Murray Mallee, south-eastern Australia. Landscape and Urban Planning 117, 81-91.
Clarke MF, Avitabile SC, Brown L, Callister KE, Haslem A, Holland GJ, Kelly LT, Kenny SA, Nimmo DG, Spence-Bailey LM, Taylor RS, Watson SJ & Bennett AF (2010) Ageing mallee eucalypt vegetation after fire: insights for successional trajectories in semi-arid mallee ecosystems. Australian Journal of Botany 58, 363-372.
- 100 years of habitat change: an animated fire ecology
- Reading the bush: juxtapositions in history
- Growing old in a shrubland: gravity always wins