Global ‘gigantism' hotspot: Tasmanian tree standing at almost 100m tallest in the country
Most of the largest and tallest were Eucalyptus regnans, commonly known as mountain ash, including a tree known as 'Centurion', measuring 96 metres, according to new research in the Australian Journal of Botany.
Located in the state's Huon Valley, Centurion was once the world's second tallest specimen, behind 'Hyperion', a coastal redwood in California measuring 115.6 metres. But that was before bushfire damage in 2019 cost the mountain ash almost 4 metres in height and several rungs on the global ladder.
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It is now ranked sixth among the tallest living species, behind the coastal redwood, a Himalayan cypress, sitka spruce, douglas fir and a tropical species from Malaysia, Shorea faguentiana.
This made Australia, and especially Tasmania, a 'global hot spot of gigantism in plants', according to co-author Dr David Bowman, a professor of fire science at the University of Tasmania with a background in eucalypt ecology.
Bowman said Tasmanian eucalypts were the 'kings and queens of the forest' that were achieving 'the physiological limit of what a giant tree can be'.
They were able to reach their enormous size – in height and mass – due to Tasmania's 'cool nights, beautiful growing conditions in the day, an abundance of moisture', and had grown over hundreds of years without disturbance.
Most of the 25 tallest trees in the state ranged in age between 320 and 500 years old, and according to the paper, they towered above the tallest known trees in other states.
Victoria's tallest tree, a mountain ash dubbed 'Slinky Sloan', measured 93 metres. Western Australia followed with a 78 metre Karri tree, while Queensland's 'Big Bob', a flooded gum, measured 72.8 metres.
In New South Wales, two different gum trees vied for top spot, both measuring 71 metres, while there were no known trees above 70 metres in South Australia, the Northern Territory or ACT.
The paper also catalogued the largest examples of trees by trunk volume. The most massive – with a trunk measuring at 463m3, and a diameter of 6.14m – was 'easily the largest known tree in Australia'.
These giants were an 'extraordinary expression of life', with significant cultural and environmental value, Bowman said.
Yet many lacked official plans for their conservation, he said, and faced multiple threats due to their age, increasingly frequent and severe fires and climate change.
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The paper's co-author, Brett Mifsud, who has been gathering data on tall and giant trees in south-eastern Australia for close to 35 years, said Tasmanian fires in 2019 had been 'utterly devastating' killing 15 of the largest 25 trees known at the time.
Fire could enter into existing hollows and cavities of very old trees and burn slowly for days, if not weeks, he said. Sometimes trees survived the fire, but later collapsed due to structural damage.
Rachel Nolan, an associate professor at Western Sydney University and an expert in forestry and fire ecology who was not involved with the study, said in order to survive and reach those heights, mountain ash trees needed long fire-free intervals.
While many forest eucalypts were resilient and resprouted new leaves in response to fire, mountain ash was a species that was often killed by bushfire, she said.
But even other large eucalypts, thought to be more resilient, were showing high rates of large tree loss due to more frequent, intense fires. 'When you get more fire, you get more injury to trees,' she said,.'If you get enough of that the trees will just fall over.'
Large, ancient trees provided valuable habitat and carbon storage, Nolan said, adding on a personal note 'there's something magical about these big, tall trees that's really inspiring and that we don't want to lose as a society'.
Mifsud agreed. 'As a human, we want to be able to see the biggest, the best, the most wonderful things, and we're in danger of losing those things through climate change.'
'To be bereft of such wonderful things because we didn't look after them, or didn't have a plan to look after them, would be a tragedy.'
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