As New South Wales and Queensland were being hit by catastrophic floods earlier this year, Tasmania’s south-west was drying up.
Last summer was the driest in some parts of the south-west since records began, according to the Bureau of Meteorology. This is bad news for the region’s unique trees and sensitive ecosystems in part of the extensive world heritage area that covers about a fifth of the state.
While the floods in New South Wales and Queensland were driven by a La Niña event, Tasmania’s dry summer was the result of a lesser-known climate driver called the Southern Annular Mode, or SAM for short.
The SAM consists of roaring westerly winds that race around the globe’s south. Sometimes the winds make their lap of the globe relatively close to Antarctica and other times they blow further north. The winds carry rain fronts. When they hang low near Antarctica, as they did last summer, Tasmania does not get much rain.
This year’s SAM contributed to Tasmania’s rainfall being 42 per cent lower than average, and the state’s third driest summer on record. Parts of the south-west had between 200 and 400 millimetres less rain than they normally would in summer.
What does this mean for the state’s world heritage wilderness?
Scientists fear the dry could have a significant impact on the south-western part of the world heritage area. Fire ecologist Dr Jenny Styger, adjunct researcher at the University of Tasmania, says the living history of the south-west, which stretches back to the time of the dinosaurs, made it particularly vulnerable to dry weather.
Many of Tasmania’s trees evolved on the supercontinent Gondwana before the state broke away from what is now Antarctica about 50 million years ago and began to drift north. Many plants unique to the island state - including King Billy pines, pencil pines, deciduous beech (known as fagus), and Huon pines - are still adapted to chilly Gondwanan conditions.
“They're like dinosaur plants, they’re of an extremely ancient lineage,” Dr Styger says. “They evolved at a time when Tasmania was much cooler and wetter than it is now and as a result they are extremely fire sensitive - they can be killed by a single fire.”
“The thing you see disappearing is the cool summers and that's disturbing.”
Dr Jenny Styger, fire ecologist at University of Tasmania
The soil the trees grow in is high in organic content. This means the ground is also extremely flammable when conditions are dry and puts the forests at increased risk of burning.
Styger says the south-west was “primed to burn” this summer and it was “really lucky” there were not wide-scale fires. The rainforests and alpine ecosystems of the south-west can take hundreds of years to recover if they burn.
Though the forest was dry and ready to blaze, disaster was dodged because there were very few ignition sources. It meant the 2021-22 summer contrasted with the experiences of 2016 and 2019, when dry lightning storms across western Tasmania sparked unprecedented fires in the world heritage area. Any fires that did start were jumped on quickly and extinguished.
While catastrophe was averted this year, climate projections published in a major report by the Intergovernmental Panel on Climate Change suggest that Tasmania will have less annual rainfall in the future.
Beyond increasing the risk of fire, drought weakens and kills trees through what is known as dieback. Styger says she has not yet observed dieback in the south-west, possibly because soils in the region might have had enough moisture stored to survive the summer, or recent dieback may simply not have been observed yet. She says it is occurring elsewhere in the state. “You see it all through the midlands and central plateau,” she says.
These dry conditions transform landscapes. “You can lose your canopy trees…and then you’ll have shrubby undergrowth growing up to form a completely different vegetation community,” Styger says.
Ancient trees: a window to the past
The Huon pine is perhaps the best known of Tasmania’s dinosaur trees. Dr Kathy Allen, a tree and climate researcher and Australian Research Council future fellow at the University of Tasmania, believes some huon pines might survive for 3000 years. “They commonly get to between 500 and 1000 years of age,” she says.
Each ring in the trunk of a huon pine leaves a record of what the climate was like in the year that ring grew. Dr Allen has been non-destructively drilling into these gnarly, ancient plants and extracting long, skinny samples across the rings that she uses to reconstruct a long-term climate record.
“I’ve been working on a 2000-year reconstruction of summer temperatures,” she says.
Her work shows a “rapid upturn” in local temperatures from the 1950s. The tree core data show while there were occasional hot summers before the middle of last century, the trend of extremely warm summers is new.
“The thing you see disappearing is the cool summers and that's disturbing,” she says.
Allen says Huon pines are highly sensitive to drought - their roots need to either be in water or be able to access water to thrive. Given the climate projections for the region, she is worried about the species’ future. “Drought is probably the biggest threat,” she says. “As you have the landscape drying, it can become more prone to fire - so that's another threat.”
Styger says there are some things that could be done. First Nations people intentionally burned the landscape in a “regular and planned” way, focusing on coastal heaths and low button grass moorlands. By keeping fuel loads low, these fire regimes protected ancient rainforests in moist gullies, along rivers and in the shade of mountains.
She says the south-west needs “serious action on climate change” by reducing carbon dioxide emissions, but “on the micro scale there are immediate actions we can take, such as improving our land management practices”.
“We need to utilise those traditional learning practices that have been developed by Aboriginal people in Tasmania for over 40,000 years,” she says.