Professor David Lindenmayer and Dr Chloe Sato, two of Australia's most respected ecologists, have been tracking the plants and animals of the forest since 1983, publishing more than 200 papers along the way. “It’s been very distressing to watch,” Professor Lindenmayer says. “Many, many of these sites that should have animals no longer do. To see many of the big trees that were there gone – that’s quite difficult to witness." In the paper, they argue the ash forest has lost the ability to sustain itself. They estimate its probability of collapse at about 90 per cent in the next 50 years. About 80 per cent of the entire forest is now designated for logging, they claim – although this figure is hotly disputed. On top of that, huge swathes of the park have been consumed in bushfires. The paper says the number of large, old trees in the forest almost halved between 1997 and 2011. The number of Leadbeater’s possums and greater gliders has more than halved. There are many fewer birds, including kookaburras, rosellas and honeyeaters, the paper says.

Almost 99 per cent of the forest is now young trees. Trees tend to reproduce mostly when they are very old, says Professor Lindenmayer. Without old trees, there are no new trees. Young trees are also much more flammable than old trees. This means the whole forest is at much higher risk of a huge blaze. And if one struck, the trees would not grow back, the paper finds. The trees would likely be replaced with wattle and acacia shrubland, which would be very bad for Melbourne’s water supply, the paper finds. Some of the city’s reservoirs – the Upper Yarra Reservoir, the Maroondah Dam – are right in the middle of the forest. Rain falls on the trees and filters into the dams. But young trees soak up much more water than older ones to fuel their growth. Replacing the ash with shrubland would “take enormous quantities of water out of the system,” says Professor Lindenmayer.

Melbourne Water, the authority on the city's water supply, did not respond to repeated requests for comment. But Professor Rod Keenan, from Melbourne University's school of forest sciences, slammed the paper. "It is not very rigorous – and I am surprised it was accepted for publication in this journal," he said. The paper relies largely on one marker to predict collapse: the number of old trees in the forest. Government datasets show the forest can recover, and that animal numbers have not fallen as dramatically as claimed. This paper ignores them, says Professor Keenan. About 70 per cent of the park is protected from logging, he claims.

"The senior author has been a vocal advocate for a cessation of timber harvesting and creation of a new national park in this region for some time," he said. A spokesman for the Minister for Energy, Environment and Climate Change, Lily D’Ambrosio, said logging was allowed in less than 1 per cent of the state's water catchments. Large areas of the mountain ash forest were available for harvesting, the spokesman said, but all old-growth sections were protected.