Rare Australian sandalwood trees that are more than 200 years old are dying in South Australia's outback.

Key points: It has been observed that mature sandalwood trees that have been alive for hundreds of years are dying in South Australia's outback

It has been observed that mature sandalwood trees that have been alive for hundreds of years are dying in South Australia's outback Drought and climate change has been blamed for the trees demise

Drought and climate change has been blamed for the trees demise Citizen scientists are encouraged to help track the impact of drought on trees by uploading them online via a project called Dead Tree Detectives

Ecologist John Read spotted the dying trees on his property at Secret Rocks, between Whyalla and Kimba, on the state's Eyre Peninsula.

The trees had been seen by the explorer Edward John Eyre in 1840.

"We probably walked past a dozen of these big old mature sandalwoods near our house and at least seven of them were dead and the others looked really stressed," Mr Read said.

"We've sort of controlled kangaroo and rabbit numbers to help them regenerate, and they've been doing pretty well over recent years, but in the last three or four months, we've noticed a lot of them are dying."

Sandalwoods are a threatened plant species because they are harvested extensively to make incense and other aromatherapy products.

'Indicator of climate change'

Mr Read said be believed climate change and drought were responsible for the tree deaths.

"I'm pretty sure it's because we've had a very hot and dry period which is a real indicator of climate change," Mr Read said.

"It really struck home when these trees that have been around for many centuries can no longer tolerate the conditions.

"If it was just a single species of tree, you might think it would be a pest or disease that's gone through, but we've noticed quite a significant die-off of wattles and long-lived pine trees."

Mr Read called for national action on climate change and said: "old trees don't lie".

A citizen scientist submitted this photograph of trees dying at Girraween in QLD, near the NSW border. ( Supplied: Dead Tree Detective project )

Dead Tree Detectives project

In 2018, scientists launched a citizen science project called Dead Tree Detectives, to better understand the damage dry conditions were having on trees.

Participants take photographs of dead trees and upload them to the Atlas of Living Australia website.

Scientists involved with the program then log details including where the picture was taken and what species of tree had been identified.

Brendan Choat, Associate Professor at the Hawkesbury Institute for the Environment at Western Sydney University, said it was an important project.

"Knowing where and when trees have died will really help us work out what the cause is, identify which tree species are vulnerable and help us to assess future risk," Professor Choat said.

"We have hundreds of observations that have been uploaded to the site and they show trees that have died or trees that are very stressed as a result mainly of the drought."

Entire hillside of dead Eucalyptus trees at Tallarook in central Victoria ( Supplied: Dead Tree Detective project )

Tree deaths 'unprecedented'

Professor Choat said he had seen an increase in the number of trees that were dying in the last six months.

"Initially it was one or two trees dying here or there in an otherwise healthy-looking canopy, but just over the last few months people have been uploading some really shocking photos where whole hillsides are dying," he said.

"Old timers who have lived in these areas for a long time are saying they've never seen anything like this.

"Words like 'unprecedented' are being used and to us that's really surprising and concerning.

"Given all of the images of mass tree dieback from the drought we are seeing posted on the dead tree detective, I don't think there is much doubt that water stress and drought are the primary cause of mortality in this case.

"This drought seems to have pushed a lot of very resilient vegetation over the edge."

He said some people were calling these "hot droughts" or "global change-type droughts" but he thought it was "just the new normal".

This hillside close to the NSW-QLD border on the Bruxner Highway is covered in dead Eucalyptus trees and a few green Callitris pines. ( Supplied: Dead Tree Detectives project )

Professor Choat said now that the badly affected trees had been identified, the plan was to evolve the project in the next three to six months.

"What we're going to do next is try and get out to these sites and evaluate which trees are dying and see if there are any signs of recovery," he said.

"We'll then conduct experiments to try and understand exactly why the trees are dying and what are the physiological mechanisms that are underpinning that mortality.

"Then that will give us further insight into how things might be affected in the future and how we might best approach revegetation."