Tree deaths rising across Australian forests as climate stress intensifies, study finds

Tree mortality is accelerating across Australia’s forests as a result of chronic stress caused by a warming climate, according to a new scientific study.

The research, published in the journal Nature Plants, analysed records spanning 83 years from more than 2,700 forest plots across four major forest biomes: Tropical savanna, tropical rainforest, warm temperate forest and cool temperate forest.

The study found that tree deaths have increased steadily over recent decades across all forest types, regardless of location or vegetation structure.

Researchers reported that mortality rates rose most sharply in hot, dry regions and in dense forests where trees compete more intensely for water and light. “This temporal trend persisted after accounting for stand structure and was exacerbated in forests with low moisture index or a high competition index,” the authors wrote in the paper.

Species with traits associated with rapid growth, including low wood density, high specific leaf area and shorter maximum height, showed higher average mortality. However, the rate at which mortality increased was broadly similar across different functional groups, suggesting a widespread climatic driver rather than species-specific causes.

A global pattern

The findings mirror similar trends observed in tropical rainforests in the Amazon, temperate forests in the western United States and Europe, and boreal forests in Canada. The scientists said this collective increase in tree mortality threatens forests’ ability to absorb and store carbon, a key buffer against climate change.

The study focused on what researchers described as “background tree mortality”, deliberately excluding deaths caused by logging, land clearance or major fire events. This allowed them to assess what changed over the decades and impacted the forests. 

The authors noted that the world has warmed by an annual average of about 1.2 degrees Celsius since the pre-industrial era, and said rising temperatures were the most likely cause of the increased tree deaths.

Across Australia, the warming trajectory, combined with rising atmospheric aridity and more frequent drought, has intensified stress on forests, creating conditions that undermine their long-term resilience.

Declining carbon storage

In a press statement accompanying the research, the authors said the number of trees dying each year increased by an average of 3.2 per cent, rising from around 15 per 1,000 trees in 1996 to nearly double that figure by 2017.

Belinda Medlyn, a professor at Western Sydney University’s Hawkesbury Institute for the Environment and a co-author of the study, said the losses were not being offset by new growth. “That makes it very likely that the overall carbon storage capacity in the forests is declining over time,” she said.

The researchers said climate change is now imposing physiological stress that overwhelms even the most resilient species, pushing forest systems beyond their capacity to recover naturally.

“Our findings suggest that ongoing climate change has driven pervasive shifts in forest dynamics beyond natural recovery in a range of forest biomes with high resilience to disturbance,” the study concluded. “This threatens the enduring capacity of forests to sequester carbon under current and future climate scenarios.”

Vulnerable trees and limits of adaptation

In tropical savannas, the study found that both very small and very large trees were particularly vulnerable, often due to the combined effects of fire damage and cumulative drought stress.

The authors noted that many trees have evolved specialised traits such as deep root systems and fire resistance to cope with natural variability. However, the relentless increase in what the researchers described as “thermal stress” from rising temperatures is pushing trees beyond their physical limits. Even relatively small changes in moisture availability or episodic storms can be enough to trigger tree collapse under these conditions. 

Because the pattern of increasing mortality was consistent across all four forest biomes studied, the researchers said it was likely that similar processes are already underway in other parts of the world.

“Our study suggests that forests’ capacity to act as a buffer against climate change will decline over time,” professor Medlyn said.