Tropical trees are ‘dying faster’ in Australia with potential consequences for climate change

Tropical forests help to slow global heating but are also vulnerable to it

Northeast Australia’s relict tropical rainforests, one of the oldest and most isolated rainforests in the world. (Alexander Schenkin)

Tree deaths appear to be accelerating in some regions of the tropics, a reality that could have profound consequences for efforts to limit global heating to below 2 degrees Celsius, according to a new study.

Tree deaths in North Queensland, Australia have doubled between 1984 and 2019, according to the paper published in the scientific journal Nature, indicating that tree life expectancy has potentially halved in this region.

“It was a shock to detect such a marked increase in tree mortality, let alone a trend consistent across the diversity of species and sites we studied,” said David Bauman, a plant ecologist at the University of Oxford. “A sustained doubling of mortality risk would imply the carbon stored in trees returns twice as fast to the atmosphere.”

Northeast Australia’s relict tropical rainforest. (Alexander Schenkin)

Tropical forests are critical to the global carbon cycle as they absorb carbon dioxide from the air and are known to influence the pace of climate change, according to the study.

Dr Bauman and his colleagues analysed a record of tree deaths in 24 forest plots over 49 years - 1971 to 2019 -  in North Queensland. They looked at more than 74,000 trees from 81 different species, and found that annual tree death risk has doubled on average across all plots and species over the last 35 years, between 1984 and 2019.

What exactly is driving the tree deaths, and whether particular species are particularly vulnerable remains unclear, authors of the report said. They said, however, that they believe that the increase in the rate of tree mortality could be linked to climate change and wind-disturbance events due to cyclones.

Trees in drier local climates were found to have a higher average mortality risk, meaning atmospheric water stress, driven by global heating, may be a primary cause of the uptick in tree death in moist tropical forests, the report said.

An improved assessment of tree health would help to establish the cause of individual tree deaths, it added.