A major new study has revealed that forests across Australia are losing trees at accelerating rates, marking a continent-wide shift in vegetation dynamics in response to a rapidly warming climate.
Published today in Nature Plants, the Western Sydney University-led study draws on 83 years of records from more than 2700 forest plots around Australia.
James Cook University Associate Professor Michael Liddell was a co-author on research, which provided the first comprehensive, continent-wide synthesis of background tree mortality – tree loss not caused by fire, clearing or logging – across Australia's diverse forest ecosystems, including tropical rainforests, savannas, and temperate eucalypt forests.
The research shows a persistent increase in background mortality since the 1940s, with the pattern strikingly consistent across forest types.
Over the same period, tree growth has remained the same or slowed, indicating that the increase in mortality is not part of a natural cycle of renewal but evidence of an emerging imbalance between tree growth and tree loss.
Assoc Prof Liddell was a co-author and contributor of tropical rainforest data for the paper and said the findings highlighted the real value of multi-decadal forest research across Australia.
"As climate change reshapes the questions we ask of forest systems, long-term datasets are becoming increasingly important for answering them with confidence," he said.
"Some of the data underpinning this work were collected at places like Robson Creek Rainforest SuperSite, and this reinforces a key message: measuring trees is where we start, but on its own it isn't enough.
"We know tree mortality will have flow-on effects, so we need to track carbon fluxes, and faunal biodiversity to better understand how ecosystems are responding to a rapidly warming climate."
He said the decades-long rise in tree mortality closely tracks Australia's warming and drying climate, with rising temperatures the dominant driver of change.
Tree mortality has increased most rapidly in hot, dry regions and in dense forests where competition for water and light exacerbates stress.
Senior author and Western Sydney University Distinguished Professor Belinda Medlyn, from the University's Hawkesbury Institute for the Environment, said the increasing tree mortality rates point to growing stress in Australia's forests.
"Australians rely on their forests for a wide range of ecosystem services, from cultural values and recreation to timber for houses," she said.
"Increasing tree mortality in our unique forests will affect all of these. A particular worry is that the forests' ability to store carbon will decline. This has significant implications for Australia's net carbon balance.
"Forests worldwide absorb about one-third of human carbon dioxide emissions. If mortality continues to rise while growth stagnates, that buffering capacity will erode."
Distinguished Professor Medlyn said there are many opportunities to adapt forest management to help safeguard forest health but added understanding how forests respond to these management approaches requires a commitment to long-term monitoring of forest function.
She said over the past quarter century the number of forest plots being monitored has fallen sharply, undermining capacity to detect and respond to these accelerating changes.
"Our results highlight the critical need for ongoing forest monitoring that is designed to detect long-term trends, in order to guide effective forest management for the future," Distinguished Professor Medlyn said.
