A new study indicates a rising natural death rate among trees in major Australian forests, a trend attributed to higher average temperatures linked to climate change. Published in the journal Nature Plants, the research suggests that this increase in tree mortality may lead to reduced carbon dioxide sequestration by forests in the future, alongside existing threats such as megafires, diseases, and invasive species.
Research Findings
Belinda Medlyn, a plant physiological ecologist and senior author from Western Sydney University, reported that the research team observed a steady increase in tree death rates over the past six decades, affecting various Australian forest types from cool temperate zones in southern Tasmania to tropical savannas in the north. This pattern suggests a global phenomenon of altered forest function.
The study analyzed 83 years of state government and independent data, encompassing 203,721 individual trees, 958 species, and 2,724 sites across Australia's eastern states, the Northern Territory, ACT, and northern Western Australia. Sites were categorized into tropical savannas, Queensland's tropical rainforests, warm temperate forests, and cool temperate forests.
Between 1963 and 2020, the documented natural tree mortality rates showed significant increases:
- Tropical rainforests: Annual mortality more than doubled from 0.5% to 1.3%.
- Tropical savannas: Annual mortality increased from approximately 1.5% in 1996 to 2.7% in 2017.
- Warm temperate forests: Mortality rates more than tripled from 0.2% in 1943 to 0.7% in 2018.
- Cool temperate zone: Mortality rates rose from 0.4% in 1966 to 0.7% in 2019.
Implications for Carbon Storage and Ecosystems
Medlyn stated that these increases, while seemingly small, signify a fundamental change in forest function, leading to a reduction in stored carbon due to faster tree turnover. David Lindenmayer, a forest ecologist at Australian National University not involved in the study, commented that accelerated mortality has major implications for ecosystem integrity, carbon storage levels, and biodiversity suitability. He noted that rising temperatures are driving profound changes in forest structure via increased mortality, a situation further impacted by altered fire regimes.
Furthermore, Medlyn indicated that higher tree mortality could lead to an overestimation of carbon stored in revegetation and carbon credit schemes. Political geographer Kate Dooley, from the University of Melbourne, emphasized that forests alone cannot offset ongoing fossil fuel emissions, advocating for separate accounting of carbon removals from forest restoration and reductions in fossil fuel emissions. Dooley also suggested that Australian forest management should prioritize monitoring diverse ecological traits to inform biodiversity and climate goals, rather than relying solely on modeled carbon sequestration rates.
Unclear Death Mechanisms and Monitoring Challenges
The precise mechanism by which increased temperatures contribute to tree death remains unestablished. Medlyn noted that several hypotheses require testing, including increased water stress or cumulative damage from frequent heatwaves. Understanding these mechanisms could inform forest adaptation strategies.
Ongoing research faces challenges due to the abandonment of numerous monitoring sites, many of which were initially established as part of native forestry operations now curtailed in several states. Medlyn highlighted a radical reduction in monitored plots, particularly in warm temperate forests, where numbers have decreased from 900 to fewer than 100. Current remote sensing technologies, such as satellites, do not yet adequately capture individual tree deaths, raising concerns about the availability of data to track forest changes.