Aboveground biomass in Australian tropical forests now a net carbon source
Article Date: 2025-10-18
Article URL: https://www.nature.com/articles/s41586-025-09497-8
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Summary
Long-term monitoring across permanent plots in Australia’s tropical rainforests shows a worrying shift: aboveground biomass (AGB) has stopped being a net carbon sink and is now, on balance, releasing carbon. The study attributes this reversal to increased tree mortality and more frequent/intense disturbances — notably cyclones, drought and heat stress — together with limits on growth benefits from rising CO2 (nutrient and physiological constraints) and the loss of large trees that store disproportionate amounts of carbon. The finding aligns with similar regional trends reported elsewhere in the tropics.
Key Points
- Australian tropical forest aboveground biomass has moved from net sink to net source of carbon based on long-term plot data.
- Rising tree mortality and storm damage (cyclones) are primary drivers of the biomass decline.
- Extreme climate events (drought, heat, increased vapour pressure deficit) reduce growth and raise respiration, lowering net carbon uptake.
- Loss of large-diameter trees disproportionately reduces carbon stocks and accelerates source status.
- CO2 fertilisation effects appear constrained by nutrient availability and other ecological limits, so rising CO2 is not offsetting losses.
- Results have implications for national carbon accounting, climate mitigation targets and forest management in northern Australia.
Content summary
The authors synthesised decades of inventory from permanent rainforest plots combined with climate and disturbance records. Using established allometric methods and statistical models, they tracked changes in tree growth, recruitment and mortality to estimate aboveground biomass trends. The analysis shows a decline in AGB and an increase in carbon losses through mortality and post-disturbance decomposition, such that net ecosystem carbon balance for aboveground biomass is now negative in the monitored region.
The study highlights the role of intensified disturbance regimes — especially stronger or more frequent cyclones and droughts — and physiological stress from higher temperatures and vapour pressure deficit. It also notes constraints on any potential CO2 fertilisation benefit, including nutrient (phosphorus) limits and microbial/soil responses. Authors flag spatial and temporal variability across sites and acknowledge that belowground carbon pools and soil responses remain uncertain and require further study.
Context and relevance
Although Australia’s tropical rainforests are a comparatively small portion of global tropical forest area, they play a critical role in national carbon budgets, biodiversity and regional climate regulation. The result mirrors findings from other tropical regions (Amazon, parts of Africa and SE Asia) where long-term sinks are weakening or reversing. For policymakers and land managers this matters: assumptions that mature tropical forests will continue to offset emissions are now less secure and national accounting and mitigation strategies must reflect increasing disturbance-driven emissions.
For researchers the paper adds to a growing body of evidence that climate-driven stressors, nutrient limits and disturbance regimes can overwhelm growth gains from higher CO2. For conservation and forestry practitioners, it underlines the importance of protecting large trees, reducing compounding human pressures (invasion, land-use edges, fire) and investing in monitoring and adaptive management to retain carbon stocks.
Author perspective
Punchy take: this isn’t a minor blip. The study is a clear alarm that parts of Australia’s tropical forest are already flipping from helpers to emitters in the carbon ledger. If you care about credible emissions accounting or nature-based climate solutions, the details here matter — urgently.
Why should I read this?
Short answer: because it’s the kind of study that changes how you should count forests in climate policy. If you’re into climate targets, land management, or Aussie biodiversity, this paper saves you time — it lays out why these forests can no longer be assumed to mop up carbon and points to the drivers behind the switch.