Carle, Hannah, Bauman, David, Evans, Michael N., Coughlin, Ingrid, Binks, Oliver, Ford, Andrew, Bradford, Matthew, Nicotra, Adrienne, Murphy, Helen, and Meir, Patrick (2025) Aboveground biomass in Australian tropical forests now a net carbon source. Nature, 646 (8085). pp. 611-618.

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Abstract

Tropical forests act as important global carbon sinks¹, and Earth System Models predict increasing near-term carbon sink capacity for these forests, with elevated atmospheric carbon dioxide concentration thought to stimulate tree growth^2,3. However, current forest inventory data analyses suggest that the carbon sink capacity of intact tropical forests may be in decline, portending a possible future switch from carbon sinks to carbon sources^{3, 4, 5, 6–7}. Here we use long-term forest inventory data (1971–2019) from Australian moist tropical forests and a causal inference framework^{8, 9–10} to assess the carbon balance of woody aboveground standing biomass over time, the demographic processes accounting for it, and its climatic drivers, including cyclones. We find that a transition from sink (0.62 ± 0.04 Mg C ha⁻¹ yr⁻¹: 1971–2000) to source (−0.93 ± 0.11 Mg C ha⁻¹ yr⁻¹: 2010–2019) has occurred for the aboveground woody biomass of these forests, with sink capacity declining at a rate of 0.041 ± 0.032 Mg C ha⁻¹ yr⁻¹. The transition was driven by increasingly extreme temperature and other climate anomalies, which have increased tree mortality and associated biomass losses⁴, with no evidence of the carbon fertilization (stimulation) of woody tree growth. Forest dynamics underlying carbon sink capacity were also punctuated by cyclones, with impacts of a similar magnitude to long-term climate-induced changes. Our findings suggest the potential for a similar response to climate change by woody aboveground biomass in moist tropical forests globally, which could culminate in a long-term switch from carbon sinks to carbon sources.

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