Aboveground biomass in Australian tropical forests now a net carbon source.

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^3-7. Here we use long-term forest inventory data (1971-2019) from Australian moist tropical forests and a causal inference framework^8-10 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.