Despite the past stability of the African carbon sink, our most intensively monitored plots suggest a post-2010 increase in carbon losses, delayed compared to Amazonia, indicating asynchronous carbon sink saturation on the two continents. A statistical model including carbon dioxide, temperature, drought and forest dynamics accounts for the observed trends and indicates a long-term future decline in the African sink, whereas the Amazonian sink continues to weaken rapidly. Read more...

Tropical forests account for approximately one-third of Earth’s terrestrial gross primary productivity and one-half of Earth’s carbon stored in terrestrial vegetationThus, small biome-wide changes in tree growth and mortality can have global impacts, either buffering or exacerbating the increase in atmospheric CO2. Models, ground-based observations, airborne atmospheric CO2 measurements, inferences from remotely sensed data and synthetic approaches each suggest that, after accounting for land-use change, the remaining structurally intact tropical forests (that is, those not affected by direct anthropogenic impacts such as logging) are increasing in carbon stocks. This structurally intact tropical forest carbon sink is estimated at approximately 1.2 Pg C yr−1 over 1990–2007 using scaled inventory plot measurements. Yet, despite its relevance to policy, changes in this key carbon sink remain highly uncertain.

Globally, the terrestrial carbon sink is increasing. Between 1990 and 2017 the land surface sequestered about 30% of all anthropogenic carbon dioxide emissions. Rising CO2 concentrations are thought to have boosted photosynthesis more than rising air temperatures have enhanced respiration, resulting in an increasing global terrestrial carbon sinkYet, for Amazonia, recent results from repeated censuses of intact forest inventory plots show a progressive two-decade decline in sink strength primarily due to an increase in carbon losses from tree mortality. It is unclear if this simply reflects region-specific drought impacts, or potentially chronic pan-tropical impacts of either heat-related tree mortality, or results from internal forest dynamics as past increases in carbon gains leave the system. A more recent deceleration of the rate of increase in carbon gains from tree growth is also contributing to the declining Amazon sink. Again, it is not known whether this is a result of either pan-tropical saturation of CO2 fertilization, or rising air temperatures, or is simply a regional drought impact. To address these uncertainties, we (1) analyse an unprecedented long-term inventory dataset from Africa, (2) pool the new African and existing Amazonian records to investigate the putative environmental drivers of changes in the tropical forest carbon sink, and (3) project its likely future evolution.

We collected, compiled and analysed data from structurally intact old-growth forests from the African Tropical Rainforest Observation Network (217 plots) and other sources (27 plots) spanning the period 1 January 1968 to 31 December 2014 (Extended Data Fig. 1; Supplementary Table 1). In each plot (mean size, 1.1 ha), all trees ≥100 mm in stem diameter were identified, mapped and measured at least twice using standardized methods (135,625 trees monitored). Live biomass carbon stocks were estimated for each census date, with carbon gains and losses calculated for each interval (Extended Data Fig. 2).

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