« La forêt amazonienne ne peut pas accumuler plus de carbone »

Finally, we put our results in a global perspective. According to global records, the land carbon sink has increased since the mid-1990s (refs 1, 2). While tropical land contributed significantly to this global sink during the 1980s and 1990s, our results show that the total net carbon sink into intact Amazon live biomass then decreased by 30% from 0.54 Pg C yr−1 (confidence interval 0.45–0.63) in the 1990s to 0.38 Pg C yr−1 (0.28–0.49) in the 2000s (see Methods). If our findings for the Amazon are representative for other tropical forests, and if below-ground pools have responded in the same way as above-ground biomass (AGB), then an apparent divergence emerges between a strengthening global terrestrial sink on one hand1, 2 and a weakening tropical sink on the other. However, from an atmospheric perspective we also note that some of the effects of the Amazon changes are yet to be observed, as little of the carbon resulting from increased mortality is immediately released into the atmosphere30. Instead, dead trees decay slowly, with a fraction also moving into a long-term soil carbon pool. The Amazon forest sink has therefore become increasingly skewed towards gains in the necromass pools, inducing a substantial lag in the probable atmospheric response. On the basis of the observed long-term increase in mortality rates, we estimate that the atmosphere has yet to see ~3.8 Pg of the Amazon necromass carbon produced since 1983 (see Methods), representing a 30% increase in necromass stocks. The modelled increase in Amazon necromass is twice the magnitude of the cumulative decadal decline in the live biomass sink from the 1990s to the 2000s (from 5.4 to 3.8 Pg C).

In summary, we find that the Amazon biomass carbon sink has started to decline, due to recent levelling of productivity increases, combined with a sustained long-term increase in tree mortality. This behaviour is at odds with expectations from models of a continually strong tropical biomass sink6, and underlines how difficult it remains to predict the role of land-vegetation feedbacks in modulating global climate change7, 10. Investment in consistent, coordinated long-term monitoring on the ground is fundamental to determine the trajectory of the planet’s most productive and diverse biome.

#forets #climat

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