"This finding is significant firstly because the Amazon is such a vast area and an important part of the global sink: the Amazon accounts for ca. 20% of the terrestrial carbon sink," Roel Brienen of the University of Leeds, UK, told environmentalresearchweb. "If this sink shuts down, growth rates of atmospheric carbon dioxide may go up faster. If other forests in the tropics are acting in the same way – and we do not know this yet – then it may go even faster."

The finding also has implications for climate models, which generally assume that the carbon sink provided by vegetation will continue to increase well into the mid 21s century or beyond, because of the carbon-fertilization effect, which sees higher carbon-dioxide levels in the atmosphere boost vegetation productivity.

"It shows that basically there is a limit to the carbon-dioxide fertilization effect on forests, and that the capacity of these productive systems to take up carbon from the atmosphere may be saturating rather sooner than later," said Brienen. "These models often do not account for the observed increases in mortality, which may offset growth increases."

To come up with the results, Brienen and colleagues recorded the growth, death and recruitment ("birth") of all trees bigger than 10 cm in diameter at a height of 1.3 m over time across 321 permanent sample plots, as part of the RAINFOR forest census.

"This is a labour-intensive method, but is one of the only reliable methods to quantity changes in the balance of tropical forests over time," said Breienen. "Forests, and in particular tropical forests, are an important component of the Earth system, taking up carbon from the atmosphere and thus putting a brake on the rate of increase in carbon dioxide and climate change. Tropical forests like the Amazon have historically been responsible for a large part of this uptake of atmospheric carbon dioxide."

The measurements revealed that the net rate of biomass increase in the Amazon has declined since 1983. Both mortality and productivity rates rose across the same period, although the productivity increase appears to have stalled since 2000.

"Our study does imply that if the observed trends continue, then even deeper cuts in carbon emissions are needed," said Brienen.

Global records indicate that the land carbon sink has increased since the mid 1990s, whereas this study finds that the Amazon contribution to the take-up decreased from 0.54 Pg of carbon per year in the 1990s to 0.38 Pg of carbon per year in the 2000s.

The team believes that these Amazon changes will not yet greatly affect the atmosphere, because the dead trees will decay slowly and some of this carbon will move into the soil. The researchers estimate that the atmosphere has yet to see about 3.8 Pg of the Amazon necromass carbon produced since 1983.

Now, Brienen and colleagues, who reported the results in Nature, hope to continue monitoring what is happening to forest in the Amazon and "other important tropical forests in Africa and Asia".

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