Letters

The potential of secondary forests

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Science  08 May 2015:
Vol. 348, Issue 6235, pp. 642-643
DOI: 10.1126/science.348.6235.642-c

TROPICAL FORESTS ARE increasingly modified by human activities. Centuries of human–forest interactions have led to a diverse array of forest areas in different phases of succession. In recent decades, forest conversion to cattle pasture or agricultural fields, followed by land abandonment, has led to large areas of second-growth forest in the Amazon. These forests grow rapidly and sequester large amounts of carbon in their biomass, but they tend to be ignored, as most of the debate on the carbon balance of the Amazon basin tends to revolve around old-growth forests.

For example, a recent study has shown that the net carbon uptake of Amazonian old-growth forests has declined by a third per decade from 1990 to 2010 (1, 2). When extrapolated over the whole Amazon basin, these results translate into a reduced role of intact tropical forests in climate change mitigation. This alarming conclusion, however, completely ignores the important role of regenerating forests as carbon sinks. For instance, in 2010 about 25% of formerly deforested areas in Para, Brazil, were occupied by second-growth forests. Although re-growing forests have lower carbon stocks (45 to 48% of old growth forest), their net carbon sequestration rate is up to 20 times higher (4.6 to 5.8 Mg carbon ha−1 year−1) (3) than old-growth forests (1). Additionally, about one-quarter of the forests in the Amazon basin are managed for timber production. Net carbon sequestration rates after timber extraction are high, and the application of reduced-impact logging techniques further increases carbon sequestration rates (2.8 Mg ha−1 year−1 compared with 0.5 for conventionally logged areas) (4). Consequently, it is essential to incorporate the carbon sequestration potential of second-growth, logged, and managed forests in future assessments of the Amazon basin as a global carbon sink.

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