Interannual Variability of Carbon Uptake of Secondary Forests in the Brazilian Amazon (2004-2014)

Tropical secondary forests (SF) play an important role in the global carbon cycle as a major terrestrial carbon sink. Here, we use high-resolution TerraClass data set for tracking land use activities in the Brazilian Amazon from 2004-2014 to detect spatial patterns and carbon sequestration dynamics of secondary forests (SF). By integrating satellite lidar and radar observations, we found the SF area in the Brazilian Amazon increased from approximately 22 Mha (10(6) ha) in 2004 to 28 Mha in 2014. However, the expansion in area was also accompanied by a dynamic land use activity that resulted in about 50% recycling of SF area annually from frequent clearing and abandonment. Consequently, the average age of SF remained less than 10 years (age similar to 8.2 with one standard deviation of 3.2 spatially) over the period of the study. Estimation of changes of carbon stocks shows that SF accumulates approximately 8.5 Mg ha(-1) year(-1) aboveground biomass during the first 10 years after clearing and abandonment, 4.5 Mg ha(-1) year(-1) for the next 10 years followed by a more gradual increase of 3 Mg ha(-1) year(-1) from 20 to 30 years with much slower rate thereafter. The effective carbon uptake of SF in Brazilian Amazon was negligible (0.06 +/- 0.03 PgC year(-1)) during this period, but the interannual variability was significantly larger (+/- 0.2 PgC year(-1)) If the SF areas were left to grow without further clearing for 100 years, it would absorb about 0.14 PgC year(-1) from the atmosphere, partially compensating the emissions from current rate of deforestation in the Brazilian Amazon.