Abstract
Despite sustained declines in Amazon deforestation, forest degradation from logging and fire continues to threaten carbon stocks, habitat, and biodiversity in frontier forests along the Amazon arc of deforestation. Limited data on the magnitude of carbon losses and rates of carbon recovery following forest degradation have hindered carbon accounting efforts and contributed to incomplete national reporting to reduce emissions from deforestation and forest degradation (REDD+). We combined annual time series of Landsat imagery and high-density airborne lidar data to characterize the variability, magnitude, and persistence of Amazon forest degradation impacts on aboveground carbon density (ACD) and canopy structure. On average, degraded forests contained 45.1% of the carbon stocks in intact forests, and differences persisted even after 15 years of regrowth. In comparison to logging, understory fires resulted in the largest and longest-lasting differences in ACD. Heterogeneity in burned forest structure varied by fire severity and frequency. Forests with a history of one, two, and three or more fires retained only 54.4%, 25.2%, and 7.6% of intact ACD, respectively, when measured after a year of regrowth. Unlike the additive impact of successive fires, selective logging before burning did not explain additional variability in modeled ACD loss and recovery of burned forests. Airborne lidar also provides quantitative measures of habitat structure that can aid the estimation of co-benefits of avoided degradation. Notably, forest carbon stocks recovered faster than attributes of canopy structure that are critical for biodiversity in tropical forests, including the abundance of tall trees. We provide the first comprehensive look-up table of emissions factors for specific degradation pathways at standard reporting intervals in the Amazon. Estimated carbon loss and recovery trajectories provide an important foundation for assessing the long-term contributions from forest degradation to regional carbon cycling and advance our understanding of the current state of frontier forests.
Highlights
Changes in Amazon forest carbon stocks are a significant source of greenhouse gas emissions from human activity
Using a large sample of intact and degraded forests, we provide the first comprehensive look-up table of degradation emissions factors for Amazon forests to guide the incorporation of forest degradation within REDD+ MRV
aboveground carbon density (ACD) in degraded forests varied by two orders of magnitude across the study area, providing clear support for the creation of multiple classes of forest degradation within REDD+ or other carbon accounting frameworks based on degradation frequency, severity, and timing
Summary
Changes in Amazon forest carbon stocks are a significant source of greenhouse gas emissions from human activity (van der Werf et al 2009, Pan et al 2011, Aguiar et al 2016). Annual deforestation rates in the Brazilian Amazon have declined by 80% since 2004 (Hansen et al 2014, INPE 2015), forest degradation from fire and logging remains a threat to forest carbon stocks across the Amazon arc of deforestation (Morton et al 2013). The magnitude of carbon losses from forest degradation is large (Longo et al 2016), but the long-term consequences of fire and logging on forest structure and composition remain uncertain (Andrade et al 2017). Decades of Amazon frontier expansion have left a mosaic of degraded forests along the Amazon arc of deforestation (Asner et al 2005, Morton et al 2013). Canopy damage in logged forests can increase vulnerability to additional disturbances, including fire (Uhl and Vieira 1989, Holdsworth and Uhl 1997), but the feedbacks and synergies among disturbance agents, as well as the long-term impacts of degradation, are still largely unresolved
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