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Quantifying long-term changes in carbon stocks and forest structure from Amazon forest degradation

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Quantifying long-term changes in carbon stocks and forest structure from Amazon forest degradation
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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.
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Transkript: Englisch(automatisch erzeugt)
Deforestation in the Brazilian Amazon has declined by over 80% in the past decade. A large win for both conservation and climate. Forest degradation, however, from fire and logging remains widespread along the Amazon arc of deforestation.
Which begs the question, just how important is forest degradation for carbon stocks and habitat? Decades of frontier expansion have left a complex mosaic of degraded forests along the Amazon arc of deforestation. Fire is particularly damaging to tropical forests because trees do not have adaptations to survive even low-intensity surface fires.
Large, slow-moving understory fires leave a ghost forest of standing dead trees, fundamentally altering the structure, composition, and function of degraded forests. Degradation impacts very spatially across these vast landscapes and vertically up through the forest canopy.
Quantifying changes in forest structure from forest degradation is very difficult from the ground. As logged and burned forests are typically a dense tangle of regrowing trees and lianas. Yet assessing forest damages and recovery over time is essential to understand the long-term consequences of forest degradation on forest carbon stocks and habitat. Understanding whether logging and fire represent a small or large source of carbon emissions from the Amazon region
is critical to balance the global carbon budget, improve climate projections, and maximize the effectiveness of climate mitigation measures. In this paper, we targeted those critical data gaps by quantifying carbon loss and recovery associated with forest degradation.
We combined field measurements, airborne lidar, and satellite time series to disentangle the complex legacy of forest degradation by providing the first comprehensive look-up table of carbon emissions factors for Amazon forest degradation. We also made corresponding measurements of habitat structure to better understand the biodiversity co-benefits of avoided forest degradation.
Airborne lidar uses millions of laser pulses to generate 3D information on forest structure. The unique perspective of high-resolution lidar can help us bridge the scale gaps between field inventories and satellite data by providing detailed estimates of above-ground carbon stocks in habitat structure over large areas.
For example, carbon stocks in burned forests declined by almost 50% following a single fire and more than 80% following two fires. Forests that endured three or more fires retained only 10% of their estimated original carbon stocks. Overall, we found that the carbon consequences of forest degradation were larger, more persistent, and more variable than previously reported.
Given the slow recovery of degraded carbon stocks and the ubiquity of forest degradation along the arc of deforestation, our results reaffirm the need to track multiple classes of forest degradation for REDD+, or other carbon accounting systems, to achieve their mitigation goals. We invite you to read our paper for a more in-depth assessment of the largest risks to frontier forest carbon stocks in habitat.