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Estimating net emissions from forest clearance and degradation
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Degradation and forgone removals increase the carbon impact of intact forest loss by 626%

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We estimated emissions from forest clearance that occurred in intact forests between 2000 and 2013 using a “combine and assign” approach (61). Pulse emissions from a disturbance process d in a fragmented parcel i were measured using$Ed,i=0.5*(Ad*Pd*Bi)$(1)where A is the area of disturbance d (in ha), P is the proportion of biomass lost due to disturbance d (i.e., the “carbon emission factor”), and B is the mean aboveground biomass estimated inside parcel i (in T ha−1). The factor of 0.5 is the proportion by weight of carbon in aboveground dry biomass.

In a second time period between 2013 and 2050, we estimated committed emissions from forest degradation processes that require years or decades to take effect and accounted for carbon sequestration that may occur from reforestation beyond 2013. We note, however, that a small fraction of the committed emissions we estimate for this second time period would actually occur in the first time period (2000–2013), given that some forest clearance occurred early in the 2000s.

We used Eq. 1 to estimate committed emissions between 2013 and 2050 from selective logging, fragmentation, and ecological changes triggered by defaunation. Carbon emission factors for selective logging in countries that lost intact tropical forest between 2000 and 2013 were drawn from the peer-reviewed literature (table S1) (20). Where carbon emission factors for selective logging were not available for a particular country, they were set to the median of estimated values from countries within the same geographic region.

Emissions from defaunation in lost parcels were also estimated using Eq. 1. We estimated the area within tropical moist broadleaf forests affected by defaunation, ADFAUN, in parcel i using$ADFAUN,i=(Ai−AFC)*Ti$(2)where Ai is the forested area of parcel i (in ha), AFC is the area of forest clearance in parcel i (in ha), and Ti is the relative abundance of tree species vulnerable to defaunation in parcel i. We derived Ti values and carbon penalties for defaunation from Osuri et al. (table S3).

The committed period also accounted for carbon sequestration by reforestation that is likely to occur between 2013 and 2050 at sites that were cleared between 2000 and 2013. In tropical countries, the majority of land for agricultural expansion comes from forests, not from previously cleared lands (48), and pressure to continue agricultural expansion in tropical countries is likely to increase in the coming decades (62). We therefore assumed that reforestation will occur in 28% of areas that were deforested in lost parcels located in tropical and subtropical ecoregions (14). We based this estimate on the observed ratio of forest loss to gain in the tropical forest domain for the 2000s (3.6 for >50% tree cover) (14). We assumed that regrowing forests will sequester carbon at rates estimated for the 2000s (table S4) (4).

Most selective logging in the tropics operates on 20- to 40-year harvest cycles (20). We assumed that selective logging footprints within lost parcels will be logged at least once between 2013 and 2050 (37 years). Emissions from selective logging at a specific site are cyclical—an immediate release followed by gradual recovery—but we applied carbon penalties for conventional logging and responsible logging linearly across 37 years. This simplification was based on the typical long-term carbon impact of selective logging activities (20) and is not likely to affect our estimated regional or pantropical emissions from selective logging.

Time scales and trajectories for emissions from fragmentation remain poorly characterized. Previous studies show that emissions of the magnitude we considered (i.e., 25% loss in aboveground carbon penetrating 500 m from a newly created edge, and the first 100 m of this 500-m edge incurring a 50% loss in aboveground biomass) can occur within 10 to 17 years (27). We conservatively assumed a loss residence time for fragmentation of 37 years and, for simplicity, assumed that this loss occurs linearly (table S5). The time frame over which emissions from defaunation may occur is also unknown at this time. These emissions are likely to be much longer term than more immediate drivers of carbon losses such as deforestation or logging (17), yet ecological changes within tropical forests with the potential to result in biomass changes of the magnitude we considered for defaunation (i.e., 0.5 to 13.9% loss in aboveground carbon) can occur within 20 to 30 years (63). We chose to set a conservative time scale for defaunation emission to 100 years and also assumed that this loss occurs linearly. We did not account for any defaunation that potentially occurred in intact forests before year 2000 (64).

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