Degradation in carbon stocks near tropical forest edges

Abstract

Carbon stock estimates based on land cover type are critical for informing climate change assessment and landscape management, but field and theoretical evidence indicates that forest fragmentation reduces the amount of carbon stored at forest edges. Here, using remotely sensed pantropical biomass and land cover data sets, we estimate that biomass within the first 500 m of the forest edge is on average 25% lower than in forest interiors and that reductions of 10% extend to 1.5 km from the forest edge. These findings suggest that IPCC Tier 1 methods overestimate carbon stocks in tropical forests by nearly 10%. Proper accounting for degradation at forest edges will inform better landscape and forest management and policies, as well as the assessment of carbon stocks at landscape and national levels.

Figure 1. Edge effects on tropical forest carbon.

Regression model for (a) demonstration purposes, at different scales, , and for a given magnitude, M=θ₂ exp(−0.232θ₃)/θ₁; (b) all pantropical forests; (c) moist broadleaf forests; and (d) dry broadleaf forests. Nonlinear least squares regression models were based on the entire set of pixels within the forest biomes, with a separate regression derived for each of the 10,000 km² sub-regions (N=2,836). Plots show only a subset of the points (a random sample of over 1,000,000 pixels for the pantropics and 100,000 for each biome) to aid in display; grey shading in b–d denotes where the heaviest density of points lay. The curves in each plot are based on the model associated with the sub-region having the magnitude and scale closest to the weighted average of the whole pantropics (b), the moist broadleaf biome (c), or the dry broadleaf biome (d). The weighted averages for these regions are listed in the lower right of each panel.

Figure 2. Magnitude and scale of forest carbon edge effects across the tropics.

Edge effect relationships derived for tropical forest within each 10,000 km² sub-region (N=2,836), with redder colours denoting stronger edge effects both in terms of magnitude of difference (% difference between biomass at forest edge and in forest interior) and scale of the edge effect (the distance from forest edge at which biomass is within 10% of the asymptotic biomass, or that seen in the interior of forests; in km).