A globally consistent negative effect of edge on aboveground forest biomass.

Because of widespread forest fragmentation, 70% of the world's forest area lies within 1 km of an edge. Forest biomass density near edges often differs markedly from biomass density in the interior. In some biomes, these 'edge effects' are responsible for substantial reductions in forest carbon storage. However, there is little consensus on the direction and magnitude of edge effects on forest biomass across the globe, which hampers their consideration in forest carbon stock accounting. Here we examined eight million forested locations to quantify variability in edge effects on biomass at a global scale. We found negative edge effects across 97% of examined areas, with aboveground biomass density on average 16% lower near edges than in interior forests. Higher temperature, precipitation and proportion of agricultural land were linked to more negative edge effects. Along with differences in the spatial scale of analysis, this variation can explain contrasting observations among previous studies. We estimate that edge effects have reduced the total aboveground biomass of forests by 9%, equivalent to a loss of 58 Pg. These findings underscore the substantial impact of forest fragmentation on global biomass stocks and highlight the critical need to account for edge effects in carbon stock assessments.