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Deforestation in the tropics is not only responsible for direct carbon emissions but also extends the forest edge wherein trees suffer increased mortality. Here we combine high-resolution (30 m) satellite maps of forest cover with estimates of the edge effect and show that 19% of the remaining area of tropical forests lies within 100 m of a forest edge. The tropics house around 50 million forest fragments and the length of the world's tropical forest edges sums to nearly 50 million km. Edge effects in tropical forests have caused an additional 10.3 Gt (2.1–14.4 Gt) of carbon emissions, which translates into 0.34 Gt per year and represents 31% of the currently estimated annual carbon releases due to tropical deforestation. Fragmentation substantially augments carbon emissions from tropical forests and must be taken into account when analysing the role of vegetation in the global carbon cycle.

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Patterns of carbon retention and distribution across human-modified landscapes have been poorly investigated. In this paper carbon distribution across three forest habitats of a fragmented Atlantic forest landscape in northeast Brazil is examined. Data on tree assemblages (DBH ≥10 cm) inhabiting forest interior stands, forest edges and fragments (2.05–365 ha) were obtained via information from 59 0.1-ha plots (a total of 4,845 stems and 198 tree species), and it was further incorporated in four allometric equations for estimation of above-ground biomass and carbon. Stocks of carbon were highly variable within habitats of Serra Grande, but forest interior plots retained almost three times more carbon (202.8 ± 23.7 TonC/ha) than edge and fragment plots, while these edge-affected habitats exhibited similar scores. Moreover, emergent tree species accounted for the majority of the carbon retained (59.13%) in interior plots with understorey species playing a minor role. However, carbon retained by emergent species decreased by a half across forest edges and forest fragment since large stems (> 70 cm DBH) and very tall trees (> 31 m height) were very rare in these habitats. Finally, a forest cover mapping revealed the occurrence of 213.19 km2 of forest interior habitat in the whole Atlantic forest of northeast Brazil. This figure means that only 8% of total remaining forest habitat has a full potential for carbon storage, with the other 92% (edge-affected habitats) storing just a half of that. Our results suggest that habitat fragmentation and the consequent establishment of edge-affected habitats (forest edges and fragments) drastically limit forest capacity for carbon storage across human-modified landscapes since the loss of carbon due to reduced abundance of large trees is not compensated by either canopy or understorey species.

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