High resolution analysis of tropical forest fragmentation and its impact on the global carbon cycle

Abstract

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.

Figure 1. Fragment size distribution for tropical forest.

Distribution of total forested area, core area (green) and edge area (blue; edge depth=100 m, blue numbers: edge area/total area (%)) for different fragment size classes (a) and different continents (b).

Figure 2. Total carbon losses due to tropical forest fragmentation.

Comparing carbon-loss scenarios assuming different assumptions for relative carbon losses e in the forest edge area, for different edge penetration distances d and different underlying forest cover maps. The numbers represent the total carbon loss due to fragmentation for different scenarios. (a) Carbon loss for different assumptions of relative carbon losses in forest edges e assuming edge depth d=100 m; (b) carbon loss for different assumptions of edge depth d assuming relative carbon losses in forest edges of e=50%; (c) comparison of results based on different forest cover maps assuming d=100 m and e=50% (for additional details, see Supplementary Tables 2 and 3).

Figure 3. Worldwide carbon emissions due to fragmentation of tropical forests.

(a) Colours represent the estimated carbon losses for each fragment, setting edge depth d to 100 m and relative carbon losses in forest edges e to 50%. Insets illustrate exemplary regional carbon emissions for (b) tropical America (89.752 W, 13.515 N), (c) tropical Africa (17.206 E, 4.499 S) and (d) tropical South-East Asia (103.898 E, 3.091 S).

Figure 4. Annual carbon emissions from tropical forests.

Annual carbon emissions from tropical deforestation (grey bars) and additional emissions from tropical forest edges due to fragmentation from this study (orange bars). The percentages represent the fraction of tropical fragmentation carbon emissions and tropical deforestation carbon emissions.