Tropical forest fragmentation and greenhouse gas emissions

Rainforest fragments in central Amazonia have been found to experience a marked loss of above-ground biomass caused by sharply increased rates of tree mortality and damage near fragment margins. These findings suggest that fragmentation of tropical forests is likely to increase emissions of CO2 and other greenhouse gases above and beyond that caused by deforestation per se. We estimated committed carbon emissions from deforestation and fragmentation in Amazonia, using three simulated models of landscape change: a 'Rondonia scenario,' which mimicked settlement schemes of small farmers in the southern Amazon; a 'Para scenario,' which imitated large cattle ranches in the eastern Amazon; and a 'random scenario,' in which forest tracts were cleared randomly. Estimates of carbon emissions for specific landscapes were from 0.3 to 42% too low, depending on the amount and spatial pattern of clearing, when based solely on deforestation. Because they created irregular habitat edges or many forest perforations which increased tree mortality, the Rondonia and random-clearing scenarios produced 2-5 times more fragmentation-induced carbon emissions than did the Para scenario, for any given level of clearing. Using current estimates of forest conversion, our simulations suggest that committed carbon emissions from forest fragmentation alone will range from 3.0 to 15.6 million t/year in the Brazilian Amazon, and from 22 to 149 million t/year for tropical forests globally. (C) 1998 Elsevier Science B.V.