Carbon loss, from tropical forests

Globally, the net carbon loss from tropical forests into the atmosphere from tropical deforestation and changes to secondary forest, pasture and crops, has been estimated (Seiler and Crutzen, 1980 Dickinson, 1981) to be about 1.0 X 10 kg carbon, or equivalent to about 17% of that due to fossil fuel burning. This amount tends to support the view that the current increase in atmospheric CO2 is primarily due to increased fossil fuel combustion (Niehaus, 1979 Broecker et d., 1979). Nevertheless, there is sufficient uncertainty in the basic data, e.g. the rate of deforestation and the amount of biomass, to permit wide variation in the final estimates. Thus, other estimates have suggested that the release of carbon due to tropical deforestation is 3 X 10 kg carbon or even more, with overall releases from biota estimated to approach 6 X 10 kg carbon (Wood-well, 1978 Woodwell et d., 1978). This latter amount is more or less equivalent to that released by fossil fuels. 

The burning of slash following deforestation, whether intentional or unintentional, results in far greater direct and indirect losses of nutrients than deforestation alone. This is particularly true in many tropical forests where only a small fraction (if any) of the aboveground biomass is removed prior to burning. Carbon losses from slash fires in the tropical dry forest were 4-5 fold greater than C losses from wood export (Table IV) (55). Slash fires in tropical dry forests resulted in N losses of 428-500 kg ha whereas fuel wood export of the relatively N-poor coarse woody debris amounted to approximately 41 kg N ha" Losses of P increase with increasing fire severity. P losses of 10-77 kg ha" as a result of severe fires is not uncommon (Table TV) (53, 58, 60). 

Tropical forests and savannas are the primary source of C emissions that originate from biomass burning (73, 75). However, temperate forests are also sources of atmospheric carbon. Harmon et al. (77) reported that conversion of primary temperate forests to younger, second-growth forests lead to increases in atmospheric CO2 levels, due to losses in long-term carbon storage within these forests. They ascertained that timber exploitation of 5 million hectares of primaiy forests in the Pacific Northwest of North America during the past century has resulted in the addition of 1,500 Tg of C to the atmosphere. 

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