A comparison of two algorithms for estimating carbon dioxide emissions after forest clearing

The clearing of forest during the last two centuries has contributed to the growth of carbon dioxide in the atmosphere and the prospect of global warming. The OECD, under the auspices of the IPCC, has developed a set of standard procedures for estimating greenhouse gas exchanges, including carbon dioxide exchange from forest clearing and regrowth. We present an alternative algorithm for estimating carbon dioxide exchange from forest clearing and regrowth based on a simple plant/soil model (DCDT) and compare it with the IPCC/OECD algorithm using two worked examples: constant clearing and constant regrowth of a semi-arid mallee forest in Australia. In these examples, the initial and final (equilibrium) carbon in the plant and soil are the same in both algorithms. The algorithms differ in their time scales of response. The IPCC/OECD algorithm, when compared with the DCDT algorithm, overestimates the carbon dioxide emissions from converting mallee forest to wheat cropping by up to 33%, averaged over a decade. The IPCC/OECD algorithm, when compared with the DCDT algorithm, underestimates the emission from regenerating the cleared land by as much as 100% over a decade. These differences arise when the rates of land clearing and regeneration vary on a time scale shorter than that for the biosphere to equilibrate after a land use change or disturbance. This study shows that the improved quantification of the processes of carbon dynamics in the plant/litter/soil system may be as important as improving the mapping of areas cleared and current carbon pools in reducing the uncertainties associated with CO2 emissions from forest clearing. (C) 1997 Elsevier Science Ltd.