One of the assumptions underlying efforts to convert cropping land, especially marginal crop land, to plantations is that there will be a net reduction in greenhouse gas emissions, with a gas "sink" replacing a high energy system in which the breakdown of biomass is routinely accelerated to prepare for new crops. This research, based on case studies in Kingaroy in southeast Queensland, compares the amount of greenhouse gas (GHGs) emissions from a peanut/maize crop rotation, a pasture system for beef production and a spotted gum (Corymbia citriodora) timber plantation. Three production inputs, fuel, farm machinery and agrochemicals (fertilizer, pesticides and herbicides) are considered. The study extends beyond the farm gate to include packing and transportation and the time period is 30 years. The results suggest that replacing the crops with plantations would indeed reduce emissions but that a pasture system would have even lower net emissions. These findings cast some doubt on the case for farm forestry as a relatively effective means of ameliorating greenhouse gas emissions.