Forestry's long production period entails compound interest, often making investments unprofitable. Several plausible but dubious arguments purport to excuse forestry from bearing compound interest: the existence of externalities, the invalidity of social discounting, the ability of previous revenues to bear replanting costs. The so-called "allowable cut effect" permits comparison of improvement expenditures with immediate yield. In a somewhat similar way, planting forests to absorb carbon dioxide permits almost-immediate burning of fossil fuel, a benefit offering simple interest on the planting costs. Such carbon-fixing plantations appear to be economic even when it is uneconomic to plant for fuelwood production. In one case study, the unit cost of growing wood for burning was £356 per tonne coal equivalent, while the cost of carbon fixing was only £76 per tonne of coal burned. The economic acceptability of such planting is not, however, established: particularly, fossil fuel burning may have other malign effects.