In both scenarios there are some energy needs for gathering the resource and converting it into useful energy, but, as the arrows on the transportation system suggest here, these are generally comparatively small. Arrows provide a qualitative indication of the magnitude and direction of carbon flows. By comparing the results of these and other scenarios under a variety of initial conditions, biomass growth rates, and end uses, we begin to get some clues to the most carbon-efficient ways to manage forest or other lands and to the potential for biomass fuels to mitigate the increase in atmospheric carbon dioxide. The comparisons show that when the amount of forest biomass on the land in the beginning is very large and the productivity of the land is low, the most effective strategy is to allow the trees to grow, to stand, and to store carbon. Similarly, the net carbon balance on degraded lands with low productivity is best when they are reforested, without harvesting, to store carbon.
Results are quite different on lands that can support high growth rates.
There, the net reduction in carbon dioxide emissions is far greater if the trees are harvested and used as a fuel, with prompt replanting, than if the trees are left unharvested for carbon storage. Using current technologies, the most efficient way to convert biomass to useful energy, and thus to maximize the carbon dioxide savings, is to burn the biomass for heat or electricity generation, displacing coal. In all scenarios, carbon dioxide benefits increase as biomass growth rates increase and as utilization efficiency increases. The Biofuels Feedstock Development Program at ORNL aims to increase the productivity of tree and grass crops and improve the efficiency of biomass feedstock supply systems. Improvements in these areas offer a large payback both in the economics of biomass fuels and in the potential for net reductions in carbon dioxide emissions.