Forests managed by long-rotation forestry already play an important role as a source for renewable biomass and energy. The biomass output from these forests may be increased through higher harvest levels, but to be sustainable over time, harvest levels cannot exceed forest growth. A large proportion of the tree biomass (tops, foliage, branches, stumps, and small and unmerchantable trees) is left on the logging site in conventional stemwood harvest, representing a considerable amount of available biomass. The realizable biomass potential is lower, due to environmental and techno-economic constraints. If more technically adapted, efficient, and environmentally friendly biomass harvest and transport systems are developed, the realized biomass potential can be considerably increased. By complementing current stemwood harvesting technologies with systems that include logging residues, stumps, and small trees, a larger proportion of the potential will be available. In a longer term, increased forest growth can raise the biomass output. Many silvicultural treatments are available, including improved site preparation, vegetation control, insect control, browser control, genetically improved seedlings, fast growing tree species, fertilization, irrigation, denser stands, and so on. Inexpensive silvicultural measures that can be applied on large areas at a fast rate, such as using genetically improved seedling stock, have the highest potential to increase total forest production. Because of the long-rotation periods, it will take decades to make a difference in forest production and thereby in the biomass output potential. In addition, a more efficient use of biomass in both forest and energy industries and in biorefineries of the future holds potential. © 2012 John Wiley & Sons, Ltd.