[en] If energy sources are to be called renewable, they must be used on the conditions set by nature and natural balance must be maintained. The ash from biofuels from forests contains almost all the minerals and nutrients (except nitrogen) which were present in the fuel and should be recycled to the forest. This conference paper describes the composition and properties of ash, its production and handling, and its ecological effects when recycled. The chemical properties of ash depend largely on the content of minerals, nutrients and heavy metals in the fuel, and on how the combustion process is conducted. Ash from bioforests or from waste combustion may contain heavy metals and may be unsuitable for recycling. Ash is strongly alkaline and needs processing before recycling. In forests that grow on peat the addition of ash causes a strong growth of trees. On firm ground, however, the effect of adding ash is more in the nature of long-time forest care. 4 refs