Wood is a bio-fuel. This means that it comes from living things. Lawn grass, field crops and garden plants and flowers are all in the bio-family of living things. In fact, there is no difference between the wood of today and fossil fuels of aeons ago, such as coal and oil except time. Millions of years ago all fossil fuels were lush green plants, thriving on nutrients and moisture in the earth and the photosynthesis process. These have long since died but have been preserved beneath the surface of the earth. Actually, the wood of today and the fossil fuels of aeons ago trace their origin to energy from the sun. The plant's tissue is a storehouse of some of this energy. The energy remains stored until plants are eaten, burned or decayed, thus releasing the energy. Decay results when microbes and fungus digest the plant tissue.
Turner, George M., "Energy in Agriculture: Energy Resource Series for Youth and Adult Energy Programs: 9. Wood" (1981). Agricultural Engineering Energy Series. 17.