Wood is one of the most common and versatile building materials. Under certain circumstances, it can also be considered sustainable, but in general the over-harvesting of wood from the forests around the world has exceeded the ability of forests to regenerate themselves. The result of this has been an environmental catastrophe in many regions, with considerable loss of species, topsoil, and natural beauty. Log, timber framed, and stick framed homes can be made with certified sustainably harvested trees. This means that the forests where the trees are harvested are carefully monitored to make sure that the health and character of the forest is maintained. Only certain trees are culled periodically, leaving the remaining trees to grow and contribute to a healthy ecosystem. Unfortunately, the American Forest and Paper Industries has begun to mislead the public with their own SFI (Sustainable Forestry Initiative) label, which would better be considered the "Same-old Forest Industry" label. The Forest Stewardship Council (FSC) legitmately certifies forest products, so look for the FSC label.
Another method of building with wood that can be done sustainably is with cordwood, which uses short sections of tree limbs and trunks that would otherwise likely be used just for firewood. This means that a large percentage of any given tree can be utilized for building, and many species of trees that are not typically milled for lumber can be used. Cordwood walls also provide an excellent balance of insulation and thermal mass quite naturally, without the need for further treatment either inside or out.