Reaction wood is formed in trees or limbs not growing parallel to the pull of gravity. Reaction wood is hard and heavy due to shorter cells with thicker walls then in normal wood. Conifers develop reaction wood on the underside of trunks or branches, compression wood. In hardwood trees, reaction wood forms on the upper side of the leaning trunk, tension wood.
Reaction wood has been used in different ways. Its great longitudinal shrinkage has made spruce branches the most used wood for barrel hoops. Skis and sled runners from pine are smooth against the snow; saamis made bows from pine reaction wood and it was used in boatbuilding.