Have you ever wondered how a tree supports itself as it reaches towards an opening in the canopy? Or how a long limb holds itself horizontal? The answer is reaction wood.
A cut limb from a conifer tree showing reaction wood. Notice how the “center” of the branch is actually very near the top of the limb. Compression wood has formed on the underside to keep the limb horizontal.
Reaction wood is a special kind of wood that forms on the top or bottom of a limb or leaning trunk. In general conifers “push” and hardwoods “pull”. This means that in gymnosperms (conifers) this specialized wood forms on the underside of the limb or trunk and is called compression wood. In angiosperms (hardwoods) it is formed on the top of limbs and branches and is called tension wood. Reaction wood has different properties than normal growth, being more brittle, denser, prone to cracking, etc. depending on whether it’s a conifer or hardwood.
In the sub arctic, where there is no good bow wood, natives of Finland, Norway, Sweden and Siberia, used compression spruce, pine, and larch for making the bellies of stron hunting and war bows.
The belly of a bow is in compression so this amazing wood worked very well for this. The back ( part of a bow facing away from you) was made of a strip of birch, which is strong in tension and weak in compression. So in the land of no good bow woods, they came up with a solution using the wood from 2 trees, with differing characteristics to make superior bows.
I have made and am still making working replicas of these bows from birch, with larch and spruce compression wood on the bellies.
There are other advantages to this belly wood. Miss bows get weak and soft in higher humidity, but the conifer reaction wood elongates and makes the bow stronger. The back and joints of fish glue on these bows were covered with birch bark to keep the birch back and glue lines dry. Some bows were completely wrapped with strips of birch bark to keep them dry.
Sorry I missed a couple typos.
As a former logger, I know the mills avoid reaction wood as it binds in the saws and warps badly. So there is no commercial use of reaction wood.
Tension wood in hardwoods can make a good bow also. You don’t need to laminate hardwood tension wood. It can take compression in most cases because of cell structure.