Growth Stress and Reaction Wood

In practice growth stresses are not measured directly (Archer, 1987). It is easier to release the stress and measure the strain relief (e), together with the elastic modulus or stiffness, E. The growth stress (a) is calculated assuming simple elastic theory, E = o/e. Where growth stresses are severe the longitudinal tensile strain at 

Maximum compressiva. stress, due to creep / and compression / failures over time 

Summation of both growth stresses and wind loading 

If the stem of a young sapling is bent into a loop in the vertical plane, then at the top of the loop the uppermost part of the looped stem is stressed in tension and its underside is in compression whereas at the bottom of the loop the stresses are reversed. In such a looped stem softwoods form compression wood tissue on the underside at the top of the loop (where the bending stress is compressive) and on the underside at the bottom of the loop (where the bending stress is tensile), whereas hardwoods form tension wood on the upper part of the stem in both positions, i.e. compression wood can form in that part of the stem that is in tension as well as that part of the stem that is in compression, and similarly for tension wood. 

Compression wood forms on the underside of all softwood branches and tension wood on the upper side of hardwood branches. Thus reaction wood is responding to and resisting the additional incremental mass of branch wood that is weighing it down and which would otherwise result in the branch drooping from its preferred inclination. Most curious, if a branch is forced upwards compression wood forms on the upper side with softwoods, while tension wood forms on the lower side with hardwoods. Reaction wood is seeking to maintain some pre-determined, natural branch angle. 

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Growth stresses and reaction wood, while normal in the sense that they occur with some frequency, are generally considered atypical and their presence can create problems in utilization. The very large growth stresses in severe compression wood or tension wood, contrasting with reduced growth stresses in opposite wood, are extreme examples of the asymmetric distribution of growth stress around the... 

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