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Reaction wood

A vertical trunk forms normal wood plus some reaction wood when moved in the wind. Horizontal branches and leaning branches must form reaction wood in an attempt to prevent them from bending and cracking under their own weight.

Compression wood forms on the underside of branches and contains more lignin than normal wood. Wood with a high lignin content is especially strong in compression. Tension wood forms on the upper sides of hardwoods and contains more cellulose than normal wood. Wood with a high cellulose content is especially strong in tension and can resist bending downward. Both strategies appear to work most of the time.

Trees in the woods suddenly exposed when most surrounding trees are removed can bend and sometimes break. See: more on this story.

Hardwoods such as oaks form a type of reaction wood called tension wood on the upper side of the branch. Once this fails to prevent the branch from drooping, they switch to develop more wood on the undersides, called normal wood, as shown here. This results in an oval cross section.

Conifers form a type of reaction wood called compression wood on the undersides of horizontal limbs. Compression wood attempts to prevent the branch from drooping. The pith in this pine is clearly way above the center point on this branch indicating much more development under the branch than on top. These branches form an oval in cross section.

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