Reaction wood

Trees differ from other plants in their ability to grow tall and large which enables them to obtain more sunlight than their smaller neighbors.

As a tree enlarges, it develops sapwood and heartwood. The sapwood is sturdy and flexible, while the heartwood is strong and ridged. This woody structure helps to support the weight of the tree and allows it to expand to a large size.

The spreading root system also helps balance weight and keeps the tree anchored to the ground. Trees, like other plants, produce growth hormones which regulate the rate at which they grow. In most cases, a tree will grow only fast enough to support itself and carry out its normal functions. A straight tree has a better chance of staying erect than a tree that is twisted or leaning.

Large tree branches can also place a lot of force on the tree. Trees compensate for a lean and for heavy branches by producing reaction wood, forming in place of normal wood in response to gravity and the deposition of growth hormones. Reaction wood is much denser and stronger than normal wood, and its purpose is to balance the forces of gravity.

Cone-bearing trees (“gymnosperms”) such as pines, spruce, and fir produce a type of reaction wood known as “compression wood,” when a tree is leaning or a large branch extends from its trunk. This compression wood is located on the downhill side of the lean or the underside of the branch. The compression woods help push the tree or branch back towards level, thereby preventing it from drooping further. Compression wood is full of the chemical lignin, which creates hard, brittle wood.

Hardwood trees (“angiosperms” or “dicots”), such as oaks, hickories, and maple, produce tension wood to compensate for leaning and large branches, as well as for heavy winds. This tension wood is located on the back side of the lean or the top of the branch. The tension wood helps pull the branch back to the straight and narrow. Tension wood has larger amounts of the long stringy cellulose fibers present, which provide strength and elasticity.

When examining growth rings in a cross section, reaction wood tends to be wider than normal wood. While the production of reaction wood helps the tree from bending further or cracking under pressure, this type of wood is not ideal for lumber because it can be very dense and brittle, producing rough surfaces that cup, warp, twist, or break apart. Reaction wood may separate from normal wood, resulting in ring shakes along the growth rings.

Sometimes this wood is so strong that it will break or bend saw blades as they cut into the wood.

Article by FCFCDB

Nature Note for 9/30/2022