Reaction wood, knots and growth stress

Features built into the tree

Reaction wood, knots and growth stress are not processing faults. The mill can only recognise them early, route each log to a suitable product and avoid making them worse.

Reaction wood

Trees that lean, or whose crowns are pushed by wind or slope, build special wood to straighten the stem. Softwoods form compression wood on the underside of the lean; hardwoods form tension wood on the opposite side (Penn State Extension, North America; the Wood Handbook places it on the upper side). Compression-wood tracheids have a large S2 microfibril angle, high lignin and high longitudinal shrinkage; tension-wood fibres have a gelatinous layer (Wood Handbook).

On the log. The clearest sign on a log end is eccentric growth: an off-centre pith with wider rings on one side, the reaction wood on the wider side. Severe reaction wood usually looks darker and is denser and harder (Wood Review, Australia).

In the board. Tension wood fibres are partly torn out rather than cut clean in sawing, it can stain unevenly, and it collapses more readily in drying (Penn State; Wood Review).

How large is the longitudinal shrinkage? The sources disagree. Penn State gives up to six times normal for compression wood and three to five times for tension wood. Wood Review says up to 20 times for both. The Wood Handbook gives 0.1 to 0.2 % for most species and about 2 % for reaction and juvenile wood, which by own arithmetic is 10 to 20 times. The conflict is not resolved.

Consequences. Johansson (2003, Wood and Fiber Science) cut 12 studs into 3,600 sticks of 10 x 10 x 200 mm and measured longitudinal shrinkage between 18 and 8 % MC. Sticks with severe compression wood shrank significantly more than sticks with none or mild compression wood, and his model suggested that variation in longitudinal shrinkage across the section and along the stud explains most of the change in bow and crook.

Knots

A knot is the embedded base of a branch. Sound (live) knots are intergrown with the stem; dead (loose) knots are encased and can drop out as the board dries. A knot removes load-carrying clear wood, deflects the grain around it and concentrates stress.

Position matters as much as size. A finite-element study of Norway spruce beams (Saad and Lengyel, 2023, Hungary) found that a mid-span knot cut load-bearing capacity by about 39 %, and the largest loss, about 69 %, occurred with the knot at the lowermost, tension-side position. Knots in the compression zone had only a marginal effect. These are model results for particular geometries: a ranking of mechanisms, not design values.

A Lithuanian visual-grading study (Baltrušaitis and Pranckevičienė, 2003; about 4000 m3 of pine and spruce) found that knots, distortion and fissures predetermined the grade in 62.4 % of cases, and sawing accuracy and drying quality together in 24.1 %.

Growth stress and shake

Standing trees carry maturation stress. When a log is cross-cut it releases as end splits, and sawn boards can spring. In a study of 440 mature European beech from nine stands in five countries (Jullien and others, 2013), mean maturation stress was about 9.64 MPa; in 45 % of trees it was 15 MPa or more, and only 15 % were below 8 MPa. Slenderness was the best predictor of high stress, and a large crown of low stress.

Shake is a separation along the grain: ring shake follows a ring, star shake runs radially from the pith. For oak in Great Britain, Price (2015) cites an estimate that about 21 % of high-forest oak trees were affected. These are British oak figures and must not be transferred to conifers.

What it means at the saw and in the kiln

• At the saw. Read the log end first: an eccentric pith signals reaction wood, so steer such logs away from long, thin, straight products. Judge knot position as seriously as size.
• In the kiln. Segregate suspected reaction wood and pith boards, or dry them with restraint. Expect dead knots to loosen and checks to start at knots, and grade appearance products after drying.

Key points

• Compression wood (softwood, underside) and tension wood (hardwood) shrink far more along the grain than normal wood; the multiple is source-dependent.
• An eccentric pith with wide rings on one side is the log-end sign of reaction wood.
• Knot position matters: a mid-span or tension-side knot is worse than one in the compression zone (spruce model, Hungary).
• Beech growth stress varies more than fourfold between trees.
• Shake figures are for British oak and cannot be transferred.