Juvenile wood, growth rate and variability

The core is different wood

Wood formed in the first rings around the pith, while the tree is young and has a large live crown, is called juvenile wood or corewood. Compared with mature wood further out it typically has lower density, shorter cells, a larger microfibril angle and higher longitudinal shrinkage (Lesson 2). The ring count is a convention: Donaldson's review defines conifer juvenile wood as the inner 10 to 15 rings, and Conners gives the same range for shrinkage.

A worked example from Japan

Loblolly pine, a North American species, grown at Tochigi, Japan (Tanabe and others, 2016; five 40-year-old trees, boundary at the 14th ring): basic density 0.33 in juvenile against 0.38 g/cm3 in mature wood; S2 microfibril angle 26.4 against 17.7 degrees; latewood tracheid length 3.02 against 4.33 mm. Lumber classified by ring width had static MOE 4.56 against 6.53 GPa, bending strength 40.7 against 51.3 MPa and air-dry density 0.47 against 0.49 g/cm3. By own calculation juvenile lumber was about 30 % lower in MOE and 21 % lower in MOR, but only 4 % lower in density. Density barely shows the problem; stiffness does. This is a small sample of one species, and the values are not general.

There is no single boundary

Different properties mature at different rates. In lodgepole pine in central British Columbia (Mansfield and others, 2009; 60 trees) the transition was about 15 years for microfibril angle, about 18 years for tracheid length and about 31 years for wood density. A mill that defines juvenile wood by a fixed ring count uses a convention, not a biological fact.

How much of the log is core?

The share of corewood rises steeply as logs get thinner. In 25-year-old New Zealand radiata pine (Haslett and others, 1991), the first 10 rings made up 44 % of log volume in logs over 400 mm, 50 % at 300 to 399 mm, 75 % at 200 to 299 mm and 99 % at 150 to 199 mm. Of 100 x 50 mm framing, about 35 to 36 % of lengths were rejected for twist overall, from 18 % for the largest logs to 70 % for the smallest. In a subsample, dressing to 90 x 45 mm more than halved twist rejection (38 % to 16 %), a dressing effect and not a log-sorting result.

Growth rate and variability

Fast growth does not mean poor wood in all species, but in spruce and many pines it tends to lower density (Lesson 3). Fertilised Swedish spruce also had a higher microfibril angle, which compounds the loss in stiffness (Lundgren, 2004).

Position in the stem matters too. In Finnish spruce, at the same cambial age ring density increased with stem height, but for rings formed in the same calendar year it decreased with height (Jyske and others, 2008). Both are true because the upper stem is younger at a given calendar year. A top log is mostly young wood near the pith, so the same ring number is not equivalent at different heights.

Stiffness, warp and grading

Juvenile wood combines a high microfibril angle with low density, which depresses stiffness. It also shrinks more along the grain, and with spiral grain and compression wood nearby, boards from near the pith are the most likely to bow, crook and twist (Johansson and others, 2003, Sweden). Visual grading by ring width and knots does not capture this, whereas machine stiffness grading does.

What it means at the saw and in the kiln

• At the saw. Treat the first 10 to 15 rings of a fast-grown conifer as a different material. Sort logs by small-end diameter and by stand or supplier, and sort boards with the pith separately where product standards allow.
• In the kiln. Expect more twist, bow and crook from core boards. Dry them with restraint and do not mix them with outer-wood boards in a charge for straight products. Burdon and co-authors (New Zealand, abstract only) report that careful segregation of juvenile wood may greatly reduce processing conflicts.

Key points

• Juvenile wood has lower density, a larger microfibril angle and more longitudinal shrinkage.
• In loblolly pine from Japan juvenile lumber was about 30 % lower in MOE but only 4 % lower in density.
• The juvenile-to-mature transition is property-specific (lodgepole pine: about 15, 18 and 31 years for microfibril angle, cell length and density).
• Smaller logs are mostly core: in radiata pine twist rejection rose from 18 % to 70 % as log diameter fell.
• Sort by log diameter, stand and supplier, and use stiffness grading.