Juvenile wood, growth rate and stiffness

Two logs of the same species and diameter can saw into very different timber. One gives stiff, straight boards; the other passes visual grading but fails the stiffness limit or twists in the kiln. Much of that difference comes from where in the stem the wood grew and how fast the tree grew.

Juvenile wood

The first rings around the pith, formed when the tree is young with a large live crown, are called juvenile wood (or corewood). Compared with mature wood further out, juvenile wood typically has lower density, shorter cells, a larger microfibril angle (MFA) and higher longitudinal shrinkage. MFA is the angle at which cellulose microfibrils wind in the main cell-wall layer.

A well-documented example is loblolly pine grown in Tochigi, Japan (a North American species; five 40-year-old trees; Tanabe and co-authors, 2016). Lumber was classified by a ring-width threshold of 5.0 mm:

Worked example: how much weaker is the juvenile lumber?

The percentage lower is (mature - juvenile) / mature.

• MOE: (6.53 - 4.56) / 6.53 = 0.30, about 30 percent lower
• MOR: (51.3 - 40.7) / 51.3 = 0.21, about 21 percent lower
• Density: (0.49 - 0.47) / 0.49 = 0.04, about 4 percent lower

The lesson: density barely differs, but stiffness and strength fall a lot. Density alone would not have warned you. This is a small sample of five trees, so the direction is typical for pines but the values are not general.

There is no single juvenile wood boundary

Different properties mature at different rates. In lodgepole pine in central British Columbia (60 trees; Mansfield and co-authors, 2009, North America) the estimated transition was about 15 years for MFA, 18 years for tracheid length and 31 years for density. A mill that defines juvenile wood by a fixed ring count is therefore using a convention, not a biological fact. Practical working rule from the portal: treat roughly the first 10 to 15 rings of a fast-grown conifer as a different material from the outer wood.

Growth rate and density in spruce

In spruce and many pines, faster growth means more low-density earlywood and therefore lower density. Two datasets:

• Sweden (fertilised and irrigated Norway spruce trials; Lundgren, 2004): more than a doubling of ring width went with only about a 20 percent density decrease, so dry matter production still rose, but MFA was higher and calculated strength lower.
• Germany (case study of 100 Norway spruce trees in five plots; Höwler and co-authors, 2022, Europe):

Density fell by (511 - 415) / 511 = 0.188, about 19 percent, while median MOE fell by a factor of 15,900 / 7,830 = 2.03, about two. The authors stress that each site had one treatment, the sample was small and only the lowest 12 m was analysed, so age, site, treatment and juvenile-wood share are confounded. Do not conclude that wide rings alone cause a halving of stiffness.

Consequences in the mill

• Stiffness: juvenile wood combines high MFA with low density, which depresses MOE. Machine stiffness grading captures this; ring-width rules do not.
• Warp: boards containing the pith or core are most likely to bow, crook and twist.
• Sorting: variation between trees within a site is often larger than between sites, so sort by stand or supplier and monitor the pass rate.

Key points

• Juvenile wood has lower density, shorter cells, higher MFA and higher longitudinal shrinkage than mature wood.
• In the loblolly example juvenile lumber was about 30 percent lower in MOE and 21 percent in MOR but only 4 percent in density.
• The juvenile boundary is property specific (15, 18 and 31 years in one lodgepole pine study); a ring count is a convention.
• Faster growth lowers density in spruce, but the German plot contrast mixes age, site and treatment.
• Sort by stand and supplier, and use stiffness measurement rather than ring width alone.