Density, growth and juvenile wood: where the variation comes from

Density with a stated moisture state

Density is mass divided by volume (kg/m3). Specific gravity is density divided by that of water, so it is a pure number. Both need a stated moisture state, because mass and volume change with moisture content (MC). MC is defined on the oven-dry basis: MC (%) = (wet mass - oven-dry mass) / oven-dry mass x 100, so it can exceed 100%.

The USDA Wood Handbook (North America) uses oven-dry mass with different volumes:

• Basic specific gravity (Gb): oven-dry mass over green volume, the most repeatable reference;
• G12: oven-dry mass over volume at 12% MC, common in strength tables.

Conversions

Use 1000 kg/m3 for water.

• Density at 12% MC: rho12 = G12 x 1000 x 1.12.
• Green density: Gb x 1000 x (1 + MC/100).

Example 1. A species with G12 = 0.60: 0.60 x 1000 x 1.12 = 672 kg/m3.

Example 2. Spruce with Gb = 0.40 and sapwood at 120% MC: 0.40 x 1000 x 2.2 = 880 kg/m3 green. This is the number that matters for truck loading.

Example 3. A sample weighs 1.50 kg wet and 1.00 kg oven-dry: MC = (1.50 - 1.00) / 1.00 x 100 = 50%.

Never compare G12 and density at 12% directly; they differ by the factor 1.12.

Why density matters for grading

Density is a grade-determining property and an input for machine grading. Across species, higher density generally means higher strength, but the link to stiffness is weaker. Within one species the scatter is large: in a Polish study of 27 Scots pine boards from one stand, density at 12% MC ranged from 469 to 686 kg/m3, while stiffness ranged from 7.25 to 16.15 GPa, more than a factor of two.

Where density varies

European spruce data show where to expect variation (Finland, 85 Norway spruce, Jyske et al. 2008):

• 49 to 80% of total density variation lay inside annual rings (earlywood versus latewood);
• 11 to 27% between rings;
• only 3 to 6% between heights in the stem.

Density in conifers generally rises from pith to bark, mainly through latewood.

Juvenile wood

The first rings around the pith, formed under a large live crown, are juvenile wood (corewood). Compared with outer mature wood it typically has lower density, shorter cells, a larger microfibril angle and higher longitudinal shrinkage.

A Japanese study of 40-year-old loblolly pine (a North American species grown in Japan, five trees, a small sample) measured lumber means:

Worked example: what does juvenile wood cost?

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

Density barely changed while stiffness fell by almost a third. The larger microfibril angle of juvenile wood is a likely contributor, and density does not capture it; that is a reason to measure stiffness too.

The juvenile-to-mature transition is property-specific: in lodgepole pine from British Columbia it occurs at about 15 years for microfibril angle, 18 for tracheid length and 31 for density. A fixed ring count for the boundary is a convention.

Growth rate and silviculture

In spruce, faster growth tends to mean lower density because more earlywood is formed. In a Swedish study (Lundgren, 2004), more than doubled ring width went with only about a 20% fall in density, but fertilised trees also had a higher microfibril angle. A German case study of 100 spruce trees found a strong negative correlation between ring width and density (rho = -0.754), but age, site and treatment were confounded, so it is a case study, not a rule.

Checklist for a raw-material density routine

• Cut a clear sample from the same position on each log; note whether the pith is included.
• Weigh, measure, oven-dry to constant mass, weigh again.
• Record the MC at which dimensions were measured and track results by supplier.

Key points

• Always state the moisture basis: rho12 = G12 x 1000 x 1.12, green density = Gb x 1000 x (1 + MC/100).
• Density varies mostly inside annual rings and between trees; species tables hide this.
• Juvenile wood (loblolly pine, Japan): about 30% lower MOE and 21% lower MOR but only 4% lower density.
• Faster growth lowers spruce density, with a smaller loss than the growth gain; effects of age, site and treatment are confounded in the case studies.