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Juvenile wood
A tree’s age can affect the quality
of the sawn timber. In Radiata Pine, for example, the density
of the timber laid down in each successive growth ring is
slightly greater than the one before. Therefore, the density
of the timber produced in the first 10 years of growth—the
central ten growth rings—may be up to 40 per cent lower
than that of the wood being laid down in the outer growth
rings. The early growth of pine, or ‘juvenile wood’,
also has shorter fibres and higher fibre angles, which reduce
its strength and make it difficult to dry. Because of this,
many pine millers may prefer logs older than 25 years due
to concerns about wood quality. The timber laid down by young
eucalypts is also lower in density than that laid down by
older trees. However, since the wood of many mature eucalypts
is much heavier than that of internationally recognised cabinet
species like Blackwood and Oak this loss may actually be an
advantage as the lighter timber is easier to work.
Juvenile wood is sometimes called ‘crown wood’.
It has been shown that it is the proximity of the active canopy
that determines the characteristics of the wood cells laid
down. Stem pruning, by lifting the green canopy, has been
shown to reduce the incidence of juvenile type wood in the
lower trunk.
Whilst it is true that if fast grown trees are harvested when
young their wood density will be lower than older trees of
a similar size, in most species, including pine and eucalypts,
growth rate has little effect on the density of the wood in
each growth ring. In fact, fast grown oak (and other ring
porous hardwood species) is actually denser than slow grown
oak. Poplar is one of the few species for which faster growth
appears to lead to lower wood density irrespective of age.
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