For strength, the annual growth ring of conifers is ideally 1–1.5 millimetres thick. The proportion of summer- wood is at its largest in such a growth ring. This kind of a proportion between summerwood and springwood comes about in slow-growing Nordic coniferous forests.
The properties of trees can vary, for example depending on the growth site. Because of this, the logs arriving at the sawmill and the finished sawn timber are graded for different purposes. Grading is mainly based on two primary criteria: appearance and strength-related aspects.
Juvenile wood Located close to the core, juvenile wood consists of the core that emerges during the first year of growth, as well as the 10–15 annual growth rings around it. It has more cracks and branches that have stopped growing than the other parts of the trunk, which makes it weaker. Thanks to their slow growth in diameter, Nordic conifers do not contain much juvenile wood and their strength proper- ties are evenly distributed.
Annual growth ring The new wood that emerges during the growing season is called the annual growth ring. Successive growth rings are formed by the springwood and summerwood that are created during the growing season, and can be clearly distinguished in the cross section of the tree.
Phloem Phloem is the inner layer of the bark around the tree. Its vascular tissue conducts photosynthetic products from the needles to the other parts of the tree.
Heartwood When trees grow, the cells in the inner layers die off. The resulting heartwood is composed of dead cells, from which the reserve substances have been used to generate compounds that protect the tree. The high resin and pitch content of heartwood increases its resilience to rot and pest insects. Window and door frames are typical end uses of heartwood.
Sapwood Sapwood, where tree growth takes place, is living wood that conducts water and nutrients from the roots to the top. While pine sapwood is permeable to water, spruce sapwood is not. This makes spruce sapwood an ideal material for the external cladding of buildings.
Bark The outermost layer
of conifers – the bark – protects the inner parts from damage and disease.
Cambium A thin layer of cells beneath the outer bark that produces new wood cells inwards and new phloem cells outwards.
Sources: PuuProffa.fi, The Finnish Timber Council (Puuinfo Ltd.), The Helsinki Term Bank for the Arts and Sciences, Jääskeläinen, Anna-Stiina; Sundqvist-Andberg, Henna / Puun rakenne ja kemia Photo source: PuuProffa.fi
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