Wood structure and features

All wood consists of cellulose, hemicellulose and lignin, which are just different ways of combining the elements carbon, hydrogen and oxygen – derived from air and water. In other words, to grow a huge tree requires very little input from soil nutrients, but it does take a lot of rain and a lot of carbon dioxide.

Copyright University of Canterbury, 1996. Artwork by Mark Harrington

Nature has discovered how to create a very strong though relatively light material from these simple building blocks. Wood comes in two main forms: hardwoods and softwoods. The words “hardwood” and “softwood” cause constant confusion to those who don’t realise that they are just biological categories and don’t necessarily refer to hardness or softness.

Softwoods, or conifers, have a quite different microscopic composition to hardwoods (broadleaves) and the wood is useful for different purposes. For example, softwoods are better at making strong paper whereas hardwoods are preferred for glossy paper.

Inside a tree there is sapwood, which is the pale-coloured band of wood closest to the bark. This can be wide or narrow depending on species. At common rotation ages, radiata pine is predominantly sapwood – which is actually an advantage because sapwood is easily penetrated by artificial preservatives or dyes.

Inside the living sapwood band, there is the dead heartwood, which is a repository for a wide range of chemical waste-products (extractives) that tend to reduce its permeability. Thanks to the extractives, heartwood has two main advantages depending on species: it may be an attractive colour, and it may have some degree of natural durability. The latter is because the heartwood, although dead, must provide structural support to the tree for a considerable time despite constant attack by insects and fungi.

Most people will be aware of the annual growth rings in a disc of wood. The light-coloured part in each ring is called earlywood and is laid down at the start of summer, whereas the denser dark-coloured band (latewood) is laid down in autumn. Wide rings usually indicate “fast growth” (ie a high level of wood production) but will not necessarily say much about wood density or any other feature of wood quality.

There are species such as Douglas-fir where the bands of latewood are extremely dense compared to the earlywood, and these are usually best at providing structural timber; then there are species with a smooth transition from earlywood to latewood (such as radiata pine), which are superior for making appearance-grade products with uniform surfaces suitable for paints and varnishes.

Being a biological product, no two pieces of timber are identical. This is frustrating for engineers and architects, but does provide some interest and variety to the consumer. In particular, the size and pattern of knots can be important for both appearance and strength. Knots indicate where a branch has grown, and may seamlessly merge to the rest of the wood if the branch was alive at harvest but will be only loosely connected if the branch had died previously.

The density of wood will also vary depending on species and where the tree was grown. High density generally means harder, stiffer and stronger wood, but this is not always critical or even desirable. It is sometimes more important for timber to be lightweight (eg balsa for model aircraft) or stable (Western Red Cedar for window sashes), or easy to process and manufacture into a wide range of products (radiata pine).