Understanding Timber: The Anatomy of Trees

A timber board may look simple on the surface, but do not be fooled. Beyond the basics of what we can see lies years of evolution and cell growth, with as much biology at play in an Oak desk or Pine dining table as in our own bodies. The anatomy of the trees whence your timber comes is fascinating, and our team at EcoChoice thinks it sheds light on just why this is such an incredible and versatile building material.

Intrigued? Find out more about the anatomy of timber and the trees it comes from in our insightful guide.

Looking at Tree Trunks

If you have ever looked at the stump of an old tree and marvelled at the details on display, you are not alone! The cross-section of a tree trunk reveals amazing features. Moving from the outside to the inner centre, you will see:

• The bark: The tree’s outer protective layer, shielding it from weather, pests and physical damage.
• The cambium: A thin, active layer just under the bark where new cells develop, helping the tree grow and expand in diameter.
• The sapwood: A layer which transports water and nutrients along the axis of the tree, and is often lighter and more susceptible to pests.
• The heartwood: The strong, inner core providing structural support, which is denser and darker than the sapwood.
• The core (or pith): The innermost part, a small, soft section from the tree’s earliest growth.

When looking into timber for construction and engineering, you will receive the heartwood or sapwood of the tree depending on your intended application. The inner 5 to 20 growth rings are often called juvenile wood as they were formed in the first growing phase of the tree. This area of timber is not as durable or dense as the more mature timber, so we do not recommend it for construction.

What Are Tree Rings?

Tree rings are a pretty distinctive part of the cross-sectional trunk. They form as the tree grows, with one ring for every year and yes, you really can count the age of a tree using its rings!

These rings form as each growing season is defined by different rates of growth. In the earlier part of the season, the tree grows more quickly and the wood both less dense and lighter in colour. Later in the season, it slows down and becomes darker, creating the distinction needed for rings to form.

Interestingly, ring width can also be used to indicate the strength of timber. When the tree grows quickly it becomes less dense and forms wider rings, telling us that the timber will be of lower quality and less suitable for construction.

Down to a Cellular Level

If you look at timber under a microscope, you will notice that the cells are long and tube-shaped, with each cell being around 2-5mm long and 10-50mm wide. These are known as fibres, and they line up to form the grain of the timber.

The cell walls themselves are made up of several layers which work together to give wood its characteristic strength and hardness. Each layer is made of cellulose fibres embedded in a mix of lignin and hemicellulose, with the fibres oriented differently in each layer. The thickest layer has fibres running parallel to the cell, which is great for handling tension forces. The other layers have fibres at angles between 50–70°, which helps the wood resist buckling under compression.

In softwoods, such as larch and douglas fir the walls of the cells are stuck together by a glue-like substance called the middle lamella, which is rich in lignin. The centre of each cell, known as the lumen, is where nutrients are transported.

Hardwoods such as Oak or Ekki take a different approach, using large-diameter, tube-like cells called vessels to move nutrients. Hardwood fibres usually have smaller lumens and thicker walls than softwood fibres.

The way that the fibres of timber are aligned along the trunk ensures it can tackle the challenges of tree growth with ease, without bending in the wind or buckling under its own weight. This also makes it a great material for construction, bringing strength, durability and load-bearing characteristics to a variety of applications.

Find Your Favourite Timber Species

If you are considering choosing timber for your next project, we can help. At EcoChoice, our timber experts know everything there is to know about wonderful wood, and we will be happy to match your application to the ideal timber species. Get in touch today and let’s talk more about your timber needs.