What is 'wood'?

Does this question leaf you stumped?
19 November 2021


Collection of different sized logs


Iacopo Russo, one of our interns at The Naked Scientists, walks us through the nearby forest to the Naked Scientists office while explaining various definitions of 'wood'.

Iacopo - When I'm deep into a project and I can't see the wood for the trees, I like to go for a walk to clear my head. The Naked Scientist office is based in Maddingly, just outside Cambridge. We're lucky enough to have a patch of woodland just across the road. It's a crisp autumnal day. There's leaves everywhere on the ground and I'm surrounded by trees, which as we all know are made of wood. Of course we're familiar with it, but what exactly is wood? Well, it's made of three main parts, cellulose, lignin, and hemicellulose. Cellulose is a chemical made out of sugar molecules and makes fibres that are very strong in tension. These fibres are like chains, which means if you pull on them, they will resist. But if you try to squash them, they will crumble. This would be very bad news for the tree. So to prevent it, trees strengthen their wood with lignin, a stiff polymer with a messy structure that stops the cellulose chains from collapsing under their own weight. Lignin is also what gives wood it's brown colour. Finally, to stick the two together, there is hemicellulose, which gives wood flexibility.

To the right of me here, there's a beech tree with its lovely red and yellow leaves. And to the left is a spruce which will hold it's needle leaves all winter long. Deciduous trees like beech produce what we call hardwood. Whereas coniferous trees like spruce make softwoods. These two groups of trees have been evolving separately for hundreds of millions of years, so their woods have very different structures with hardwoods being a lot more complex. However, just to confuse things, not all hardwoods are hard and not all softwoods are soft. Balsa wood, which is a very soft wood used to make model airplanes, is technically a hardwood. Whereas yew trees, which you often see in graveyards, give a very hard wood, but are categorized as softwoods.

Ahead of me I see a tree that's been cut down with the characteristic rings that give wooden furniture the beautiful grain with dark and light lines. This is all to do with the amount of lignin, which you may remember was the brown bit in wood and is found on the outside of the wood cells. In spring and summer trees grow fast and the cells are large to allow nutrients to travel from the roots to the canopy. This means there's less space for lignin. In autumn and winter trees grow more slowly. Being strong and standing up to harsh weather is more important to them, so a lot of lignin is produced, which explains the darker colour.

I've always found baffling about wood is that it may look sturdy, but actually at least half of a tree is made from the air. Thanks to photosynthesis, trees transform carbon dioxide from the atmosphere into the sugars that become the wood. And get this, in order to grow 1 kilogram of wood, a tree has to draw down 1.6 kilograms of CO2. That's why everyone talks about trees as one of the main solutions to oppose climate change. But we have other uses for trees beyond carbon sequestration. Of course, it's easy to think of houses made of timber and of wooden furniture. But in fact, in the USA half of the harvested timber is used in wood pulp and paper production. And even in places like Europe, a quarter is burned as fuel. I've got to get back to the office now, but it does make me think, are there better ways we could be using this marvellous material?


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