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Q & A: Density of different woods

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Most recent answer: 10/22/2007
Q:
Why does some wood have a greater density than other wood?
- Brittney Nelson A (age 13)
Fred Moodry Middle School, Anaconda, Mt. United States of Am
A:
Hi Brittney,

Woods from different species of tree or bush have different properties because of the makeup of the cells inside. The density of wood depends on how much of each kind of material is in the wood. One component that affects the density very much is the amount of air in the wood.

Tree trunks and bush branches have two main functions (and many others, too!) -- supporting the plant and providing nutrients, particularly water, from the soil. (They also offer protection, fire and insect resistance, a place to put chlorophyll during the long winters, and lots of other things). To transport water, wood has many small tubes which draw water through them by capillary action. See our discussion on for more details.

When wood is inside a living tree, these tubes are full of fluid (water and tree sap). When the wood is dried, the tubes are left with air and it is much lighter. So the density of wood depends quite a bit on how dry it is, but it also depends on how many and how big those tubes are. The remainder of the material in the cells (cellulose and other constituents) also varies in composition from tree to tree depending on the needs of the tree -- tall trees with thin trunks may need stiffer support, while short trees may be different. Some trees, like eucalyptus trees, have a lot of oils stored in the wood, and oils are less dense than water.

Tom

(published on 10/22/2007)

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