1. On a cross-section of a tree stem you can usually see a dark-coloured centre and light-coloured wood nearer to the bark. The light-coloured wood is called sapwood. It is made of living cells and is in communication with the cambium and inner living bark. The dark-coloured wood in the centre is called heartwood. It consists of dead cells. Its main function is mechanical support of the stem and the crown.
2. When the tree is young it has no heartwood. The formation of heartwood begins when the tree grows to a certain height. This process begins at different time in different species. It also depends on conditions in which the tree grows.
3. As the tree becomes older the heartwood grows wider. The tree, of course, retains a layer of sapwood under the bark which provides sap conduction. The layer of sapwood may consist of only a few growth rings in some species and of more than 100 in others. The sapwood is usually widest when the tree is young and becomes narrower as the tree increases in diameter. The sapwood is wider in the upper portion of the trunk than in the base of the tree.
4. In most trees heartwood can be clearly seen because ofits darker colour, but some species such as spruce, true firs, hemlocks, aspen and poplar usually do not show a darkening of the heartwood. In some other species the formation of heartwood begins much later and the stems of smaller size may be composed of sapwood only.
5. The formation of heartwood is an enigma. There are a lot of questions about it that still have no answers (e.g. At what time does the formation of heartwood begin in a tree? Why cannot some sapwood be transformed into heartwood? etc.). It should be mentioned that one part of a growth ring may be in the sapwood and the other in the heartwood.
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When does the formation of heartwood begin?
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