Tag Archives: tree

Sapwood and Heartwood

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Until recently, I didn’t realise that there was more than one type of wood inside a tree. The difference was brought to my attention by Earth Science Photo of the Day‘s photo from April 4th.

ebony-sapwood-heartwoodSource: David K. Lynch

The photograph above shows a cross-section through a branch from an ebony tree. The heartwood in the centre is what we traditionally think of as being ebony – almost dark black in colour, whilst the sapwood surrounding it is the more “usual” pale brown colour.

All wood begins as sapwood, and it is sapwood that grows just under the surface of the bark, forming growth rings in the process. Sapwood, as its name suggests, carries sap (transported in tubes called xylem) which the tree uses to store and transport water, sugars (maple syrup is made by reducing xylem sap from maple trees to concentrate the sugars), hormones and nutrients.

In young trees all wood is sapwood, but in older trees, as the tree grows in diameter, less cross-sectional area is required for the transport of sap, and greater structural support is required to keep the tree upright. The sapwood in the centre of the tree dies, forming heartwood, and as the cells die they release chemicals that change the colour of the wood, as well as making the wood stronger and more resistant to attack by insects.

sapwood-comparison

The ratio of sapwood to heartwood depends on how many leaves the tree has and how fast it grows: more leaves and faster growth require more water and therefore more sapwood, and not all trees form any heartwood at all. In the photograph above, a cross-section of a maple tree is on the left and a cross-section of a black locust tree on the right: maple trees have very large leaves, and the black locust trees have small leaves, hence the very obvious difference in their sapwood to heartwood ratios.

 

Baobab

Baobab is the common name of the eight species of tree of the genus Adansonia. Baobabs are well-known for their distinctive broad and swollen trunks.

Two photographs of Grandidier’s Baobab, Adansonia grandidieri.

All Baobab’s occur in arid regions, and survive by storing water (sometimes as much as 120000 litres) inside their trunks, making them a useful source of water for human populations. Their bark is used for cloth and rope, their leaves for medicines and condiments, their fruit (“monkey bread”) as food and their seeds to replace coffee. It is not for nothing that the Baobab is sometimes referred to as the Tree of Life.

The unusual trunk structure makes it very difficult to date Baobab trees, as they do not produce annual growth rings, but radiocarbon dating has shown some to be more than 2000 years old.