Shades of Green: Earth's Forests -- Forest Life

Right: Cross-sections of two coniferous and one broad-leaved tree trunk. Photos by Maya Walters.

The central column of wood in a tree is called the "heartwood". All the cells in this layer are dead and clogged with resin, and are therefore unable to move nutrients through the tree. The heartwood's only job is to provide structural support. The outer layer of wood is called the "sapwood". This is the living layer of the tree, where all the nutrients are channeled. This explains how an old hollow tree can remain alive, for it is only the central heartwood that has rotted away. The diameter of either layer compared to the total size of the tree trunk varies depending on the species.

There is a major difference between the wood of most coniferous and broadleaved trees. Broadleaved trees have "pipes" (called vessels) running through the wood to carry nutrients between the roots and the leaves. Coniferous trees don't have these vessels. Instead, nutrients are moved up the trunk through small chains of special cells within the wood, and down the trunk from the leaves to the roots through cells in the bark. The vessels in broadleaved trees are many times larger than the chains of cells in coniferous trees (which can be as small as .2 millimeters), and therefore nutrients can flow much faster: sap can move at rates of 20 meters an hour in some oak trees, compared to only half a meter during the same amount of time in coniferous trees.

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