How trees grow

Trees are the tallest free-standing organisms in the world. They live longer and become more massive than any other living organism on earth. The record is held by "General Sherman", a 83.7m high Giant Sequoia in California. It is estimated that it could provide enough wood to build 40 timber framed houses. So what is their secret?

Non-woody plants grow from actively dividing cells called meristem found only at the tips of buds and roots (hence apical meristem).

But in most woody plants however, there is a complete layer or shell of cells called the cambium which wraps around all parts of the plants like a glove. This cambium allows perennial woody trees and shrubs to grow outwards as well as upwards each growing season. This secondary thickening is the answer to trees' success.

During evolution, woody plants developed this layer of special cells called the vascular cambium around the outside of their stems but below the bark. This is a key to their success.

This cambium is active on both the inside and outside.

On the inside the cells divide to lay down woody tissue or xylem which strengthens the stem and allows water to move up and down.

On its outer face, the cambium produces a thinner layer of phloem or bast - the tissue that is used to transport sugars from the site of production in the photosynthesising leaves down the stem or trunk to the roots or up to the growing shoots. The phloem is the tissue that glistens if the bark is removed.

Secondary thickening means that a new layer of xylem can be added each year in a sleeve around the existing wood inside the stem.

This all means that when they are young and small, tree saplings can concentrate on growing upwards and can then fatten or thicken their trunk later in life when structural strength is needed. Secondary thickening also permits branching so the tree can spread sideways and provide a larger surface for its leaves to capture sunlight.

To protect the delicate phloem tissue, trees have a second cambial layer outside the main one. This cork cambium produces bark that protects the delicate phloem from mechanical damage and many pests, insulates the trunk from changes in temperature and protects it from fire and other damage.

Bark is a comparatively waterproof layer which protects the tree from drying out and from insects, pests and fungal diseases. As the trunk grows fatter, the bark spreads and cracks, often giving it a gnarled appearance.

Each growing season, the cambial cells divide to form a new layer of phloem to the outside and an annual ring of new wood to the inside. The phloem is like a sleeve or glove towards the outside of all of the woody parts of the tree.

In places like the UK with a marked difference or seasonal climate, trees do not grow all year. The seasonal growth pattern is shown by growth rings in the wood. These are visible if the trunk is cut horizontally. These rings occur because trees lay down a larger proportion of wide, thin-walled cells in spring to help water transport up the stem; later in the year, narrow, thick-walled cells are laid down which help strengthen the growing trunk. And most are dormant in winter.

In climates with a marked summer and winter, there is an annual cycle of tree growth. The first new activity is below ground. In spring, as the soil warms, new tiny, fine root hairs grow and begin absorbing water and dissolved nutrients to pass up the rest of the buds, shoots and leaves. In the leaves, the green pigment chlorophyll captures solar energy and uses it to make carbohydrates from water and carbon dioxide through photosynthesis.

These carbohydrates from the leaves are transported to other parts of the tree and are the building blocks for making more complex products like starch or cellulose.

Every growing season, the cambial cells divide to produce multiple layers of new cells - to the inside these form the new wood cells or xylem - to the outside they make the thinner phloem or bast.

The first new xylem cells or spring wood each year are large with thin walls. By mid-summer the cells added become smaller and often darker - the summer wood - and by autumn these can be tiny, dark and thick-walled. The system then shuts down for the winter and starts up again with a new layer of spring wood the following year.

So if you cut a tree trunk, a clear annual ring is evident.

This happens each year, so counting the annual rings tells you how old a tree is or was.

The medullary rays visible are lines of larger living cells which radiate out from the centre of the trunk like irregular wheel spokes. The rays cross from one year's growth ring or sleeve of wood or xylem to the next providing lateral strength by binding it all together. Sugars are stored in the rays too. The rays link the xylem and phloem systems. More: The following books provide an excellent overview of how trees work, how they adapt to different climates and environmental pressures, and the pushes, pulls, and strains and stresses they have evolved to cope with so successfully.

"Trees" (2001) by Roland Ennos published by the Natural History Museum;

"Trees: their Natural History" by Peter Thomas (2000) Cambridge University Press; "Trees: their Mechanical Design" by Claus Mattheck (1991) Springer-Verlag, Berlin.

"The Body Language of Trees" by Claus Mattheck & Helge Breloer (1994) London:HMSO.

Any more recent ones are on our Links & Latest pages.