Wood structure & properties

Producing a woody trunk and branches is costly in a tree's energetic budget. But trees are long lived so can justify the investment. Wood is a strong, long-lasting material. Expensive for a plant to produce in energy terms, it is then cheap to maintain because most of the cells are dead.

Wood Structure

The two major groupings of trees - the Gymnosperms or conifers and the Angiosperms or broadleaved trees - have different types of wood structure - and are often known as softwoods and hardwoods respectively.

Conifer or softwood is made up of many thin tubes or tracheids, each between 0.1 and 10mm long and 1.2 thousandths of a millimetre wide. These are lined up longitudinally in the wood. Each tracheid is a closed unit which joins to its neighbours by tiny holes. Water and dissolved nutrients pass up the trunk through this myriad system of tracheids. This system is good for trees with a very harsh season of the year such as a long cold winter or hot dry summer.

In angiosperms, the vessels making up the wood are wider in diameter and the cells are open-ended, abutting each other to form vessels that may be several metres long although very thin. This means they can transport water more efficiently, an important advantage in wet moist climates. However this presents problems in harsher climes where conifers do better.

Sapwood and Heartwood

With both soft and hardwoods, as the tree grows older, the first xylem tissue laid down dies to produce a dead core or darker material in the centre of the trunk. Known as heartwood, that contrasts with the younger living sapwood which still conducts water up the tree.

As the heartwood is formed, the vessels in it are blocked and the wood cells are filled with resins and gums which help stiffen up the stem and provide resistance to pests and diseases.

Production of new sapwood and expansion of dead heartwood are ongoing processes in a living tree.

Weight for weight, wood has probably the best engineering properties of any material. Many of its structural properties result from the microscopic layout of its cells and cell walls.

The water in a living tree usually weighs more than its timber. After a tree is felled, the sap slowly evaporates and the timber shrinks a bit - and may split too. When this process, called seasoning, is finished the timber is much more stable and can be used for joinery.

Annual Rings

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.

Rays

These 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.

Grain

Most cells in the xylem or wood are orientated vertically - along the line of the trunk. This is the 'grain'. So cutting at right angles to the trunk cuts across the grain.

More: TRADA.
How trees grow; Wood based products.