There are three main parts of a tree being roots, trunk and crown and each has it’s own particular function.
1. The Root System
The root system is a large branch like network providing a firm anchorage against forces exerted above the ground by storms and strong winds. The root system is also vital in that it supplies the tree with essential water and mineral salts extracted from the soil.
2. The Trunk
The trunk is what poles are cut from, it is important to understand biologically what makes up its structure. The trunk conducts water and mineral salts to the leaves and manufactured food materials back from the leaves. It also stores food and provides the rigidity necessary to retain the crown above competing vegetation.
There are several components to the trunk, which are:
The outer bark is a corky layer of dead tissue with the main purpose of protection of the tree against external damage. This bark also assists in the reduction of water through evaporation.
The inner bark or phloem is comparatively soft and moist and is the tissue through which food manufactured in the leaves is conducted down to the branches, trunk and roots.
Underneath the inner bark is a thin layer of cells called the cambium. This zone is invisible to the naked eye and is responsible for all growth in trunk thickness. It builds new wood cells on the inside and new bark cells on the outside. Any increase in diameter or length of the tree is due to this addition of new cells. A tree that dies from ringbarking is due to cutting of this cambium layer.
Immediately below the cambium layer is the sapwood zone, composed of living cells whose function is to conduct water and mineral salt solutions from the roots to the leaves. This is achieved via a series of long conducting channel known as vessels. This zone can vary greatly in width and is usually lighter in colour than the truewood or heartwood. Some species can develop an intermediate layer of sapwood next to the heartwood, which may take on the appearance of heartwood but still contains living sapwood cells.
Heartwood is derived from sapwood by blockage of the conducting channels and conversion of stored food materials into tannins, resins and other substances. It is these changes, which makes heartwood more durable than sapwood. The component cells in the heartwood zone are dead with its main function being the provision of rigidity for the trunk and support for the crown.
3. The Crown
The main function of the crown is the manufacture of food materials (glucose). Carbon dioxide absorbed through the leaves and water and nutrients supplied from the roots are processed through photosynthesis and changes into complex food materials for transmission to growth tissues. Oxygen is a resulting waste product and discharged back into the atmosphere.
4. Wood Cells
Like other living organisms wood is composed of individual units known as cells. They are intended to act in either:
- Conduction of water and food materials
- Food storage, or
- Provision of mechanical strength
The wall of the wood cell is formed principally of cellulose’s which form the bulk of the cell, lignin which is strongly bonded to the cellules and a variable amount of incorporated water.
The lignin is a resin like adhesive substance bonding the framework of the individual cells to each other.
5. Water in Wood
All live trees or freshly cut timber contains water. The moisture content of freshly sawn timber is in the range of 60% to 100% which means the water component can be as high as one part water to one part wood substance.
Freshly cut timber will gradually loose the bulk of its moisture until it reaches a level similar to that of the surrounding atmosphere. (normally 12-14%) This loss of moisture is referred to a seasoning.
Poles that are to be pressure treated with preservatives in the sapwood must be seasoned to allow sufficient room for the preservative treatment to replace the water. Poorly seasoned poles will result in uneven and low levels of preservative treatment. Only the sapwood can be successfully treated with preservative treatment as the vessels, which provide a pathway for the treatment solution, are blocked in the heartwood and therefore stop penetration.
The level of moisture in wood will affect the properties of the wood and therefore will influence its suitability for different purposes.
The weight of green timber can be as much as twice that of seasoned timber.
As a rule, dry timber is considerable stronger than green timber.
Timber having a moisture content of less than about 20% will not decay. An established ratio of both water and air are necessary for fungal activity to take place and hence cause decay.
6. Defects in Timber
Timber will lose moisture more quickly from the end grain than across the grain. As timber near the end grain shrinks quicker than adjacent timber that has not lost the same amount of moisture, stresses will be formed that can only be relieved by the formation of small cracks. The size and seriousness of this cracking will depend to a large extent on the wood species.
Large lateral splits in power poles can affect both the strength and durability of the pole. If cracks develop after the pressure treatment process has been completed, areas of untreated wood will be exposed to fungal spores, insects etc and be potential problems in the later life of the pole.
Growth stresses can also produce serious splits in the ends of poles and these occur because the outer portion of a tree trunk is often in longitudinal tension while the inner section is in compression.
Seasoned timber that is constantly exposed to outside weather conditions can have the outside layers absorb moisture and subsequently swell. This outer layer will compress and when dried will re-open original drying cracks. Repeated wetting and drying over the life of the timber will see these cracks enlarge providing possible access for fungal infection. This often occurs on horizontal timber surfaces such as the top of crossarms.
Twisting of timber can occur during the drying process and this is mainly due to an unusually high level of spiral structure within individual cell walls.
