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Timber

Structure of a tree and production of timber
by

Antoni Cantone

on 10 August 2011

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Transcript of Timber

Root system Structure of a tree Anatomy of a tree Classifications Hardwood Softwood Plantation Timber Logging Selective Felling Seasoning Kiln Drying Products In most plants, the root system is a below-ground structure that serves primarily to anchor the plant in the soil and provide sustenance.

Fine roots within the system take up water and minerals while large roots store the nutrients.

The storage function is of primary importance in some species, such as the Mallee. Long and harsh conditions, or devastating fires, make the presence of a pool of resources vital for tree survival. The inner bark, or “phloem”, is a pipeline through which food is passed to the rest of the tree. It lives for only a short time, then dies and turns to cork, to become part of the protective outer bark. The Cambium cell layer is the growing part of the trunk. It annually produces new bark and new wood in response to hormones that pass down through the phloem with food from the leaves. Sapwood is the tree’s pipeline for water moving up to the leaves. Sapwood is new wood. As newer rings of sapwood are laid down, inner cells lose their vitality and turn to heartwood. Heartwood is the central, supporting pillar of the tree. Although dead, it will not decay or lose strength while the outer layers are intact. A composite of hollow, needlelike cellulose fibers bound together by a chemical glue called Lignin. The outer bark is the tree’s protection from the outside world. Continually renewed from within, it helps keep out moisture in the rain, and prevents the tree from losing moisture when the air is dry. It insulates against cold and heat and wards off insect enemies. Look closely at each growth ring, and you'll see a wider, light-colored area to the inside of a thin dark area.

The light-coloured area is growth that occurred during the summer months, when sap flowed freely through the tree and growth occurred very quickly.

Conversely, the darker-colored section occurred during the winter, as the tree naturally built up a layer to protect the fresh summer growth.

This outer layer of the growth ring works with the cambium layer and bark to protect the tree from the cold of winter.

To prepare for the winter months, tree will also pull the majority of it's sap from the upper reaches of the tree to help keep it from freezing. In general, hardwoods come from deciduous trees that lose their leaves annually, and softwood comes from a conifer, which remains evergreen throughout the year. Hardwoods tend to be slower growing and are usually of a higher density that softwood, however this is not always the case.

Botanically speaking, hardwoods are identified as having 'covered seeds', and refers to the fact that the seeds are contained in gumnuts or flowers. Softwoods fall into the botanical category having 'uncovered seeds'. The seeds are generally found in cones, which open up to allow the seeds to blow away in the wind and germinate.

The essential difference between the timber from hardwoods and softwoods is the presence of vessels in hardwoods. These are continuous pipes running the length of the tree and serve as conduits for water and nutrients in the outer layers of wood in a growing tree. In hardwoods, the cells are closed and cannot function as conduits. In softwoods, the cells have openings to other cells. Softwoods do not have pores in their cell structure. Instead, most of the woody tissue is made up of long narrow cells called tracheids, which transport the sap and also provide strength to the stem. Sap is also allowed to travel between the cells through pits, which are little valves in the cell walls. Most hardwoods have two main types of cells that run vertically. Sap is carried upwards in the pores (also called vessels) which join together one on top of another to form pipes. Strength is provided by fibres, which have thick cell walls and make up the bulk of the wood. To help reduce the environmental impact of cutting trees for timber, special plantations of fast-growing trees are planted. These are to be specifically harvested when they have matured. When the trees are harvested, new ones are planted. This makes plantation timber environmentally sustainable, as no natural forests are cut down.

The most readily available plantation timbers in Australia are softwoods such as Hoop Pine, Radiata Pine, and Slash Pine. The latter of the two are introduced species. 'Track Feller Bunchers' fell and stack trees. 'Timber Harvesters' fell trees, remove branches, and cut logs to specific lengths to be loaded onto trucks. Harvesting Felling can be achived with a chainsaw and wedges. Logging is the process in which certain trees are cut down by a Lumberjack. This can be accomplished with a chain saw and wedges, or machinery. Clear felling is a method of harvesting, a coupe in which all useful trees, apart from those to be retained for wildlife habitats are removed.

Following the cutting and removal of logs rehabilitation is carried out by contractors. During harvesting seed capsules are collected from the felled trees. The seeds are tested and stored in anticipation for sowing operations. Before sowing, slash burning is undertaken. What is left on the forest floor after logging is set alight. This produces an ideal seed bed for the new forest. The collected seeds are then sown.

Maximum sunlight reaches the ground because there are few trees to provide shade. The ash on top of the soil provides nutrients and an uncompacted surface for seedlings to take root. As trees develop many of the seedlings can't compete with others to get enough light, water or nutrients and consequently die, therefore natural thinning of regrowth occurs. Often selected trees or saplings need to be removed to allow stronger trees to mature. Clear Felling Selective felling is a method of harvesting in which small patches of trees or single mature trees are removed. This occurs on a periodic basis, so the forest remains of an uneven age. Regeneration usually occurs after each felling so there is a wide variety of ages among the trees in the forest.

The forests where this system is used are typically mature, open, and uneven aged. If the area was clear felled, valuable regrowth timber would be wasted. Individual trees can be felled and removed without damaging those left standing. The retained trees have more opportunity to grow and increase timber volume. This increases the amount of timber that can be grown in a given forest.

This system of managing the harvested trees replicates the natural forest life cycle. Coupe: An area of forest of variable size, shape and orientation from which logs for sawmilling or other industrial processing are harvested. Conversion Natural Drying 'Natural Drying' or 'Air Drying', is a traditional method of seasoning timber. It is simple and inexpensive, however it is dependent on favourable weather conditions.

The timber is staked carefully in piles in an open or louver sided shed. The sloping roof protects against direct sun and rain.

The boards are separated by sticks of a standard size placed one above the other and at regular intervals along the timber. Air is allowed to circulate freely resulting in moisture evaporation. ‘Artificial Drying’ or ‘Kiln Drying’, provides a quick, controlled, and reliable method of seasoning timber. It offers a rapid turnover and is used by manufacturers to process most hardwoods.

Timber is stacked as for natural seasoning, but on trolleys, before being put in the kiln. After sealing, the kiln steam is introduced which soaks and penetrates the timber. After a time, pressure and humidity are introduced and the steam is drawn out by fans.

Heat is gradually introduced and the temperature is raised. Finally, hot, dry air is circulated until the moisture content is reduced to the required level. Once a tree is felled it should be converted into usable timber. The two main methods of conversion are ‘Through and Through’ and ‘Quarter Sawn’. 'Quarter Sawn' is more expensive technique of conversion because of the need to double handle the log. This method also produces more wastage. It is however more decorative and less prone to distortion. There is a variety of ways to saw each quarter of the log. 'Through and Through' produces mostly tangentially sawn timber and some quarter sawn. Tangential timber is the most economical to produce because of the relative repetitive production methods. It is used extensively in the building industry. Trees have a high moisture content and before their wood can be used it needs to be seasoned. This is a process whereby the wood is dried to remove excess moisture and serviceable timber is produced. Defects At various stages in the timbers life cycle, from germination to useful product, it can develop undesirable characteristics that are considered to be defects and can lower the timbers value. Defects in the timber can affect it both structurally and visually. These defects have five main categories. 1. Conversion Defects Chip mark: this defect is indicated by the marks or signs placed by chips on the finished surface of timber

Diagonal grain: improper sawing of timber

Torn grain: when a small depression is made on the finished surface due to falling of some tool

Wane: presence of original rounded surface on the finished surface 2. Fungi Defects Fungi attack timber when these conditions are all present:

A. The timber moisture content is above 25% on a dry-weight basis
B. The environment is warm enough
C. Air is present

Wood with less than 25% moisture can remain free of decay for centuries. Similarly, wood submerged in water may not be attacked by fungi if the amount of oxygen is inadequate.

Fungi timber defects:
• Blue stain
• Brown rot
• Dry rot
• Heart rot
• Sap stain
• Wet rot
• White rot Dry Rot Heart Rot Brown Rot 4. Natural Forces Defects 3. Insect Defects Termites Carpenter Ant Many insect species are able to use wood as a food source. In doing so they can cause serious damage to timber by tunnelling into standing trees, freshly felled logs or wet decaying timber. This weakens the structural integrity of the timber. The excrement left in the tunnels by theses insects can promote decay and fungal growth in the timber. A very small number of beetles are able to attack timber in the more or less dry conditions found in buildings.

Following are the most common insects responsible for the decay of timber:

A. Beetles
B. Marine borers
C. Termites
D. Carpenter ants Insect Damage 5. Seasoning Defects There are two main natural forces responsible for causing defects in timber: abnormal growth and rupture of tissues. Bark Inclusion Sap Stain Wane When timber is seasoned most of the moisture trapped in the timber is removed. As the moisture is drawn from the timber it decreases in volume. Because timber is an organic substance, densities in the timber are not uniform across the entire tree. This means that some sections of the timber will dry at different rates. Heartwood, sapwood and knots in timber can be points of change and points of defect. Common seasoning defects are:

Bow
A deviation from flat along a straight line drawn end to end. Measured at the point of greatest distance from the line

Crook
A deviation edgewise from a straight line drawn end to end.

Cup
A deviation in the face from a straight line drawn edge to edge.

Twist
A deviation flat wise, or a combination of flat wise and edgewise in the form of a curl.

These kinds of defects are categorised by how severe the change is from the original shape of the timber. This is measured as the greatest distance an edge has deviated from a flat surface. Cup Twist Crook Bow Seasonal Changes
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