TIMBER GROWTH AND FELLING

Wood is obtained from trees. When it has not been processed for use, it is typically referred to as timber. Trees are renewable materials, although they grow very slowly, some good quality hardwoods require 50 or more years to mature. Because of this, the planting of trees to replace the ones that are cut down is encouraged by many government agencies and environmental groups all over the world.

Before wood can be used, it has to be obtained and processed. There is no other known way of obtaining wood apart from through felling of trees. To facilitate the felling of trees in Nigeria, land is categorized into two namely, free areas and forest reserves.

 

1) Free area

This is a portion of forest where a contractor can be permitted by a government representative to cut down a tree of his choice after paying some money called tariff. In such an instance, a properly registered tree logger identifies matured trees of interest and therefore approaches the appropriate agency of government or government representative to request permission to pull down the tree. If the permission is given, he moves in his machines and equipment to pull down the trees.

 

2) Forest reserve

A forest reserve is a portion of land where a forest has been intentionally established, usually through a deliberate planting of trees. Forest reserves exist all over Nigeria, with many of them having been created several decades ago. In a forest reserve, the government invites the contractor to come and fell trees in a designated area. This happens whenever the trees in that particular section of reserve have matured or they may otherwise need to be replaced with newly planted ones. The contractor will be required to pay some money. The amount of money paid will depend on the volume of the timber felled otherwise called outturn volume.

All over the world, felling of trees is carefully controlled because indiscriminate felling can negatively affect the environment. Trees are among the most useful living things on earth and they produce wood which is one of the world’s most useful materials. They cannot be quickly replaced. Many portions of the earth’s surface where trees have been indiscriminately felled with little or no control are now bare with attendant consequences of erosion and other threats to the environment.

 

Conversion and Seasoning of Timber

A) Conversion

After trees are felled, they are transported to a saw mill by rail or road. In some areas of the world, transportation is accomplished by floating the logs along a river. At the saw mill, the felled trees or logs are put through a process called conversion. Conversion is the process of cutting tree trunks or logs into desired sizes. This is usually done before the logs dry and consequently shrink. When converting the logs, allowances are made for eventual shrinkage on drying and final planing. The methods of conversion are as follows:

i) Plain sawing 

This is also known by names such as flat sawing, slash sawing, Through- and-through sawing etc. It involves sawing across the medullary rays of wood. However, planks of wood so converted are more liable to warp or split than those of other methods. In plain sawing, planks are cut parallel to the axis of the tree trunk.

 

ii) Quarter sawing

In this method, the log is cut along the medullary rays. Quarter sawn planks are more stable than those of plain sawn. In quarter sawing, planks are cut at right angles to the growth rings.

 

iii) Tangential sawing

Tangential sawing may be considered a variation of plain sawing. It involves cutting planks in a manner that allows the wider edges of their cross-sections to be tangential to the annual rings.

 

B) Seasoning

A freshly felled tree has high moisture content. Moisture content is the scientific term for expressing the amount of water present in a material. Wood of high moisture content is not of much use because of the following:

1. It is not stable. This means it will shrink with the passage of time and its quality will consequently be lowered.
2. In most cases, wet wood is weaker than dry wood.
3. Wet wood contains sap (starch, water etc.). As a result, it is more readily attacked by fungi, insects and other pests, which feed on the sap.
4. Wet wood cannot be easily painted, polished, planed or finished.
5. Wet wood does not readily absorb substances usually applied on wood to make its life longer (i.e. preservatives). Thus it is not easily preserved.
6. It is much heavier than dry wood. This makes it more burdensome to transport.

To avoid the above listed disadvantages, it is necessary to reduce the amount of water in wood to a tolerable level. This process of reducing the moisture content of wood to a desirable level is called seasoning. The objective of seasoning is not to remove all the water in the wood, in fact, for each type of wood there is usually a certain moisture content beyond which further seasoning will result in permanent damage. The knowledge of the eventual use of the wood is necessary in order to determine the moisture content to which the wood should be dried.

Moisture content is determined by weighing a sample of a piece of wood when wet as well as when it is dry and then calculating using the following equations.

If:

• m1 = mass of wood sample when wet
• m2 = mass of same wood sample when dry
• m.c = moisture content

Then,

 

There are two methods of seasoning namely:

1) Natural seasoning

This involves stacking the planks in open air , therefore, the method is referred to as air seasoning. A roof is provided to keep off rain. Proper air circulation is achieved by separating the planks with small wooden blocks called stickers and clearing the surrounding area of vegetation. The ends of the planks are painted so as to ensure even rates of drying throughout the lengths of the planks.

 

 Advantages of Natural seasoning 

• The method is cheap. All that is required is to make a shed for the stacked timber.
• It requires minimal attention. Once the wood is stacked up, it is left for the elements of weather to act upon it.

 

Disadvantages of Natural seasoning 

• It takes a long period of time to accomplish. This period can be up to one year or more, depending on the thickness of the timber planks.
• It may not dry the wood enough when very low moisture contents are desired.

 

2) Kiln seasoning

A kiln is a man-made heated chamber usually constructed of bricks in which the planks are stacked. Because the speed, the moisture content (humidity), and the temperature of the air entering the kiln are controlled by the operator of the kiln, this method is called artificial seasoning.

 

Advantages of Kiln seasoning 

• It is a very fast method, taking only a few weeks at most.
• It is a more exact method since any desirable moisture content of wood can be attained.
• Wood-destroying agents such as fungi, woodworms, insects and other pests are killed by the heat produced in the kiln.

 

Disadvantages of Kiln seasoning 

• It is costlier to operate. Much money is spent on fuel or electric power, which is used to run the kiln. Some kilns use dry waste wood as fuel to reduce this cost.
• It requires greater attention. The operator has to continually monitor the artificial conditions maintained in the kiln and make adjustments whenever necessary.

 

Sometimes, wood is seasoned by beginning with the natural method and completing with the artificial method. This is because the natural method may not be able to dry wood to very low moisture contents whenever such are desirable.