Timber measurement

Age

The age of a tree can be roughly estimated from measuring the diameter of the trunk, and observing the conditions in which it is growing. In its early life, a tree tends to grow fast, increasing in height, spread and circumference of the trunk. As the tree ages, growth slows, and height and spread increase may stop, but the trunk continues to grow slowly in girth. The mean growth in girth of most native broadleaved trees, grown in open ground and with a full crown, is 2.5cm (1”) per year (Mitchell, Alan, 1974). Therefore an open-grown tree 2.4m (8’) in girth is usually about 100 years old. Yew tends to conform to the 2.5cm rule for the first 100 years, but then slows to less than half this rate over the next 4-500 years.

Table 5d: Estimate of girth in relation to age for open-grown BROADLEAVED trees

Trees put on girth more slowly when in competition with other trees, so that a tree growing in a woodland will show a mean growth in girth of 1.5cm
(1/2”) per year or less. Species also vary in their speed of growth. The table on page 44 is based on Forestry Commission estimates for trees grown in closed woodland.

Table 5e: Approximate relationship of tree diameter to age (trees in woodland)

An accurate age can be obtained from counting the annual rings of a felled tree. An increment borer, available from forestry suppliers, can be used to take a core of wood from a living tree, from which the annual rings can be counted. For notes on estimating age of veteran trees, see here.

Length and height

All lengths should be measured in metres, rounded down to the nearest tenth of a metre for lengths up to 10m, and to the nearest whole metre for lengths greater than 10m. The length of a piece of timber should be measured with a tape following the curvature of the log.

The total height of a standing tree is the vertical distance from the base to the uppermost point (tip). The total height of a felled tree is the straight line distance from the base to the tip.

The timber height (or timber length) of a tree is the distance from the base of the tree to the lowest point on the main stem where the diameter is 7cm overbark. In hardwoods, and occasionally in conifers, this may be the lowest point at which no main stem is distinguishable.

There are several types of instrument which can be used to measure the height of a standing tree, and which are available from suppliers of forestry equipment. Two alternative methods of estimating height, using simple equipment, are as follows.

• Cut a cane or suitable piece of wood to the exact distance measured from your eyeball to the farthest stretch of your grasping finger and thumb. Stand at a distance from the tree to be measured, holding the cane vertically at this same outstretched reach. Then walk back or forth until the tip and base of the tree are exactly in line with the upper and lower end of the cane. Mark the ground directly below the cane. The height of the tree is equivalent to the distance from this mark to the base of the tree.
  
• This method requires two people, and a 30cm ruler, clearly marked at the 3cm point. One person holds the ruler vertically at arm’s length, and moves back or forth until the tree is aligned vertically with the ruler. The second person holds an easily visible marker against the tree, and moves it up or down, guided by the calls of the first person, until the marker aligns with the 3cm mark on the ruler. This point is marked on the tree and is then measured from the ground, and this measurement is one tenth of the total height of the tree.
  

Note that it is only in spire-topped trees that the apparent top shoot is the true tip of the tree. On wide-spreading trees, the shoots on the nearest branches will appear higher than the actual tip. Where possible, walk around the tree and study it from several angles before choosing the point which appears to be the top centre.

Diameter

Tree diameter should be measured in centimetres, rounded down to the nearest centimetre for individual trees. Mean diameters are recorded to the nearest whole centimetre. Diameters are usually measured with a special girth tape, available from forestry equipment suppliers, which is placed around the trunk 1.3m above the ground, and from which the diameter can be read. This measurement is called the diameter at breast height (dbh). Alternatively, use a standard tape measure, and divide the measured circumference by 3.142 to obtain the diameter.

Volume

Volume should be recorded in cubic metres, to two or three significant figures as required. Volume measurement may be required for estimating the volume of a standing tree, the volume of a stand, or the volume of felled timber.

There are several different methods of measuring volume, with varying degrees of accuracy. The method chosen should relate to:

• The reason for carrying out timber measurement. If the stand is being measured for sale, a more accurate method should be used than if the stand is being measured for drawing up a management plan.
• The value of the stand. Very high-value stands should be sold only on the basis of measurement of the felled timber. High- to average-value stands should be measured by a method called tariffing, which requires the use of tables. Low-value stands can be estimated by a method using basal area. These methods are described in the Forestry Commission publications Forest Mensuration and Timber Measurement.

The methods noted above require not only the estimate of the volume of a selected tree, but an estimate of the number of trees within the stand to give the total volume. For small projects, tree volume can be estimated from the table given below, by measuring the dbh of individual or selected trees.

Table 5f: ESTIMATE OF TIMBER VOLUME (CUBIC METRES)