Forest measurements can be used to place a value on the standing timber before the sale of a property or to negotiate the value of the timber with a sawmiller or logging contractor.

Consequently being able to measure a forest will give landholders much more control over their forest operations.

The basic measures of an individual tree are the diameter and height of a tree stem. This can be used to estimate the volume of the stem. Stem diameter (over bark) is measured using a diameter tape measuring the stem at 1.3 metres above the ground level. This is called the diameter at breast height (DBH).

Basal Area

Basal area is a very important measure of stands of trees. It is used regularly in forestry management of trees on public lands but unfortunately has been an under utilised concept on private property. Basal area expresses the density of a stand of trees. The basal area of a tree is the cross sectional area (over bark) of a tree at breast height and is measured in square metres (m˛). The basal area of a stand is the total basal of all the trees in a stand and is expressed in square metres per hectare. The following picture shows these two concepts.

Basal area measurements can be used to make management decisions such as when to thin or harvest a stand. It can also be used to set cutting limits at harvest. As a general rule, to keep a forest in a healthy and vigorous condition the basal of the forest should be within the range of 10 to 30 m˛ per hectare. This can change with different forest types.

Relascope.  Basal area measurements of stands of trees can be made with a basal area relascope. The relascope allows you to rapidly estimate the basal area at any location within a forest.

The following formula the BAF of a homemade relascope can be checked.

BAF =(50 times D divided by R) squared

Where: D is the width of the sight in mm or cm
R is the distance from the eye to the sight in mm or cm
BAF is the basal area factor

To construct a relascope the following formula will prove more convenient:
R = 50 x D divided by the square root of BAF

Where: R is the length of the stem in mm or cm
D is the width of the sight in mm or cm
BAF is the basal area factor

Note: For both formulas the units of measurement for d and w units must be the same.

Measuring Basal Area

Step	Action
Step 1.	Determine the Basal Area Factor (BAF) of the relascope you are using. Generally a basal area factor of 2 is a good starting point.
Step 2.	Stand at the point at which you intend to estimate the basal area and face north.
Step 3.	Make the basal area sweep in a clockwise direction, starting at the first tree to the right of north. A basal area sweep examines every tree within view and therefore does not have any particular plot size. Despite examining every tree in sight, a basal area sweep should only take a few minutes to do, unless the trunks of trees are obscured by undergrowth, which will need to be removed until you have a clear sighting on each tree.
Step 4.	Hold the end of the rod to your eye. Sight along the rod out to the trunk of each tree and observe whether the width of the tree at breast height is greater than, the same as, or less than the width of the upright on the end of the rod. If a tree is obscured, move sideways so you have a good view but remain at the same distance from the tree. Before measuring the next tree, move back to the central point. Each tree which appears wider than the relascope object is counted as a “hit” or an “in”. Each tree, which appears the same width as the relascope object, is counted as a “borderline” or “half in”. Each tree which appears narrower than the relascope object is a “miss” or an “out”. Observe the trees very carefully. Experienced relascope users would count no more than one tree in ten as a “borderline”.
Step 5.	Add the number of hits (or ins) to half the number of borderlines (or half ins) and multiply the sum by the Basal Area Factor (see Step 1). The total is the stand basal area in square metres per hectare. Basal area = (hits + half the borderlines) x BAF Example Hits = 14 Borderlines = 2 BAF = 2 Basal area = (14 + 1) x 2 = 30 m2/ha.

In any area of forest you need to make enough basal area sweeps so that your estimates are representative of the forest. A suggested procedure is to:

• Do a pre-survey of the area in order to stratify the forest into homogeneous forest areas.
• Superimpose a grid over each forest stratum.
• The point location for basal area sweeps should be chosen at random but the grid can be utilised to ensure good overall coverage of the area.
• As a rule of thumb denser wood areas should be sampled more intensively.
• The number of basal area sweeps necessary for each stratum will vary depending on the variability of the forest, the degree of error acceptable and the predictive confidence required for the task in hand. This decision will often need to be one of discretion made in the field. Generally the more sweeps taken the more accurate will be the sample with 25 a good average number for most situations. A rough guide for native forest is: Area (ha) Number of Sample Points 0.5 - 10 8 - 16 11 - 50 16 - 25 Over 50 25 – 40
• For landholders it is suggested basal area readings be marked on forest management plans to inform ongoing management decisions.
• For reasons of accuracy the aim should be for about 10 to 12 hits. In heavily stocked stands a relascope with a greater basal area factor (eg 4) may be necessary otherwise too many hits may be recorded. Similarly for lighter stocked stands a lower basal area factor (eg 1) may be better in order to record enough hits.

When measuring basal area on slopes greater than 10o the observed basal area must be corrected by a slope factor. Slope correction factors are as follows:

Slope (degrees)	Correction Factor
10	1.015
12 14 16 18 20 22 24 26	1.022
14	1.031
16	1.040
18	1.051
20	1.064
22	1.079
24	1.095
26	1.113

Example: A basal area sweep on land of 18o slope gives a basal area of 26 m2 per ha. Because of the slope, the reading must be corrected by the slope factor. True Basal Area = 26 x 1.051 = 27.236 or 27 m2 per ha.

Note that there is a comprehensive mathematical explanation justifying how a relascope works but that explanation is beyond the scope of this information sheet.

Permanent Growth Plots and Forests Transects

To prepare a forest inventory, including standing volumes of timber, representative growth plots or transects should be installed. They can be monitored over time and are important components in the preparation of long term forest management plans, which incorporate silvicultural management. Such plans are strongly recommended.

Permanent growth plots and transects record tree species, diameter at breast height, basal area, bole height and crown ranking and takes into account tree products, trees ready for harvest, trees to be retained for future harvest and unmerchantable stems. Biodiversity values can also be recorded.

Data recorded from representative growth plots and transects can provide statistical information on standing volume of timber to basal area ratios. Such data is representative and can be linked to random basal area sweeps taken through a forest area to provide quick, valuable information on overall standing volumes of timber.

Learning these skills is well within the ability and means of most landholders. One avenue of training is the Coombell Farm Forestry Centre near Casino NSW (telephone 02 6663 7158) which offers courses on native forest management including hands on instruction on how to measure basal area, install growth plots, record forest data and prepare forest management plans.

Courses are provided at reasonable cost and from 2002 course costs for landholders will be 75% subsidised under the jointly funded Commonwealth Government/NSW Rural Assistance Authority FarmBis Program. Such training is strongly recommended for people managing private native forests

References

• State Forests of NSW Technical Paper No. 59 'Field Methods Manual'. Research Division, Sydney 1995
• Coombell Farm Forestry Training Course Manual 2001
• Australian Master Tree Grower. The Farmers Log 1999

Prepared November 2001

Bruce Cole-Clark, Natural Resource Project Officer, Department Land & Water Conservation, Grafton,  Justin Black, Farm Forestry Advisory Officer, Northern Rivers Private Forestry and Bryce Gorham, Natural Resource Project Officer, Department Land & Water Conservation, Grafton