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Trees per hectare (ha)
Plots – Used to find out how many trees per hectare (ha).
A circular plot with a radius of 5.6 metres = 0.01 hectares
Within the 5.6m radius plot count the number of trees and multiply by 100 to
give ‘trees per hectare’.
If you want to know how many trees are to be felled, count only the marked
trees and multiply by 100 to give the number of trees to be felled per hectare.
How many sample plots?
Area of stand (ha)
Uniform stand
0.5 -2.0
6
2-10
8
Over 10
10
Variable stand
8
12
16
5.6m
Tree volume in cubic metres (m3)
To obtain an estimate of tree volume you need to know two factors.
a. Tree height and;
b. Diameter at breast height (DBH) Breast height = 1.3 metres from the ground.
a. Tree height – Used to calculate the volume of a tree by multiplying by the
area of a circle. For accurate tree heights use a clinometer or hypsometer.
To estimate tree height: (You will need two people)
1. First person walks back away from the tree and aligns the top and bottom
corners of this A4 page with the top and bottom of the tree. The top of the
tree should be measured to where the stem is no less than 7cm diameter.
A baked bean tin has approximately a 7cm diameter.
2. The second person stands next to the tree and is guided by the first
person to point at exactly where the black 10% line is level on the trunk.
3. The second person now uses a tape measure to measure the height in
metres from the 10% point down to the ground. This gives you 10% of the
tree height. Multiply this figure by 10 to give the actual height of the tree.
b. Diameter at breast height (DBH) – Used to calculate the volume of a tree
by converting DBH to area and then multiplying by tree height. For accurate
DBH measurements use callipers or a girthing tape.
To obtain diameter at breast height:
1. Measure 1.3 metres from the ground, this is referred to as breast height.
2. At breast height measure the circumference of the trunk in centimetres.
3. Divide the circumference by pi (3.14). This will give you diameter at
breast height (DBH). Example: 110 cm circumference ÷ 3.14 = 35
If you think of a tree as a cylinder, use the formula pi multiplied by diameter (cm)
squared (d2) divided by 40,000 multiplied by tree height = cubic meters (m3)
Volume (m3)
=
π d2
40,000
x Length (m)
Example
π (3.14) x 35 (cm) x 35 (cm)
40,000
x 20m (tree height) = 1.9m3
10%
Please use the methods above for rough estimates only. For accurate volume calculations
use the Forest Mensuration handbook and take into account tree species, shape and form.
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