Basic Tree Measurement For Conifer Trees
Like all assets, trees are a biological asset and need to be assessed for volume.
Why measure your trees?
Like all assets, trees are a biological asset and need to be assessed for volume. Without knowing what you have in volume, it is difficult to put a value on your crop.
Who can measure your crop?
The reason foresters measure the crop is to put a value on the standing timber and to plan for future operations. They can predict the volume for thinnings and clearfell using models such as GrowFor. However, this is far too technical for your average forest owner. There are a few simple measurements a forest owner can take themselves and by doing this, the owner can see how well their trees are growing and put a rough estimate on the volume of the stand. It by no means replaces the detailed measurement that a forester can do.
What equipment is needed to do basic tree measurement?
Think of a timber shaped cube. If you got all the trees in the stand and squashed them into a box (cube) then you would be able to say how many metres cubed you had of saleable timer. To measure the width of a tree, calipers are used. These are formatted measurement tools that you place at 1.3 metres from the base of the tree and read directly from the bar which have figures in centimetres. The next measurement you need to take is the height of the tree. Due to the shape of a tree, which is conical (cone shaped), the height has to be converted to form height using tables. Clinometers are used to measure angles, from these angles and a lot of mathematics, the tree height can be measured. Hypsometers are clinometers from which you can read the tree height directly without a lot of mathematical calculations. The final piece of equipment for simple measurement, is a 30 or 50 metre regular measuring tape on a large spool.
Equipment bought and ready to measure, how do you get started?
Tree measure is done based on sampling, so a map is required to randomly select places to do your measurement. Only assess one species at a time, if mixed, select the main species to measure. A plot for sampling is used. These plots are normally 0.02 ha (hectare) for thinned crops.
What are sampling plots?
You need a fixed area such as 0.02 of a hectare in order to expand into an area of one hectare. Then you can simply multiply the one hectare by the actual area you have planted in trees. There are finer measurement rules which I will omit due to the complexity of timber measurement, the measurement methods described here are for a rough measurement guide and are not intended for professional use. The basic equipment for tree height will measure to the nearest metre. Expensive professional equipment will measure to the nearest one or two decimal places of a metre.
How do you lay out a sample plot?
After first or subsequent thinnings, the forest is opened up and access is easier. A 0.02 ha (hectare) plot is normally used after first thinning has taken place. To get 0.02 ha plot, measure from the centre of the rack (the line of trees which have been removed in the thinning) straight across to the centre of the rack on the other side. This should be about 14 metres, as one in seven lines are normally taken out in a conifer crop. To get the length of the distance you need to go down, simply divide 200 metres squared by 14.00m, this will give you 14.29m. To prove the maths, 14.00m (across) x 14.29m (down) = 200 metres squared. This is a 0.02 ha plot, mark the edges with a stick and make sure the corners are at right angles to make a rectangle shape.
How do you find the number of trees per hectare?
Stocking is the forestry term used for how many trees are in a certain area. Start with counting all the trees in your 0.02 ha plot. After counting all the trees in the plot, say you have 30 trees, divide by 0.02 and this will give you 1,500 trees per hectare. Your stocking is 1,500 trees per hectare.
What is DBH and how is it measured?
DBH is short for Diameter at Breast Height. It is where the tree is measured for diameter and is 1.3 metres from the base of the tree. The base of the tree is where the tree meets the soil. Measure on yourself 1.3 metres from the ground and put a pin at that point on your clothing. This will help to find the point on the tree where the DBH measurement is taken. Using calipers (or a DBH tape) measure the tree’s diameter at this point. Write down each measurement to the nearest centimetre. Now get the average diameter, foresters will use a slightly different way to get the average, but a normal mathematical average is good enough for a rough estimate. To get a normal mathematical average, add up all the DBH’s in the plot and divide by the number of trees in the plot. An example would be, 600 cm (centimetre) for all the DBH’s added up, divide by 30 trees in the plot and this gives an average DBH of 20 cm.
Which tree do you measure for top height?
For simplicity, the top height tree can be considered the one with the biggest DBH (Diameter at Breast Height) in the plot. Make sure you can see the top of the tree from about 1.5 to 2 times the guessed height of the tree away. Using a hypsometer (direct read clinometer), sight the top of the tree and click on the stop button. Read and take note of the height from the instrument. Repeat, this time sight the base of the tree and take note of the reading. Normally on flat ground, the base of the tree will come up as a minus figure. Just add the two figures together to get the total tree height. Try to measure height from flat ground. This height must be converted to form height using tables. Form height tables adjust the height of a tree to allow for the conical shape depending on species.
Find volume with DBH, form height and stocking:
Finally, now put all the figures together to get a volume. Some mathematics have to be done now.
Units of measure are:
- Tree height converted to form height in metres
- Tree DBH (Diameter at Breast Height) of girth is measured in centimetres
- Stocking is the number of trees in the plot and expanded out to 1 ha
- Volume is in metres cubed
Mathematics:
Hubers formula for volume = 0.00007854 x form height x average diameter squared
Worked example
Measurements taken:
- Stocking = 1,500 Sitka spruce trees per hectare
- Top height = 18m (converted to form height) = 7.17m
- Average DBH = 20cm (girth)
Volume = 0.00007854 (Hubers formula) x 7.17m (form height) x 20cm2 (DBH2) x 1,500 (stocking)
Volume = 0.00007854 x 7.17m x 400cm x 1,500
Volume = 0.23m3 x 1,500
Volume per hectare = 345m3
This is a simple calculation and is for a rough guide for use by forest owners. The measurements for DBH and height can be used to monitor the tree growth without doing any volume calculations. Form height tables can be found in the “Irish thinning protocol.pdf” which can be found online. Watch your trees grow and the value of your investment increase over time, measure your trees with confidence and enjoy being active in your forest.
