Forest Mensuration. Brack and Wood

Stem analysis ©

Stem analysis refers to a technique of examining the growth rings of sections cut from a tree trunk. It enables the past growth history of a tree stem to be reconstructed. The method can be applied to any stem with truly annual rings and, with care, to one which may have false rings.

Transverse cross section of P.radiata section The adjacent photograph shows annual rings in a transverse cross section of a Pinus radiata.

Ease of counting annual rings on a cross section depends on:

species - counting is difficult if the colour differentiation between late and early wood is slight;
age of tree - ring width is very small in old trees;
environmental conditions:
- early or late frosts and drought sometimes promote formation of false rings;
- under exceptionally good growing conditions, there may be little differentiation into early and late wood;
extent of ring distortion - due to branch whorls, defects, compression wood, tension wood.

The analysis may be partial or complete.

Partial stem analysis

Partial stem analysis is confined to one position on the stem. It provides data on diameter growth only, e.g. take a sample sisc or core at breast height to determine the growth of DBHOB over time.

Complete stem analysis

Complete stem analysis involves a number of positions along the stem and permits diameter, height and volume growth to be deduced.

The procedure used in sampling a stem for the analysis differs somewhat depending on whether the tree is standing or felled.

Standing trees. Stem cores are the basis of analysis. Cores are obtained using an increment borer (bark to pith core) or increment hammer (core of recent growth only).
Felled trees. The analysis is based on complete stem sections (discs or "biscuits") 2 to 5 cm thick, taken at the nearest representative point below spring branch whorls (on trees that have a regular branching pattern like Pinus radiata).

The data may be tabulated and graphed in various ways. Jerram (1939) - 'Elementary Forest Mensuration' - outlines a simple, practical and effective method.

Establishing height:age curves from stem analysis

Before the tree is felled, mark a direction (e.g. north) on the bole. After felling, transfer the direction mark to each disc and record the height at which each disc is taken. The stump (or lowest) disc should ideally be taken from a position less than the expected height of the tree seedling at year 1.

Once the discs are cut and sanded, identify and count the rings on each disc. There should be one ring for every year of age on the stump. The number of rings will decrease on discs from further up stump. The difference between the number of rings at the stump and the disc cut at any other height represents the number of years taken for the tree to grow to the height of the disc. For example:

Height (m) No of Rings Age (years)
0.0
Stump 37 0 (37 - 37)
1.3 35 2 (37 - 35)
4.0 31 6 (37 - 31)
7.0 27 10 (etc.)
10.0 25 12
13.0 23 14
16.0 20 17
19.0 18 19
22.0 15 22
25.0 13 24
28.0 9 28
31.0 6 31
34.0 0 37 (Total height of tree)

A graph of height against age can then be plotted.

Determining tree shape and volume

Orient each disc in the same direction and mark 2 lines that go through the pith and are at about 90 degrees to each other. Record the diameter as the average length of these two lines for each disk. Record the diameter and height for each disc, as in the following example. (Note the age in this example is 37 years - this is the age that the tree attained the recorded diameter at the disc height.)

        Diameter (cm) under bark at various heights:-----------------------------------------
Age     Stump  B.H.   4 m   7 m  10 m  13 m  16 m  19 m  22 m  25 m  28 m  31 m
37      40.7   38.4  35.8  33.3  31.2  28.4  26.7  23.1  20.3  15.7  10.4   5.6

The above height and diameter data allows a stem profile of the tree to be drawn. The point where the line crosses the Y-axis, i.e. where the diameter equals 0, can be determined from the height:age curve developed earlier or from direct measurement of total height on the felled tree.

Count back two rings from the outermost ring on each disc and and repeat the above procedure to determine the height and diameter for the tree when it was 35 years old:

        Diameter (cm) under bark at various heights:-----------------------------------------
Age     Stump  B.H.   4 m   7 m  10 m  13 m  16 m  19 m  22 m  25 m  28 m  31 m
37      40.7   38.4  35.8  33.3  31.2  28.4  26.7  23.1  20.3  15.7  10.4   5.6
35      39.1   37.6  34.8  32.8  30.2  27.7  25.7  22.4  18.5  14.2   8.9   4.6

The total height at age 35 was about 32 m (read from the height:age curve). The stem profile at age 35 can be plotted in for the adjacent example.

By repeating the above steps, counting back at 5 yearly intervals (or any other interval of interest) a stem profile for each age can be completed. Note: as you go count back in the number of rings, the upper discs will drop out as they do not have that many ring - the tree wasn't that tall at these younger ages!

        Diameter (cm) under bark at various heights:-----------------------------------------
Age     Stump  B.H.   4 m   7 m  10 m  13 m  16 m  19 m  22 m  25 m  28 m  31 m
37      40.7   38.4  35.8  33.3  31.2  28.4  26.7  23.1  20.3  15.7  10.4   5.6
35      39.1   37.6  34.8  32.8  30.2  27.7  25.7  22.4  18.5  14.2   8.9   4.6
30      35.6   34.0  32.0  29.5  27.2  25.1  22.1  19.3  15.0   9.7   5.3   ---
25      32.2   30.7  28.4  25.9  24.1  20.6  16.8  13.0   7.4   3.0   ---   ---
20      28.1   26.9  24.4  21.6  18.5  15.5  10.4   3.3   ---   ---   ---   ---
15      24.4   22.9  19.6  15.2  11.7   5.1   ---   ---   ---   ---   ---   ---
10      18.8   16.8  11.4   ---   ---   ---   ---   ---   ---   ---   ---   ---
 5      13.5   10.4   ---   ---   ---   ---   ---   ---   ---   ---   ---   ---

Plotting these profiles as sectional area against height allows easy calculation of stem volume at any age (graphical technique) and volume increment.

http://online.anu.edu.au/Forestry/mensuration/STEMANAL.HTM
Cris.Brack@anu.edu.au
Mon, 14 Apr. 1997

Height (m)	No of Rings	Age (years)
0.0 Stump	37	0 (37 - 37)
1.3	35	2 (37 - 35)
4.0	31	6 (37 - 31)
7.0	27	10 (etc.)
10.0	25	12
13.0	23	14
16.0	20	17
19.0	18	19
22.0	15	22
25.0	13	24
28.0	9	28
31.0	6	31
34.0	0	37 (Total height of tree)