Stem analysis
ANU

 Stem analysis
Forest Measurement and Modelling.


Stem analysis refers to a technique of examining the growth rings of sections from a tree trunk. It enables the past growth history of a tree stem to be reconstructed.

The technique can be used on standing or felled trees.

  • 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") which are ideally 2 to 5 cm thick.

Annual rings in a transverse cross section of a Pinus radiata. Transverse cross section of P. radiata section
Stem analysis can be applied to any stem with truly annual rings and, with care, to one which may have false rings.

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.
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 DBHUB over time.
Complete Stem Analysis Complete stem analysis involves measurements at a number of positions along the stem and permits diameter, height and volume growth to be deduced.

Complete analysis is practical only on felled trees where discs can be cut at a number of points along the stem. These sample disks are best taken at the nearest representative point below spring branch whorls on trees that have a regular branching pattern (like Pinus radiata).

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. This information can be used to estimate a height : age curve. For example:
Height (m) Number of rings Age (years) 
0.0
1.3
4.0
7.0
10.0
13.0
16.0
19.
22.0
25.0
28.0
31.0
34.0

stump
35
31
27
25
23
20
18
15
13
9
6
0

0 (37-37)
2 (37-35)
6 (37-31)
10 (etc.)
12
14
17
19
22
24
28
31
37
Example height:age curve from stem analysis
Determining tree shape and volume Orient each disc in the same direction and measure two lines that go through the pith and are at about 90 degrees to each other. Calculate the mean diameter and mark a line through the pith that is about the same length as this mean diameter. Record the diameter and height for each disc. This disk 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 n 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 n years before the previous measurement.

The following example of a tree that was 37 years old with discs taken at stump, bh, 4, 7, 10, 13, 16, 19, 22, 25, 28, and 31 m, shows the stem profile at age 37 years (ie the outer-most ring), then 35 years (count back 2 rings from the outerside), then 30, 25, etc. (counting back 5 rings each time). The total height at each age can be determined from the height : age curve (above):
Height
(m) 		dub outer ring
(37 years) 		dub n-2
(35 years) 		dub n-7
(30 years) 		dub n-12
(25 years) 		etc... 
0.0
1.3
4.0
7.0
10.0
13.0
16.0
19.
22.0
25.0
28.0
31.0
34.0

40.7
38.4
35.8
33.3
31.2
28.4
26.7
23.1
20.3
15.7
10.4
5.6
0

39.1
37.6
34.8
32.8
30.2
27.7
25.7
22.4
18.5
14.2
8.9
4.6
-

35.6
34.0
32.0
29.5
27.2
25.1
22.1
19.3
15.0
9.7
5.3
-
-

32.2
30.7
28.4
25.9
24.1
20.6
16.8
13.0
7.4
3.0
-
-
-

etc.
Example stem profile
Examination of defects and damage in the discs, and the relative shape of the tree profile may allow a prediction of the history of the sample tree.
Example Wollemi Pine disc with damage

[stemanal.htm] Revision: 4/2001
Cris.Brack@anu.edu.au