3. Collect Growth Intercept Data
Once the opening has been stratified, the sample lines laid out and a site index species chosen, field data can be collected.
GI information can be collected concurrently with a silviculture survey (stocking or free growing) or during a separate survey.
If GI information is to be collected during a silviculture survey, no new sample lines or plots are required. Surveyors should use the same plot centre for both the GI and silviculture plots.
If sampling using the GI method on its own, surveyors will have to develop sample lines and establish plots strictly for GI information.
In either case, an adequate number of samples are required as discussed earlier.
A 5.64 m radius plot is used to collect SI data using the GI method (Figure 5.5). In this plot, one sample tree is assessed. There are two steps in the process of selecting a sample tree to measure (Figure 5.6):
- identify the largest DBH tree of the SI species
- determine if it is suitable to measure.
Figure 5.5. A 3.99 m silviculture plot nested within a 5.64 m GI plot.
Figure 5.6. Selecting GI sample trees.
In the plot, the surveyor identifies the largest DBH tree of the SI
species. This is the only candidate for sampling in the plot. Then, the
surveyor assesses whether this tree is suitable for measurement. To be suitable,
the sample tree must have the following characteristics:
- at least 3-years growth above DBH
- undamaged stem with vigorous, uniform annual height growth above breast height
- must be in the dominant or codominant crown class and not overtopped by other trees or brush
- ring width should be vigorous and uniform.
If the plot does not contain a suitable sample tree, do not collect GI information. If dropping plots results in not enough samples for a stratum, then surveyors can offset GI plots using a consistent method.
Once the sample tree has been selected, the surveyor should measure the tree as follows (Figure 5.7):
- Locate breast height (1.3 m above ground at high side)
- Measure total tree height from the ground (at high side) to the
top of the leader. For species with a droopy leader, total height
is height with the tip straightened up. Height can be measured with:
- a tape and clinometer (see Appendix 1)
- a height pole
- a carpenter's measuring tape.
- Determine breast height age. Breast height age is equal to
the number of rings at breast height.
- On small diameter trees, this must be determined by felling the
tree at breast height and counting rings.
- On larger diameter sample trees, an increment borer can be used.
The core MUST contain the pith. If it does not, redrill the
- It is possible to determine breast height age by counting annual
branch whorls above breast height for species that produce obvious
branch whorls (see Lesson 6). See Appendix
2 for additional tips on accurately determining age.
- Record the species, height and age above breast height. Current
field cards (FS 658 and FS 748) are not designed to record GI. Surveyors
will have to use a separate line under the standard survey data collected
on each plot, or record the data on a separate, blank card.
An alternate version of the GI method, not discussed in this course, estimates SI from the distance between branch whorls.
Figure 5.7. Measurements on GI sample tree.