Lesson 5 spacer Growth Intercept Method
Lesson 1
Lesson 2
Lesson 3
Lesson 4
Lesson 5

Lesson Objectives

Growth Intercept Method Notes

How to Use the GI Method

Survey Timing

Number of Plots

1. Pre-stratifying the Opening

2. Select the Site Index Species

3. Collect Growth Intercept Data

4. Summary

Lesson 6
Lesson 7
Lesson 8
Lesson 9
Lesson 10
Lesson 11
Lesson 12
Appendix 1
Appendix 2
Appendix 3
Appendix 4
Appendix 5
Appendix 6
Appendix 7
Appendix 8
Appendix 9
Course Homepage

2. Select the Site Index Species

For each stratum, surveyors have to choose the species that GI data will be collected on. This species is called the site index species.

Normally, the leading species in the stratum's inventory label is chosen as the SI species. This ensures that the SI data collected in the field is compatible with the requirements of forest cover map labels and the inventory database, as well as ISIS and MLSIS.

The inventory component leading species is not an appropriate choice for site index species when it is:

  • silviculturally unsuitable
  • suppressed, damaged, or diseased
  • scheduled to be eliminated through treatment (i.e., spacing or spraying).

Whenever this is the case, the silviculture component leading species should be chosen for the site index species.

The Code requires that SI be provided in both the inventory and silviculture labels. If the inventory and silviculture labels have different leading species, two different SI values are required. Surveyors can either collect GI data on one tree of each species in each plot or they can collect GI data on one species and use the SI conversion table to estimate SI for the other species. SI species conversion is explained in Lesson 10 of this workbook. Obviously, collecting GI data on both species will give higher accuracy.

For a stratum, a tree species is suitable as a SI species if (Figure 5.4):

  • the dominant and codominant trees have at least 3-years growth above breast height
  • the growth intercept table is available
  • more than 500 stems/ha (total trees) are evenly distributed over the entire stratum
  • sample tree height growth above DBH reflects site productivity.

Height growth above DBH may not reflect long term site productivity if the stand has experienced wide spread damage, thinning shock, fertilization or brush competition. A stand is not suitable for the GI method if these factors have altered the normal height growth. More research is needed on this subject.


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