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

Lesson Objectives

Learn how to use the growth intercept method to estimate SI.

Growth Intercept Method

The growth intercept (GI) method predicts SI from height and age measurements taken on carefully selected sample trees. As of March 1999, the GI method is available for the following species:

  • interior lodgepole pine
  • interior spruce
  • coastal hemlock
  • coastal Sitka spruce
  • coastal Douglas-fir
  • interior Douglas-fir
  • interior subalpine fir
  • interior hemlock

Although the GI method is the best way to estimate SI in many stands aged 3 to 30 years breast height age (Figure 5.1), another method may have to be chosen if the appropriate species tables are not available.

When correctly used, the GI method provides an accurate estimate of SI. However, the required input data (heights and ages) are costly to collect and determining ages may damage crop trees.

Stands suited to GI include many 3- to 30-year-old stands

Figure 5.1. Stands suited to GI include many 3- to 30-year-old stands.

The GI method cannot be used in stands where sample trees have been:

  • overtopped by trees or brush
  • seriously damaged or diseased
  • cut out by spacing
  • affected in height growth by treatment (e.g., fertilization or thinning shock).

Note: It is best to use the BEC method for the type of stands listed above.


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