Lesson 2 spacer Introduction to Site Index
Lesson 1
Lesson 2

Lesson Objectives

What is Site Index

Where is SI Used?

Site Index in Forest Cover Labels

Problems with Incorrect Site Index Determinations

When/Where is SI Recorded?

FPC and SI

SI Source Codes

Biased Site Index and Stands Growing Below Potential

Site Index and Yield

Tree Growth Response to Improved Site Quality

Lesson 3
Lesson 4
Lesson 5
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


SI Source Codes

SI estimates vary in accuracy due, in part, to the method used to estimate SI. The Ministry of Forests data capture software, data files, and data forms are being converted to accept a SI source code along with every SI estimate. The SI source codes are:

A SI from adjacent stand
C SI from site index curve
E SI from Biogeoclimatic Ecosystem Classification
H SI from stand before harvest
I SI from growth intercept
M SI from G, M, P, L site classes
O SI from provincial SIBEC rollover, Nov. 1998
S SI assigned by District Silviculture Section

As of March 1999, these codes were fully implemented in ISIS only.

Biased Site Index and Stands Growing Below Potential

Site index is a measure of the site's potential to grow trees. However, the stand that is currently on the site may not express potential, and may not achieve potential, due to:

  • overtopping by brush or trees
  • excessive establishment density
  • damage by insects, disease, snow, etc.
  • treatment such as excessive pruning or thinning-out of top height trees.

When these factors have reduced the height growth of top height trees, and when site index is predicted from top height tree height and age, the resulting site index estimate is biased. It underestimates site potential. Some have called this value the “expressed site index” or “indicated site index.” “Biased site index” is the term that will be used in this course.

A biased site index should not be entered into the site index field of any form or database, but it may be useful as input to a growth and yield model to estimate the future growth of some types of stands that are growing below their potential. Though this topic needs further research, it appears that this approach is appropriate for stands where height grows smoothly and continuously along a reduced height-age curve—like a stand growing on a poorer site. The best example of this is repressed Pl stands. Height and age measurement in a repressed Pl stand input to a site index curve, yield a site index estimate that is well below true site potential. However, because repressed Pl stands have top height development like lower SI, unrepressed stands, their future growth is best predicted with this biased SI as input to the GY model. Biased site index estimates are the best input for estimating the future growth of this special stand type.

Top height growth of repressed Pl

Figure 2.10. Top height growth of repressed Pl. (Source: J. Goudie, B.C. Min. Forests, Research Branch.)


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