Lesson 9 spacer Decision Key
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Lesson Objectives

Decision Key

Decision Key Exercises

Accuracy of Different Methods


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Appendices
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Appendix 4
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Appendix 7
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Appendix 9
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Lesson Objectives

Learn how to use the decision key to help pick a method for determining SI.

Decision Key

A decision key (Figure 9.1) has been developed to assist surveyors in determining the appropriate method to use to estimate SI (see Appendix 4 “Determining a suitable site index method”).

Decision key
Decision key
Decision key

Figure 9.1. Decision key.

For most silviculture polygons (strata), there is more than one method that can be used to obtain SI. These methods differ in their:

  1. availability throughout the province
  2. suitability for different stand conditions
  3. accuracy.

This decision key helps you choose from four methods:

  1. BEC method
  2. growth intercept method
  3. site index curve method
  4. estimate site class and convert to SI.

The decision key helps you choose a method by leading you through the following five questions:

  1. Is there an existing SI?
  2. Is it accurate enough?
  3. How old are the sample trees?
  4. For the SI species, which SI methods are available?
  5. Is the condition of the sample trees suitable for predicting SI from height and age?

Decision Key Exercises

Using the decision key, determine the best method for obtaining SI.

Exercise 1:

Use the decision key to select the method to determine SI.

You must determine SI for a standards unit in a SMP. The stand is 4 m tall Hw in the Vancouver Forest Region. The existing SI comes from the map label of the previous old growth stand. Sample trees are suitable for GI sampling.

Exercise 2:

Use the decision key to select the method to determine SI.

You must determine SI for a standards unit in a SP. The stand is old growth (age class 9) spruce-hemlock in the Prince Rupert Forest Region. The existing SI comes from the forest cover map label. Most sample trees have broken and dead tops and are over 1 m in DBH.

Accuracy of Different Methods

The relative accuracy of the various SI estimation methods is one important consideration when deciding which method to use. An assessment of the accuracy typically achieved by the different methods is built into the decision key. A recent study by Nigh [2] illustrates the typical trends in accuracy achieved by three methods of SI estimation: SIBEC, growth intercept, and site index curves (Figure 9.2). Nigh’s study is based on 44 western hemlock plots in the ICH. Though the general trends are typical, the specific error levels and the ages at which one model becomes more accurate than another, differ considerably between species.

Some trends in accuracy that are typical for all species are:

  1. For the methods based on sample tree height and age (growth intercept and site index curve), accuracy changes with sample tree age.
  2. The accuracy of SIBEC is not affected by sample tree age, as the SI estimate is not based on sampling the trees on site.
  3. In very young stands, SIBEC is the most accurate method.
  4. Once young stands achieve a certain age, the growth intercept method is more accurate than both SIBEC and site index curves.
  5. With the growth intercept method, accuracy improves with sample tree age.
  6. SI curves can be very inaccurate in young stands.
  7. The accuracy of SI curves improves to bh age 50 years and deteriorates thereafter.
  8. At some old stand age, SIBEC again becomes the most accurate method.

Figure 9.2. Average error in site index estimate over age for SIBEC, site index curves, and growth intercept in Nigh’s study of western hemlock in the ICH.

[2] Nigh, G.D. 1998. A system for estimating height and site index of western hemlock in the interior of British Columbia. For. Chron. 74(4): 588—596.

 

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