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 Objectives

Develop understanding of some important site index concepts, the uses of site index, and the legal and systems requirements for site index.

## What is Site Index?

Site index (SI) is a measure of potential site productivity —the capacity of an area of land to grow trees of a given species (Figure 2.1). It is defined as the average height that free growing, undamaged top height trees of a given species can achieve in 50 years growth above breast height. Simply, SI is the height of doms at age 50 when they have been able to grow to their full potential. On sites where suppression, repression, or damage have reduced top height growth, SI is best thought of as the top height that would be attained at bh age 50 by unsuppressed, unrepressed, undamaged top height trees.

### Top Height

Until very recently, top height trees were defined as the 100/ha largest DBH trees of a given species. Now this definition has been changed. The new definition of a top height tree is the “largest DBH tree of a given species in a 0.01 ha plot.” This definition makes sure that all the top height trees cannot be located in a cluster in one corner of the hectare.

Figure 2.1. What is site index

### Total age, breast height age, and years to breast height

The concept of SI is based on breast height age—not total age. Figure 2.2 illustrates the differences between total age, breast height age, and years to breast height. Total age is the number of years since seed germination. The number of years it took a tree to grow from seed to breast height is termed “years to breast height.” The number of years growth above breast height is termed “breast height age.”

Breast height age is the number of annual growth rings at breast height. Total age is the number of rings at the point of germination.

Breast height is 1.3 m above ground measured from the high side.

This measure of productivity is based on stand height because stand height is:

• easy to measure
• closely related to stand volume
• relatively independent of stocking density.

SI allows standardized comparisons of productive potential between sites, across a broad range of existing stand conditions.

In the past, site productivity was recorded as a site class (i.e.,good, medium, poor).

Typically, SI for different site classes is as follows:

 Coast Interior Good: > 35 m Good: > 22 m Med: 20 - 35 m Med: 15 - 22 m Poor: < 20 m Poor: < 15 m

As can be seen, these classes are very broad. The more accurate the estimate of SI, the more accurate the decisions that rely on that value will be.