## 6.6. Determining Site Index from Field Measurements

Once height and age measurements are obtained for all site trees, site index can be determined.

1. Produce height and age measurements for suitable site index trees following the protocol from Section 6.5. Data for two Douglas-fir trees are listed  in Table 6.1.

Table 6.1.  Field Data for determining site index for Douglas-fir.
Tree Total Tree Height (ft.) Breast Ht. Age* (yrs.) Site Index (ft.)
A 114 45 123
B 135 68 113
* This site index graph uses breast height age (from a core sample taken at DBH).

2. Plot each tree on the site index graph using the height and age as measured in the field.

3. Follow the curves forward or backward to the base age (50 years). In this case, “grow” Tree A to age 50, following the trends indicated by the nearest curves as shown in Figure 6.8. Figure 6.8. Tree A is “grown” from 45 years to 50 years. Estimated height at 50 years is ≈ 123 feet. Site Index for Tree A is 123. Tree B is traced back to 50 years. Estimated height at 50 years is ≈ 113 feet. Site Index for Tree B is 113. Thus two trees of unequal age can be compared on a relative basis to indicate how capable the forest site is of growing Douglas-fir trees. Source:  Redrawn and adapted from King 1966.

4. Determine the height of Tree A at age 50. This height, 123 feet, is the site index for that tree.

5. Repeat this for each site tree. In this example, Tree B is over 50 years, so is traced back to age 50 (Figure 6.8).  When a site index value for each tree has been determined, an average can be calculated for the stand. The average for the two trees here is 118 feet, which puts the trees on low site Class II land.

6. It is not correct to calculate the average age and average height of the measured trees, and plot that to determine site index. 