If a species produces annual branch whorls, it is possible to determine breast height age by counting whorls above breast height (Figure 6.2). However, mid-year branch clusters and other variations in branch growth may cause silviculture surveyors to make errors in age determination with this method. Therefore, counting rings, not counting whorls, is the recommended way to determine breast height. On sample trees with seven or fewer years growth above breast height, a count of whorls above breast height is recommended to confirm the age determined by a ring count.
Species with distinct annual branch whorls
Breast height age can be determined by counting the number of annual branch whorls above breast height plus one (for the annual whorl within the bud at the tip of the leader). Alternatively, you can count stem sections between whorlsstarting in the section that contains bh. The shape of individual branches that is created by annual branch growth is used to help distinguish an annual whorl from a mid-year cluster of branches.
Surveyors can compute confidence limits on the calculated average SI using the FS 1138 card. Site index values are input into the calculations in the same manner that well spaced trees would be, except that the plot multiplier (p) is not used.
The reliability of the SI average for the stratum can also be assessed by calculating the standard deviation of the plot SI values. Surveyors should use the following as a guide: