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