Interpreting Site Index Curves
Site index is the height to which trees will grow over a given period—usually 50 years in the Lake States. Trees are expected to grow taller on good sites than on poor sites. Site index curves can be used to estimate relative site quality from the average age and average total height of dominant and codominant trees on a particular site. Each tree species has its own set of site index curves.
To use the site index curves in this appendix, first measure the age and total height of several dominant and codominant trees of a single species on a site. Calculate the average total age and average total height of those same trees. Then refer to the site index curves for that species.
For example, refer to the site index curves for quaking aspen in Appendix B-1. If the average age of aspen trees in your stand of interest is 40 years (horizontal axis) and the average total height is 62 feet (vertical axis), then those two lines converge closest to the site index curve for 70. This means that aspen trees in your stand are expected to be 70 feet tall when they are 50 years old. They are growing on a good site.
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