Site Index (SI) is a key measure of site quality — it represents the potential productivity of a site for a particular tree species. It’s usually expressed as:
SI = height (in feet) of dominant and codominant trees at a base age
(commonly age 25 for loblolly pine in the Southeast, or age 50 for many hardwoods).
Example:
• A site index of 85 (age 25) for loblolly pine means that the tallest trees in that stand would average 85 feet tall at age 25, assuming the stand is fully stocked and unmanaged.
Notes:
• The higher the site index, the better the growth potential and timber productivity.
• It’s species-specific and location-dependent — a site may have an SI of 90 for loblolly pine but only 70 for longleaf pine.
• Site index is often estimated using height–age curves or soil and topographic indicators if direct measurement isn’t possible.
📍 Example — Lauderdale County, MS
• If a 25-year-old loblolly stand averages 90 feet in dominant height → SI = 90 → Good site.
• If a 25-year-old stand is only 70 feet → SI = 70 → Poor to Fair site, likely compacted or shallow soils.
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