Site index (SI) is a measurement commonly used by foresters to describe the productivity of a site. Typically this measurement is used to describe sites growing well-stocked even-aged forests. Site index is the average height of the dominant1 and codominant2 trees on the site, at a given age (base age). Typically, the base age for hardwoods and white pine in Virginia is 50 years, while the base age for loblolly pine is 25 years. For example, a SI of 75, base age 50, means that the average height of the dominant and codominant trees on a site will be 75 feet when they are 50 years old (SI50=75). The higher the SI, the higher the site productivity (trees will grow faster than on a site with a lower SI).

Selecting an appropriate, high-yielding variety is one of the most important management decisions that producers make. Yield potential is clearly important, but the decision is complicated by such factors as the cropping system, the need for disease resistance, end-use quality goals, year-to-year climatic variation, and the need to select multiple varieties in order to reduce risk by spreading out flowering and maturity dates.