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).