Site Index – A Measurement for Timberland Quality and Potential
Not all land is created equal, some soils and sites are more productive than others. Foresters use a metric called site index as a quick determinate of a property’s tree growth potential. This can be very valuable in helping owners understand the growth potential of different forest stands and species on their property, evaluate purchase decisions when searching for a timberland investment, project future stand growth and income, as well as understand how active forest management may improve the site index in a given area.
Site index (SI), by definition, is a measurement commonly used by foresters to describe the productivity of a site or stand of trees. This measurement usually describes well-stocked even-aged forest stands. Site index is the average total height of both the dominant and co-dominant trees in a forest stand at a given age. That age is described as a base age which is usually age 50 in natural stands and age 25 in planted stands. As an example, a planted loblolly pine stand with a site index (base age 25) of 65 would indicate the average total height of dominant and co-dominant trees in that stand at age 25 would be 65 feet. The greater the site index, the greater the productivity of the site. Site index can come in two forms; soil or expressed. Soil site index is the productivity estimate of the site in its native state. Expressed site index is the productivity of the stand following management treatments, like fertilization, or herbicide treatments. These treatments lead to increased tree growth and therefore, improved site index measurements over the native site or soil without treatment.
Site index curves for most commercially valuable species have been prepared by researchers by geographic region. These curves were created by gathering the age and height of multiple examples of the same species to build a curve that represents the height growth habits of a given species at a given age in a given area. Therefore, if a person knows the age of a tree, either by the year it was planted or by extracting an increment core and counting the annual growth rings, they can then measure the total height of the tree to the nearest one foot with a clinometer or other measuring device. With these two data points they can then plot them on the appropriate site index curve to determine the site index. By taking multiple measurements across a site an average estimate can be determined for a whole area. It is important the user select the correct reference curve for the species and geographic area. See the example curve below for Loblolly Pine Plantations in the Coast Plain region of North Carolina for base age 25.
So as an example, if you had a 20-year-old loblolly pine tree with a measured height of 34 feet. The site index measure for loblolly pine in that area would be 40 feet at 25 years old, as this is a base age 25 curve.
You plot the intersection of the two measurements and then follow the extension of the curve to the site index estimated on the right side of the graph. The height will increase as a stand ages, but the site index remains constant providing a relative indicator of site productivity.
Landowners should consult a professional forester to evaluate a given stand or property’s site index estimate. However, if you have site index information already prepared and presented to you, the information can help you understand what those number indicate and give insight into the tree growth potential of the site and consequently the future value of the timber. Site index is just one important measurement to consider among many in the purchase, sale, and management of a forest property.