Site index

Site is defined as “inherent characteristics of the physical environment” (Nyland, 2002); furthermore, the word site is used in forestry to refer to a distinct area where trees are found (Avery, 2002). Site index is used to measure the productivity of the site and the management options for that site and reports the height of dominate and co-dominate trees in a stand at ages such as 25, 50 and 100 years (Avery, 2002). For example, a red oak with an age of 50 years and a height of 70 feet will have a site index of 70. Site index is species specific. Common methods used to determine site index are based on tree height, plant composition and the use of soil maps.

The most common of the methods used to determine site index is tree height. Determining site index is achieved by measuring and averaging the total height and age of trees found on that site. Height is obtained from dominant or co-dominant trees in an even aged stand usually from an instrument called a clinometer. Age is calculated from an instrument called an increment core or from planting or harvest records. These values are then used on a graph or an equation called a site index curve.

Determining site index from plant composition is often referred to as the indicator-plant approach. Site index is determined from plant composition by, “The presence, abundance, and size of understory plants can serve as useful indicators of forest site quality” (Avery 2002). Understory plants are especially useful if they are only found in specific areas (Avery 2002). Site index can also be determined from soil maps provided by the Natural Resource Conservation Service (NRCS). Soil surveys were conducted by the NRCS and site index was measured for these soils and there is a table compiled of the relationships between different soils and different site indexes of important species of that area.

References

Avery, T.E., Burkhart, H.E. 2002. Forest Measurements. 5th ed. McGraw-Hill, New York. Nyland, N.D. 2002. Silviculture: Concepts and Applications. 2nd ed. Waveland Press, Illinoise.