Site index

Site index is a term used in forestry to describe the potential for forest trees to grow at a particular location or "site." Site is defined as “inherent characteristics of the physical environment” [Nyland, R.D. 2002. Silviculture: Concepts and Applications. 2nd ed. Waveland Press, Illinois.] ; furthermore, the word site is used in forestry to refer to a distinct area where trees are found. [Avery, T.E., Burkhart, H.E. 2002. Forest Measurements. 5th ed. McGraw-Hill, New York.] Site index is used to measure the productivity of the site and the management options for that site and reports the height of dominant and co-dominant trees in a stand at a base age such as 25, 50 and 100 years. [Avery, T.E., Burkhart, H.E. 2002. Forest Measurements. 5th ed. McGraw-Hill, New York.] 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.

Determining Site Index

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, T.E., Burkhart, H.E. 2002. Forest Measurements. 5th ed. McGraw-Hill, New York.] Understory plants are especially useful if they are only found in specific areas. [Avery, T.E., Burkhart, H.E. 2002. Forest Measurements. 5th ed. McGraw-Hill, New York.]

In the Unites States, site index can be determined from soil maps provided by the Natural Resources 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.

Examples

A trees is measured to be 60 feet in overall height, and the stand age is determined to be 50 years old. To find site index from a site index cruve (figure to the right of text), one would find age 50 along the x-axis and then find 60 feet along the y-axis. Where these two points intersect one would find the nearest line and the site index that corresponds to that line which is 60 for this example.

An example of a site index equation is:ln"S"=ln"H_d"-"b₁"("A"^-1-"A_i"^-1)

Where "S" is site index, "A_i" is index age, "H_d" is height of dominants and co-dominants and "A" is stand age. This will estimate height at index age (site index). [Avery, T.E., Burkhart, H.E. 2002. Forest Measurements. 5th ed. McGraw-Hill, New York.]

References