Site index is a term used in forestry

Forestry

Forestry is the interdisciplinary profession embracing the science, art, and craft of creating, managing, using, and conserving forests and associated resources in a sustainable manner to meet desired goals, needs, and values for human benefit. Forestry is practiced in plantations and natural stands...

 to describe the potential for forest

Forest

A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...

 tree

Tree

A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...

s to grow at a particular location or "site." Site is defined as “inherent characteristics of the physical environment”; furthermore, the word site is used in forestry to refer to a distinct area where trees are found.. 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. For example, a red oak with an age of 50 years and a height of 70 feet (21.3 m) 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 borer

Increment borer

An increment borer is a specialized tool used to extract a section of wood tissue from a living tree with relatively minor injury to the tree. The tool consists of a handle, an auger bit, and a small, half circular, metal tray that fits into the auger bit. The auger bit is usually manufactured...

 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. Understory

Understory

Understory is the term for the area of a forest which grows at the lowest height level below the forest canopy. Plants in the understory consist of a mixture of seedlings and saplings of canopy trees together with understory shrubs and herbs...

 plants are especially useful if they are only found in specific areas.

In the United States, site index can be determined from soil maps provided by the Natural Resources Conservation Service

Natural Resources Conservation Service

The Natural Resources Conservation Service , formerly known as the Soil Conservation Service , is an agency of the United States Department of Agriculture that provides technical assistance to farmers and other private landowners and managers.Its name was changed in 1994 during the Presidency of...

 (NRCS). Soil surveys were conducted by the NRCS and site index was measured for these soils. Tables were compiled of the relationships between different soils and different site indexes for important species of that area.

Examples

A tree is measured to be 60 feet (18.3 m) in overall height, and the stand age is determined to be 50 years old. To find site index from a site index curve, one would find age 50 along the x-axis and then find 60 feet (18.3 m) along the y-axis. Where these two points intersect one would find the nearest line, which represents the site index for that stand.

An example of a site index equation is:
lnS=lnH_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).