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. Understory plants are especially useful if they are only found in specific areas.
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.
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: lnS=lnHd-b1(A-1-Ai-1)
Where S is site index, Ai is index age, Hd is height of dominants and co-dominants and A is stand age. This will estimate height at index age (site index).