Determining Site Quality Using Site Index

The most direct and commonly used technique for classifying site quality is to use the tree itself as an indicator. Site index, or height, of the dominant stand at index age 50, has proved a good indicator of relative site productivity for yellow-poplar. The species is particularly suited to this technique because it normally has a single, well-defined central leader that makes height measurement easy. Intolerant of shade, yellow-poplar commonly grows in even-aged stands.  Individual trees do not tend to move up in crown position with age. They either stay in the overstory throughout their lives or eventually die out. Thus, tree height at a given age reflects the quality of the site rather than some effect of competing trees.

Four sets of site-index curves are currently used for yellow-poplar:

Southern Appalachian Curves

These site-index curves

are for a stand age of 20 to 100 years and a site index of 50 to 130 feet at 50 years. These curves are based on a sample of stands in north Georgia and western North Carolina, and on measurements of about 25 dominant and codominant trees per acre. Site index can be calculated by entering height and age in the figure at right, or from the equation:

log₁₀ site index = log₁₀ height - 9.158 (1/50 -1/age).

Suggested area of use for these curves is the Mountain province, especially south of West Virginia.

Central Appalachian Curves

Schlaegel and others (1969) developed site-index curves

from stands in West Virginia. These site-index curves cover an age range from 20 to 80 years and site indices ranging from 60 to 110 feet at age 50. The estimating equation is:

log₁₀ site index = log₁₀ height - 7.716 (1/50 - 1/age).

These curves approximate the site-index values of the southern Appalachian curves (Beck 1962) over most of the age range. Except at extreme ages (below 20 years, or over 80), the two sets of curves give estimates within one 10-point site class.

Piedmont Curves

Site-index curves by Beck (1962) are based on sample stands from the Piedmont of the two Carolinas and Virginia.

The equation is:

log₁₀ site index = log₁₀ height - 6.503 (1/50 - 1/age).

Region-Wide Curves

McCarthys (1933) site-index curves

were developed from stands distributed over most of the range of yellow-poplar. They are restricted to an age range of 10 to 50 years because of a lack of older stands at the time they were prepared. These curves are essentially similar to the Piedmont curves by Beck (1962) and can be substituted for them. It should be noted for all the site curves presented that estimates for stands less than 20 years old are subject to substantial error due to the variability of early growth. In applying the curves, care should be taken to pick trees that come from the dominant and codominant crown classes that show no signs of suppression or crown breakage, and that appear to have maintained their position in the canopy throughout their life. For greatest accuracy, the sampling intensity for site-index estimates should approximate 25 sample trees per acre (Beck 1962). The ability to estimate site index to one foot or less, by using the equations, should not create a false sense of accuracy for the estimates. Categorization into 10-foot site classes is probably the best that can be expected of any of the site curves except at ages very close to index age.