Site Index
Site index is a tool to determine the relative productivity of a particular
site or location. Site index is the height of a free to grow tree of a
given species at a base age on the site of interest. Common base ages include
25, 50, and 100, depending on the lifespan and common management practices
for that species. Obviously, not all trees are at the base age at the time
you measure the tree so curves have been published to convert other ages
in to the expected height at the base age. The relationship depends of
the observation that height growth is much less effected by variation in
density than other measures of tree size such as diameter or volume. It
should be noted that tree height growth is effected at very high densities
and and very low densities, both of these tend to reduce height growth.
Site index is species dependent and must be calculated by species and
the value is not usually equivalent between species. Therefore a site index
is usually reported as 70 for white oak but this same site may be 75 for
shortleaf pine. Some people have published conversion equations but they
are not commonly used. A very good source for site index equations for
the eastern half og the United States is Carmean
et al. (1989).
Site index equations have been constructed in three basic ways and the
method of construction effects the interpretation of the results.
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1.
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The oldest method which was develop prior to the availability of computers
to fit the equations are whole stand average site curves. In this method,
researchers collect stand average age and the top height of the stand (the
height of the dominant tree in the stand). These observations are plotted
on graph paper and "average guide curve" drawn through the points. Then
the curve was scaled to go through the known index points. Most of the
site index curves created before 1940 were done this way. This method has
a strong assumption in that the height averages the top damage contained
in the initial stands.
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2.
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The next method attempted to solve the averaging effect of the first method.
The researchers collected tree height - age pairs. The trees selected for
this method must be dominant or codominant. The curve fitting follows the
above methods except the curve are fit with polymorphic equations (this
means that each site index curve can have a different shape curve) using
statistical packages.
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3.
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The third method and the most common since 1980, is to perform stem analysis
on individual trees and fit curves to the growth pattern of individual
trees.
Selection of Site trees
Selection of suitable site trees is important to determine site index.
Site tree are free-growing, uninjured, dominant and codominant trees. Past
management practice can influence the above characteristics. Tree ages
closer to the base age will yield more accurate estimates of site index.
The ages should not vary more that 10 year. Normally, between 5 and 10
site trees are taken to estimate on site index to account for normal variation
between trees.
References
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Carmean, W. H., J. T. Hahn, and R. D. Jacobs.
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Site index curves for forest tree species in the eastern united states.
General Technical Report NC-128, USDA Forest Service, North Central
Forest Experiment Station, 1989.
Site Index curves for Missouri
Figure 1: Site index curves for Black, white and Scarlet Oak
Figure 2: Site index curves for Black Oak
Figure 3: Site index curves for White Oak
Figure 4: Site index curves for Scarlet Oak
Figure 5: Site index curves for Shortleaf Pine
Last Updated: December 2, 1999