Figure 5.2 Site index curves for shortleaf pine. Most species of economic value now have site index curves that enable the determination of land capability for that species. In this example, a 60-year-old stand in which the dominant and codominant trees are 75 feet tall is on land having a site index of 69. Better sites, such as those in coves, may tally over 90; poor, denuded and eroded lands less than 50.
of age for the dominant and codominant trees in a stand. Site Quality can be used for plantations where all of the trees are averaged at a certain height.
Where trees sufficiently large are not available for determining the site index, which requires the counting of annual growth rings, another method is needed for determining the capacity of land to produce commercial forests. To establish these curves, graduate students of T.S. Coile, a Duke University professor, fanned out across the South to sample the soils in many kinds of forests, relate edaphic qualities to land productivity, and arrive at appropriate equations. For instance, for longleaf pine in the western portion of its range, the significant criteria appeared to be soil depth to the least permeable horizon (D) and the average rainfall (R) of the area for January through June. Thus, the regression equation: Log SI = 1.8697 + 0.0002636(R) - 0.006734(D). First published in 1953, before foresters had ready access to computers.) The equation was then converted to a simple table such as the following:
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