Several characteristics determine the rate of growth of hardwood species. These include various measures of moisture relations such as imbibitional water and available soil water. Soil humus, thickness of the surface soil layer, friability of the subsurface soil, and position of trees on slopes are others. For instance, 96% of the variation in site index (SI) for black oak can be attributed to three variables. Expressed algebraically,
SI = 38.7690 + 8.8057(A) + (-0.0477)(P) + (-0.4620)S
where SI = average height of the dominant and codominant trees at age 50 years, A = depth of the Al (surface) soil horizon, P = slope position, in percent of distance from the bottom, and S = percent of sand in the Al horizon.
To determine the SI for yellow-poplar, foresters simply add 10 to 15 points to that obtained in the equation above for black oak.5
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