Determining site index
To determine site index, locate the age of your stand on the x-axis of Figure 1. Then go straight up to the height of your stand. There are 7 curves in this graph. The highest curve is for a site index of 70, then 65, 60, 55, 50, 45 and 40. The location of your stand with respect to these curves will identify your site index. For example, if your age and height point is close to the curve for site index 65, then your site index is approximately 65. If your points are between the 55 and 60 site index curve, then your site index is approximately 57.5, but 58 or even 60 is probably close enough for most purposes.
If you are having a hard time following this, recall that red pine grows best on well drained coarse soils, including old fields (assume those are site index 65 or 70). Occasional coarse yet well drained and fertile soils will show site indices exceeding 70 and even higher than 75. Poorly drained soils typically have site indices in the 40-45 range. Sandy soils typically have site indices in the 50-60 range. On wet sites you will simply not find red pine except as an isolated tree on a patch of high ground.
In other cases, roots of red pine on sandy soils may reach a shallow water table in 5-10 years; then expect show rapid growth for several decades. Stands aided by early competition control may also grow faster in terms of height than expected. Finally, stands on shallow soils may show an early (age 30-50 years) slowing of height growth, despite rapid early growth. In other words, the site curves are a helpful description of typical height growth patterns, but there are exceptions. Silviculturalists call such variation evidence of polymorphic height growth patterns.
Figure 1. Site index curves for red pine in the Lake States.
Source: Gevorkiantz, S.R. 1957. USDA Forest Service. Lake States Forest Experiment Station Technical Note 484.