Estimating and Understanding Forest Soil Productivity

Site productivity determines the volume of forest products (or any biomass) a site can produce, and how quickly it can do so. Thus, a measure of productivity is essential for gauging the value and cashflow timeline of a timberland investment.

Site Index, Foresters’ Measure of Productivity

Foresters evaluate a site’s productivity is by measuring its site index, which is a measure of the height that codominant and dominant trees of a given species will grow by a specified base age. Site index correlates closely to the potential forest product yield. Planted pine stands use a base age of 25 years. Natural stands (both pine and hardwood) use a base age of 50 years.

Using Site Index Curves

Three items are needed to determine site index:

  • the average height of codominant and dominant trees
  • tree age
  • the appropriate site index curve chart (dictated by species, stand origin, and region).

Example:  A forester samples a 13-year-old planted loblolly pine stand and determines the average codominant/dominant tree height is 37 feet. Using the appropriate site index chart (Chart 1, below), he reads across the bottom row to age 13, then reads up from there to reach a tree height of 37 feet. This intersection (red +) falls just shy of the SI70 curve, so he estimates a site index of 68 (base age 25).

Planted loblolly pine site index curves
Chart 1. Planted Loblolly Pine Site Index Curves

Comparing Differing Site Index Levels

You may wonder,”Do a few site index points make that much difference?” Using the NCSU Plantation Management Simulator, I have estimated the volume growth of identical planted loblolly pine stands with site indexes ranging from 55 to 80 (base age 25). The simulation was run to age 20 with no thinning harvest. All numbers are on a per acre basis.

Upland Piedmont Site Index Comparison
Site Index Comparison

Compare the SI60 volume growth to that of the SI65. The SI65 site produces 19.4% more tons/acre by age 20. Additionally, had I chosen to thin the stands, it would’ve been feasible to do so a year earlier for the SI65 site. Jump to the SI70 site, and the differences are even more dramatic—39.3% more volume at age 20 than SI60, and a potential thinning age of 14 rather than 16.

Below I have compared the SI60 with the SI70 in a one thinning management scenario. The stands were grown to a basal area of 120ft²/ac, thinned by harvesting every third row, then allowed to continue to grow until a basal area of 150ft²/ac was reached. See explanation of basal area here.

Comparison of Piedmont Loblolly Pine Stands
NPV Comparison of SI60 and SI70 Sites

As you can see, the SI70 site produced higher revenues at considerably earlier ages, making the net present value (NPV) of the reforestation investment 55% greater than the NPV of the SI60 site.

Dixie Reindeer Lichen and Eastern Prickly Pear Cactus, indicators of low site index
A. Dixie Reindeer Lichen (Cladonia subtenuis) and B. Eastern Prickly Pear Cactus (Opuntia humifusa). These species are indicators of a nutrient poor soil with a low site index.

Determining the Site Index for Other Species

In some cases, a landowner may want to convert a site from one species to another. How will he evaluate the growth potential of the desired species? Luckily, conversion charts are available to help estimate the site index for other species.

Example: Joe Landowner wants to harvest his natural stand of loblolly pine and convert the site to shortleaf pine.  First, his forester samples the loblolly pine stand, determining that it is 46 years old with an average codominant/dominant tree height of 88 feet. Using the appropriate site index chart (Chart 2, below), he reads across the bottom row until he reaches age 46, then reads up from there until he reaches a tree height of 88 feet. This intersection (red +) falls just above the SI90 curve, so he estimates that this site has a site index of 92 (base age 50).

Site Index Curves for Carolina Piedmont Natural Loblolly Pine Stands
Chart 2. Site Index Curves for Carolina Piedmont Natural Loblolly Pine Stands

Using his site index comparison chart (Chart 3, below), he reads up the site index scale to 92, then across to intersect with the loblolly line (Point A). From there he traces down to intersect with the shortleaf pine line (Point B), then back left to the site index scale (Point C), where he finds that shortleaf pine has a site index of 69.5 (base age 50) on this site. Knowing the site index will help him plan his stocking levels and management timeline for the new stand of shortleaf pine.

Site Index Conversion Chart
Chart 3. Site Index Conversion

Conclusion

When purchasing or managing timberland, you should always take site index into account. Combined with other soils and terrain data, it will help ensure that you manage for the right species mix and have realistic expectations for your management goals and financial returns.

Follow Tim Cartner:

Forester

Tim is a forester, real estate agent, and avid outdoorsman. When he is not managing clients’ woodland, you will find him hiking, trail running, reading, or woodworking. Motto: “Never get too comfortable–there is always room for improvement.”