Site Index – A Measure of Forest Productivity
Site index is a term used in forestry to describe the potential for forest trees to grow at a particular location or “site.” Just as a farmer might discuss soil productivity in terms of bushels per acre, forestland is compared by site index. In the good ole days, site index was measured using a base age of 50 because for the most part forest were naturally generated and grown for a long period of time. Currently with the huge improvements made with genetics, pines grow much faster and are harvested before the age of 50. Many stands are harvested when the trees reach an age of 25-35 years of age. As a result, site indexes on pine plantations are calculated using a base age of 25. For the most, site indexes are constrained to the type of soil (texture, parent material, amount of organic material etc) but forest practitioners have found way to increase site index through tillage and fertilization. Tillage is the preparation of land through mechanical means to remediate a hard pan or fracture soils. This process improves soil texture so tree roots can rapidly grow through the soil profile. This is typically done with a ‘savannah plow’.
In some low lying areas prone to flooding, a bedding plow may be used create a ridge and furrow so the trees will not be flooded. The trees are planted on the ridges to keep their feet (roots) dry.
How do you go about measuring Site Index? A tree is measured to be 60 feet (18 m) in overall height, and the stand age is determined to be 50 years old. To find site index from a site index curve, one would find age 50 along the x-axis and then find 60 feet (18 m) along the y-axis. The tree age is determined by using an ‘increment borer’ to extract a core whereby one can count the tree rings to determine the tree’s age. From there, you simply go to a ‘site index’ curve and determine the site index.
Well, enough of all the technical jargon. Why is this important? If you have narrowed your property search to 2 properties (all other things being equal), you might make your buying decision on ‘Site Index”.
Kent Morris, ALC is a Registered Forester and Associate Broker who has experience in fields such as timber appraisals, harvesting, thinnings, and timber sales. He writes articles about these fields and more in his blog Land Blog…Get The Dirt!
