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Site Index – What is it?

By: Thomas Richards, Northwest Management, Inc.

Foresters and landowners have evaluated land quality by observing the trees growing on a forest land for many years. From these observations they knew that tall trees generally indicated favorable soil and climate or areas with a relatively high potential, and that short trees indicated an area with less potential.

Site indexes and site classes are refinements of these observations that have been developed by foresters in order to classify or rank the productivity of forest lands. These indexes are productivity rankings used for management decision making, tax classification, and also for land-use planning and zoning.

Non industrial forestland owners need site classification information for the same reasons as other larger forest landowners, managers, and government officials. Site Index is the most common unit of measurement for potential productivity of forest land in the western United States. Site index is the total height to which dominant (tallest) trees of a given species will potentially grow on a given site at some index age, typically 50 or 100 years in the interior west. For example, if it is found that a particular tract of forestland has a site index for Douglas fir of 70 feet at 50 years, then we expect Douglas fir seedlings planted on that site today to be 70 feet tall in 50 years.

Site index reflects the combined effects of climate and soil on tree height growth. In general, poor sites with correspondingly low site index are found in areas with colder/drier climates and associated with soils that have limited ability to provide trees with necessary water and nutrients.

Site indexes usually are estimated for a single species although they can be calculated for any several species for any parcel of land. A property may be capable of growing grand fir, western larch, and Douglas-fir and since each of these species grows at a different rate, each one will its own site index based on its height at a given age.

Uses for site index

A forestland owner who plans to buy or sell forest land or who wants to quantify the yields and economic returns of various forest practices should consider utilizing site index. The index is an important basis for calculating and projecting future growth of forest products and for estimating harvest volumes and income at some future point in time.

Also, tax systems that require productivity to be assessed or that assign values to forest land have their foundations in site index. Recently, in some areas of the West, site index has become the basis for some land-use planning and zoning in an attempt to preserve the best forestland available for forest uses.

Site index can be calculated with reasonable accuracy for virtually any commercial tree species. A landowner should consult a professional forester to evaluate the site indexes for the tree species on a particular property. Site index information is also included in many of the comprehensive soil surveys completed within the last few years or in progress in many counties.

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