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Site index is a term used in forestry to describe the potential for forest trees to grow at a particular location or "site." Site is defined as “The average age of dominate and/or codominate trees of a even-aged, undisturbed site of intolerant trees at a base age”; furthermore, the word site is used in forestry to refer to a distinct area where trees are found. Site index is used to measure the productivity of the site and the management options for that site and reports the height of dominant and co-dominant trees in a stand at a base age such as 25, 50 and 100 years.〔 For example, a red oak with an age of 50 years and a height of will have a site index of 70. Site index is species specific. Common methods used to determine site index are based on tree height, plant composition and the use of soil maps. ==Determining Site Index== The most common of the methods used to determine site index is tree height. Determining site index is achieved by measuring and averaging the total height and age of trees found on that site. Height is obtained from dominant or co-dominant trees in an even aged stand usually from an instrument called a clinometer. Age is calculated from an instrument called an increment borer or from planting or harvest records. These values are then used on a graph or an equation called a site index curve. Determining site index from plant composition is often referred to as the indicator-plant approach. Site index is determined from plant composition by the presence, abundance, and size of understory plants. Understory plants are especially useful if they are only found in specific areas.〔 In the United States, site index can be determined from soil maps provided by the Natural Resources Conservation Service (NRCS). Soil surveys were conducted by the NRCS and site index was measured for these soils. Tables were compiled of the relationships between different soils and different site indexes for important species of that area. Direct methods of creating a site index include estimation from historical yield records, stand volume data, growth intercepts, or height–age relationships (site index). Site index has been the most widely used means of estimating site productivity in North America, despite some shortcomings. Indirect methods include estimations based on relationships among dominant species, lesser vegetation characteristics or site indicators, as well as topographic, climatic, and edaphic factors. A generalized model using both direct and indirect variables for developing composite site index equations was presented by Payandeh and Wang (1997).〔Payandeh, B.; Wang, Y. 1997. A generalized model for developing composite site index equations. Nat. Resour. Can., Can. For. Serv., Sault Ste. Marie ON, Frontline Tech. Note 99. 4 p.〕 The generalized model described 2 data sets better than did either a composite site index model (Payandeh 1991)〔Payandeh, B. 1991. Composite site-productivity functions for Northeastern Ontario black spruce. New Forests 5(1):1–12.〕 or a logistic site index model, Model 3 (Monserud 1984).〔Monserud, R.A. 1984. Problems with site index: an opinionated review. p. 167–180 ''in ''Bockheim, J.G. (Ed.). Proc. Symp. Forest Land Classification: Experiences, Problems, Perspectives. Sept. 1983, Madison WI. USDA, For. Serv., North Central For. Exp. Sta., St. Paul MN Rep. NC-102. 209 p.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「site index」の詳細全文を読む スポンサード リンク
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