Growth basal area: a field method for appraising forest site potential for stockability

Abstract

Growth basal area (GBA) is a field method for determining site potential limitations on stockability. It is defined as the basal area (BA) at which dominant trees grow 1 in. (25 mm) diameter per decade at age 100. Diameter growth is a constant used to compare sites; basal area is a variable used to express different site potentials for stockability. For example, a site with GBA = 220 ft2/acre (50 m2/ha) means dominants will grow 1 in. per decade (25 mm) in diameter at 220 ft2 BA. This is twice the stockability potential of a site with GBA = 110 ft2 (25 m2/ha). GBA is determined by relating current stand diameter growth rate and BA to a curve which permits adjustment of current BA to that for 1 in. per decade diameter growth. GBA is based on two assumptions: (i) rate of diameter growth decreases with increasing basal area and is an index of intertree competition. (ii) Change in GBA with age is related to stand age. GBA can be combined with site index (SI), shown as SI-GBA, to connote different productivities within an SI class. GBA is discussed in regard to concept of GBA curve development, assumptions made, applications, and constraints.