RELATIVE STOCKING INDEX - A PROPOSED INDEX OF SITE QUALITY

Site index is difficult to implement and interpret in multispecies, mu ltiple-aged stands, and its relationship to site factors is obscure. U sing data from the USDA forest inventory and analysis (FIA) for the La ke States, we developed log-log relationships between mean tree size a nd stand density for five cover types. Fits were good, with r2 from 0. 96 to 0.98 and slopes from -0.948 to -0.995. We define an alternative index of site quality, the relative stocking index (RSI), as the ratio of a stand's measured density to that predicted using the log-log rel ationship for its cover type (the norm). We divided the range of RSI i nto three classes for each type (<0.9 of norm, >0.91 but <1.1 of norm, and >1.1 of norm). Based on analyses of the 1977 and 1990 FIA data fr om Minnesota, class assignments for individual stands remained constan t over that 13-year period. Relationships between site factors and eit her RSI classes or analogous classes based on site index were examined in a subset of 169 stands. Temperature, precipitation, silt content o f surface soil, and calculated annual water deficit all differed signi ficantly among RSI classes, but not among site-index classes. The RSI is easy to apply, robust (resistant to change), and related to site fa ctors. It merits additional examination as an index of site quality, e specially in heterogenous stands.