RELATIVE STOCKING INDEX - A PROPOSED INDEX OF SITE QUALITY

Site index is difficult to implement and interpret in multispecies, multiple-aged stands, and its relationship to site factors is obscure. Using data from the USDA forest inventory and analysis (FIA) for the Lake States, we developed log-log relationships between mean tree size and 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 relationship for its cover type (the norm). We divided the range of RSI into 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 from Minnesota, class assignments for individual stands remained constant over that 13-year period. Relationships between site factors and either RSI classes or analogous classes based on site index were examined in a subset of 169 stands. Temperature, precipitation, silt content of surface soil, and calculated annual water deficit all differed significantly among RSI classes, but not among site-index classes. The RSI is easy to apply, robust (resistant to change), and related to site factors. It merits additional examination as an index of site quality, especially in heterogenous stands.