Testing Lake States TWIGS: five-year growth projections for upland hardwoods in northern Lower Michigan

Five-year diameter growth, basal area growth, and mortality for five upland hardwood species in northern Lower Michigan were compared to projections from Lake States TWIGS. The species studied were northern red oak, white oak, other red oak (pin oak and black oak combined), sugar maple, and red maple. The validation data consisted of individual tree measurements from 44 stands across 20 ecological land types on the Manistee National Forest. The stands were measured in 1986 and 1991; during this time interval stands experienced a drought and outbreaks of leaf defoliators. For individual dbh classes, 5 yr diameter growth was predicted within +/- 0.3 in. for all species. Mean errors for BA projections were within +/- 5 ft2/ac for all species, and mean error for trees/ac ranged from -33 for other red oak to +16 for sugar maple. Although precision was variable, Lake States TWIGS provided accurate predictions of 5 yr diameter growth for the five species tested. Projections of mortality were less accurate