Site-index Curve Shape in Spruce

Sets of site-index curves were prepared from stem analyses of white spruce (Picea glauca (Moench) Voss) and black spruce (P. mariana (Mill.) BSP.) from various regions in the boreal forest of Canada. Ordinates of the site-index curves, computed for 5-year breast-height age intervals up to 75 years, and for 10-foot site-index intervals up to 70 feet, were compared within the species for the same values of site index and age. For breast-height ages below 55 years and for site index below 70 feet, the maximum absolute difference among the ordinates did not exceed 2.0 feet in white spruce, and 1.6 feet in black spruce; the corresponding average deviations were 0.75 and 0.80 feet. For breast-height ages above 55 years, these differences increased with age and, at 75 years, reached 8.8 feet in white spruce, and 3.8 feet in black spruce; the corresponding average deviations were 4.40 and 1.53 feet.