A Parsimonious Number of Growth Curves

Construction of new site index curves is often justified by a lack of growth information for a given species and site. This justification presumes that there is a one-to-one correspondence between growth pattern and stand conditions which are determined by numerous genetic and ecological factors together with their complex interactions. Because these factors are combined in an infinite number of ways, each stand is unique and needs its own site index curve. The effort required for collecting growth information would be prohibitive. This effort is also unnecessary because many existing curves coincide with each other and are, therefore, redundant. Differences in species, site, and construction methods do not prevent the appearance of the same growth patterns. These facts indicate that unique growth conditions do not mean that each stand has a unique growth pattern. Therefore, a more productive approach to growth modeling consists of distilling these patterns from existing curves and yield tables rather than piling up more new site index curves. Earlier investigations showed that the diversity in growth curves can be reduced to a small number (15-30) of growth types. The present study demonstrates that the number of types can be further reduced to 3-5 without sacrificing accuracy of growth predictions. North. J. Appl. For. 10(3):132-136.

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.