Adjustment of the age-height relationship for uneven-aged black spruce stands

Site index (SI) is commonly used in natural stands, even when their diameter distribution deviates from that of the monospecific, even-aged, fully stocked stands used to develop basic age-height relationships. Since deviations from basic age-height trajectories can be reflected in deviations of stand diameter distribution from a bell shape, we incorporated different diameter diversity indices into an age-height equation to help improve height predictions and determine which index is best related to stand dominant height. This procedure was performed using black spruce (Picea mariana (Mill.) BSP) stands from a large network of permanent sample plots established across the province of Quebec, Canada. The age-height model that minimized the Akaike's information criterion used the Shannon evenness index (E-Sh) as an equation modifier accounting for the diameter diversity variable. The model showed that for stands established on relatively poor sites (SI = 9), no substantial differences in dominant height were found between two contrasting E-Sh values. For SI = 15, however, the larger E-Sh value increased the dominant height by as much as 1 m at 80 years. These results suggest that introduction of E-Sh into an age-height model can improve calculation of site index, particularly in regions characterized by the presence of numerous uneven-aged stands.