SOIL PRODUCTIVITY INDEX - PREDICTIONS OF SITE QUALITY FOR WHITE SPRUCE PLANTATIONS

Very few site-quality models have incorporated the biological effects of multiple soil properties on optimum root growth. One such model that was originally developed for agronomic crops and has been used for naturally forested stands is the soil productivity index (PI) model. Our objectives for this study were to determine if the PI model could be used to estimate site quality for white spruce (Picea glauca [Moench] Voss) plantations and to compare the use of PI vs. site index in equations describing changes in aboveground biomass with age. The PI model includes an estimate of optimum vertical root distributions that is used to weight the relative effects of mineral-soil characteristics (bulk density, pH, available water capacity, and aeration) on aboveground biomass. Relative topographic (percent slope) and climatic (precipitation and evaporation potential) information are also used in the model. Results indicated that site index and PI were not significantly correlated. However, PI was more highly correlated with aboveground stand biomass and mean annual biomass increment than was site index. Site index was more highly correlated with biomass estimates in older plantations (35-43 yr), whereas PI was more highly correlated to biomass estimates in younger plantations (19-34 yr). To further assess the relationships between estimates of biomass and site quality, adjusted for age, a Schumacher-type equation was used. Comparable results were observed between stand-biomass equations using PI vs. site index.