A soil-based aspen productivity index for Minnesota

An index, based on soil properties as defined by soil mapping unit and positively related to growth of quaking aspen (Populus tremuloides Michx.) in Minnesota (the aspen productivity index—APX), was developed. Aspen grows on a wide variety of soils and comprises the majority of roundwood harvest in the state. The APX was to be included in an existing information system, and the soil properties used were restricted to those in an established database. There were no co-collected forest measurements. Response curves relating aspen productivity to soil and site properties, scaled from 0.0 to 1.0, were initially developed using the abundant information in the literature and were then iteratively adjusted. Properties were aggregated into three categories; by their effects on water availability, nutrient availability, and by other (site) factors that affect growth. The APX was evaluated and validated by comparison to generalized forest productivity ratings in published soil surveys and with spatially referenced forest measurements collected at three different intensities, including a national inventory (FIA). The APX agreed well with the generalized forest productivity ratings, including both productivity classes and estimated aspen mean annual increment. Multiple and variable forest measurements within mapping units, but a single APX, reduced the explanatory power of relationships. Despite that, the APX differed significantly among productivity classes established by the FIA and those based on relative stocking. Regressions with APX as the independent variable and mean basal area and site index by mapping unit, weighted by frequency, as the dependent variable were statistically significant for basal area for all datasets and for site index for the FIA data. The relationships, evaluated over the range of the APX in the FIA dataset, predict differences in total aspen yield of about 50% in 50 years. The APX is being used to provide comparative forest productivity information for all soil mapping unit components in Minnesota (about 5800 units).