Overlaying soil and timber inventories to assess aspen productivity in northern Minnesota

Forest soil productivity is a difficult quantity to measure, yet it is an integral component of forest management decisions and an important interpretation of soil inventories conducted in forested areas. Soil-site studies are time consuming and expensive, and often cannot identify productivity differences between soil units. Merging timber-inventory data with soil-survey data may provide an efficient means for evaluating forest productivity. In this study, timber-inventory data were overlaid with soil-survey data for Beltrami County, Minnesota. Soil map units were grouped into 10 soil classes based on drainage and parent material. Productivity of aspen (populus tremuloides Michx, and P. grandidentata Michx.) was compared between soil classes using the estimates of volume, site index, and basal area collected during the timber inventory. Analysis of the volume data indicated that the soils formed in outwash deposits were significantly less productive (mean volume of stands between 40 and 55 yr = 126 m³ ha^-1) than soils formed in calcareous glacial till or calcareous deposits of stratified fine sands and silts (mean volume of stands between 40 and 55 yr = 155 m³ ha^-1). Very poorly drained soils were also in the higher productivity group, though aspen was a relatively minor cover type on these soils. The site-index data was of minimal value in separating soils into productivity groups. Basal area was highly correlated with volume estimates and may be a useful stand attribute for assessing forest productivity during land-resource inventories.