Topographic influences on soils and trees within single mapping units on a sandy outwash landscape

Topographic variation in soil properties and forest productivity within single mapping units as defined by both forest and soil type was investigated in mature northern pin oak stands (Quercus ellipsoidalis) on an outwash plain in Minnesota, USA. Tree growth, soil water, and soil and forest floor nitrogen (N) were measured as indicators of productivity at three slope positions and four aspects. Differences in soil water were highly significant among slope position (P < 0.001), in part influenced by the shallow water table at the lower slope positions. Organic matter and total N of the surface mineral horizon also differed among slope positions (P < 0.01). Organic matter and total N of the forest floor, and N released by anaerobic incubation from the surface mineral soil, differed among aspects (P < 0.1). Twenty-year radial growth rate of site-index trees was significantly different among both slope positions and aspects (P < 0.1), but total wood volume, basal area, and site index did not differ significantly with topography. Although the study sites had subdued topography and were located on single mapping units, they contained significant topographically related variation in soil properties. Stratified sampling for soil-related properties by topographic position, even in subdued terrain, would help reduce the apparent variation commonly found in these properties.