Relationships Between Andic and Spodic Soil Properties and Associated Forest Communities of the Northern Rocky Mountains.

Andisols, Spodosols, and spodic intergrades are the dominant soils in the steep, volcanic ash-mantled forested terrain of the Northern Rocky Mountain region. The objective of this study was to look at the relationships between these soil types and the structure of forest communities across 49 sites in northern Idaho. Results show that Andisols and Spodosols possess significantly different morphological and chemical properties, and support distinctly different forest communities. Spodosols support a higher proportion of tree species indicative of relatively moister and cooler conditions – 87% of site basal area is occupied by western hemlock, western redcedar, spruce, and subalpine fir. In contrast, Andisols support a higher proportion of species indicative of relatively drier environments, with 60% of the site basal area occupied by ponderosa pine, Douglas-fir, western larch, and grand fir. Spodosols and spodic intergrades support greater total basal area than Andisols and contain significantly greater quantities of organic carbon to a depth of 50 cm. Eluvial E horizons of Spodosols exhibit the lowest pH values (3.2) and greatest concentrations of exchangeable Al (8.4 cmol_c/kg) of all soils, while Andisols have significantly higher pH and lower exchangeable Al. Spodic intergrades have chemical properties similar to Spodosols, but support an assemblage of tree species that is intermediate between those of the Spodosols and Andisols. In summary, this study illustrates a clear relationship between soil morphology, chemistry, and forest communities in the Northern Rocky Mountains. These results, in combination with the known geospatial distribution of Andisols, spodic intergrades, and Spodosols, can be used to improve the quality of forest management decisions in the Northern Rocky Mountain region.