Regional-Scale Relationships between Climate and Strength of

Podzolization in the Great Lakes Region, North America

Randall J. Schaetzl and Scott A. Isard

Along a 300 km transect in northern Wisconsin and the upper peninsula of Michigan, the areal coverage of Spodosols and spodic horizon development increases markedly from south to north. This study elucidated those aspects of climate that promote podzolization in this region, through an examination of the geographic correspondences between Spodosol development and soil climate. Climatic data (1951-1991) from 21 sites along this transect were processed by a hydrologic model developed to output data on (1) soil temperatures for 0.05 and 0.5 m depths for 20 minute intervals, including data on soil freezing, (2) snowpack thicknesses under forest cover, and (3) daily water fluxes, runoff, and soil water contents at several depths.

Spodosols dominate the landscape in areas where soil frost and freeze-thaw activity are minimal and where soil temperatures rarely exceed 16-17C. Podzolization is strongest where snowpacks are thickest an association that holds at both regional and meso scales. Thick snowpacks inhibit soil frost and allow large fluxes of snowmelt water to infiltrate into already moist profiles. This type of flux (slow, steady, cold water) may be particularly effective in the podzolization process. In the southern part of the transect, where Spodosols are rare, snowmelt fluxes are 1/3 as large as in the northern snowbelt areas. The southern areas also have a small autumn infiltration peak that usually reaches to 0.3 m depth; this flux is absent in areas of strong podzolization. Mean soil water contents are low and fluxes of water into the soil are small along the entire transect during summer, underscoring the belief that the bulk of pedogenesis (i.e., translocation), in Spodosols in the study area, occurs during snowmelt.