REGIONAL-SCALE RELATIONSHIPS BETWEEN CLIMATE AND STRENGTH OF PODZOLIZATION IN THE GREAT-LAKES REGION, NORTH-AMERICA

Along a 300 km transect in northern Wisconsin and the upper peninsula of Michigan, the areal coverage of Spodosols and spodic horizon develo pment increases markedly from south to north. This study elucidated th ose aspects of climate that promote podzolization in this region, thro ugh 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 thickn esses 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 16-17 degrees C. Podzolization is str ongest where snowpacks are thickest - an association that holds at bot h regional and meso scales. Thick snowpacks inhibit soil frost and all ow large fluxes of snowmelt water to infiltrate into already moist pro files. This type of flux (slow, steady, cold water) may be particularl y effective in the podzolization process. In the southern part of the transect, where Spodosols are rare, snowmelt fluxes are 1/3 as large a s in the northern ''snowbelt'' areas. The southern areas also have a s mall autumn infiltration peak that usually reaches to approximate 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 smal l along the entire transect during summer, underscoring the belief tha t the bulk of pedogenesis (i.e., translocation), in Spodosols in the s tudy area, occurs during snowmelt.