Soil richness and endemism across an environmental transition zone in Wisconsin, USA

Soil richness was examined across the Wisconsin ecological tension zone, which constitutes a transition between the central hardwood forest and prairie biomes to the south and the northern mixed hardwood and conifer forest biome to the north. There were greater numbers (normalized by county area) of soil orders, suborders, and great groups in counties within the transition zone than in those outside the zone. A zone of soils containing glossic horizons extends 75 km to the north of the Wisconsin transition zone separates Alfisols (dominantly Hapludalfs) and Mollisols in the south from Spodosols and Glossudalfs in the north; this zone may represent the mid-Holocene position of the tension zone. Soil endemism is a concept whereby soils are restricted to a limited geographic area based on a unique set of soil-forming factors. In this study, endemic soils were identified on the basis of having a centralized distribution, lacking competing soil series, and being unique to Wisconsin and adjoining states. Although the transition zone contains 13% of the stateʹs area, it has 40% of the endemic soils. Endemic soils in Wisconsin occur in 7 orders, 14 suborders, 29 great groups, 83 subgroups, and 138 families; they include 159 soil series that cover 1.44 million ha (19% of state land area). The predominant soil properties related to soil endemism include (i) the presence of a glossic horizon (34% of endemic soils), (ii) aquic conditions or oxyaquic subgroups (28%), and (iii) bisequal profiles (20%). There are 102 vascular plant species endemic to Wisconsin, the distribution of which appears to be related more to the landform than to soil taxa.