Geophysical Evidence for Lithologic and Hydrogeological Controls on Vegetation Communities in a Large Northern Peatland

Recent conceptual models invoke hydrogeologic processes as a controlling factor in the development of the striking vegetation patterns observed in northern peatlands. These processes regulate the supply of solutes to the peat surface, controlling the surface-water chemistry and the supply of nutrients to plants. Geophysical studies in Caribou Bog, a 2200-hectare peatland in central Maine, indicate a close correlation between lithology of the underlying mineral soil and dominant vegetation. Electrical resistivity imaging along a 1 km transect across the central unit of Caribou Bog resolves underlying glaciomarine clay thickness. Ground penetrating radar precisely defines the glaciomarine interface where peat thickness is less than 10 m. Direct verification of peatland thickness and sampling at the mineral soil contact constrains the geophysical interpretation. Wooded heath interspersed with sphagnum/leatherleaf lawn occurs where glaciomarine clay accumulation is thickest (estimated to exceed 10 m in parts). Abrupt thinning of the glaciomarine clay, such that peat rests directly on bedrock in parts, correlates with a sharp transition to shrub heath dominated vegetation. The location of open pools within the wooded heath of Caribou Bog coincides with localized thinning of the glaciomarine clay and exposure of bedrock at the base of the bog. Groundwater flow cells recorded over two years suggest that the glaciomarine clay acts as a confining layer and impacts nutrient supply from the mineral soil, and hence vegetation patterns, at the bog surface.