Paleoecological aspects of the Red Lake Peatland, northern Minnesota

Abstract

The Red Lake Peatland, situated on the eastern arm of the Lake Agassiz plain in north central Minnesota, is made up of large black spruce raised bogs separated by sedge-covered water tracks marked by string patterns and teardrop-shaped forested islands. About 3 m of peat overlie a prairie soil; the basal peat is 3170 ± 100 years old (14C date). Stratigraphic distribution of pollen, macrofossils, and peat components in a series of cores along a small forested island was used to study the development of the peatland.In pollen zone RLB-1 (lowest) the upland pollen types suggest an oak savanna with a developing mesic deciduous forest. The local pollen indicates a succession from a Typha marsh to a sedge meadow. Zone 2 shows a small rise in Pinus pollen; Ericaceae pollen and Sphagnum spores indicate the development of a bog–heath vegetation type at the coring site. An increase in pollen of spruce and larch reflects the establishment of the island forest. Zone 3 is marked by a rise in Ambrosia pollen, recording agricultural land clearance in northwestern Minnesota in about 1890.The Salix–herb macrofossil assemblage occurs at the base of some cores; it is succeeded stratigraphically by the Typha–Scirpus assemblage, which also makes up the base of the remaining cores. There follows the Carex diandra – Carex aquatilis assemblage, suggesting a sedge meadow. The overlying Menyanthes–Larix assemblage continues to the surface under the sedge fen; under the island, seeds of Chamaedaphne and needles of Picea occur in this assemblage. The incoming of the Menyanthes–Larix assemblage is believed to reflect the first development of the patterned wetland.Reed peat comprises the bottom 20–50 cm of all cores; it corresponds with the lower part of pollen zone 1 and contains the Typha–Scirpus macrofossil assemblage. Pure sedge peat or sedge peat with Bryales corresponds with zone 2 and upper zone 1 of the pollen diagram and contains the Carex diandra –C. aquatilis macrofossil assemblage. Ericaceous peat occurs only under the island, and it contains the Menyanthes–Larix macrofossil assemblage. Laterally under the present sedge fen it grades into sedge peat with ericaceous components.The general upward succession of communities observed in these peat cores can be duplicated along a west-to-east surface traverse across the present prairie–forest transition. The stratigraphic succession thus records the westward movement of the prairie–forest transition across Beltrami County about 4000–2000 years ago.The area was covered by prairie more than 3100 years ago. Development of a cooler, moister climate led to a Typha marsh in the poorly drained lowland; oak savanna occupied the surrounding upland. As the water level rose in the lowland, peat started to accumulate as sedge meadow developed, and aspen parkland and conifer–hardwood forest moved into the upland. About 2000 years ago the growth of Sphagnum moss increased, accompanied by the development of patterned wetland and bog forest, with conifer–hardwood forest on the upland.