Landscape Evolution, Peatland Types, and the Environment in the Lake Agassiz Peatlands Natural Area, Minnesota

Abstract

The vegetation and peatland types of the Lake Agassiz Peatlands Natural Area are related to topography, waterflow patterns, water chemistry, and the evolution of the landscape as recorded by peat stratigraphy. Eight peatland types are distinguished: (1) minerotrophic swamp, (2) weakly minerotrophic swamp, (3) string bog and patterned fen, (4) forest island and fen complex, (5) transitional forested bog, (6) semi—ombrotrophic bog, (7) ombrotrophic bog (raised bog), and (8) raised bog drain. Consistent differences in pH, Ca, and Mg were found between waters of contrasting peatland types. These differences agree with the division of peatland types by degree of mineral soil water influence (minerotrophy). A general topographic alignment of vegetation and peatland types agrees with the hypothesis of chemical controls. Vegetation types often have sharp boundaries related to changes in water properties, peat surface configuration, and paths of waterflow. Landscape evolution included five phases: (1) Recession of Lake Agassiz about 11,700 years ago. (2) Organic sedimentation of local basins beginning 11,100 years ago. Aquatic peats eventually covered 6% of the substratum. (3) Development of fens, marshes, and carr during the postglacial warm—dry interval, beginning about 8,000 years ago. These peatlands built the sedge peats that now cover 46% of the substratum. Paludification caused water tables to rise, and most water basins were overgrown. (4) Invasion of minerotrophic swamp forests around the perimeter of the peatland and along substratum ridges, beginning about 5,000 years ago as the climate cooled and precipitation increased. These forests built the basal forest peats that now cover 48% of the substratum. (5) Capture of part of the watershed by Northeast Brook about 3,100 years ago, which caused a water—table divide and mineral depletion, initiated sphagnum invasion, and led to development of the present ombrotrophic raised bogs. As convexity grew, a sharp vegetation and chemical gradient developed along the limit of mineral soil water. Myrtle Lake rose steadily with paludification and now stands 11.8 ft (3.6 m) above the ridge 1 mile (1.6 km) north of the lake. Water tables rose 10—20 ft (3.0—6.1 m) over much of a 70—square—mile (181—km 2) area. This history does not agree with early concepts of succession, which postulate a trend toward mesophytism with peat accumulation. The only "direction" here is a possible trend toward landscape diversity.