The response of vegetation to chemical and hydrological gradients in the Lost River peatland, northern Minnesota

(1) Two peat mounds have developed in the Lost River peatland of northern Minnesota. One has the chemical and physical properties of an extremely rich fen and the other has the properties of a raised bog. The two mounds are separated by a water track with poor-fen vegetation and chemistry. (2) The vegetation at Lost River can be divided into five noda that correspond to landform units: spring-fen channel, spring-fen forest, marginal swamp forest, water track, and raised bog. These vegetation types have well-defined ranges for pH and calcium concentration. (3) Ordinations of the vascular plant and bryophyte data indicate a close relationship between the vegetation and both moisture and chemical gradients. The link between vegetation and chemistry is also supported by plots of species richness vs. pH and calcium concentration in which the peak in species numbers occurs within the rich-fen range. (4) The chemistry of the surface waters at Lost River is largely determined by the mixing of alkaline groundwater with precipitation, because the entire peatland is located at least seasonally within a discharge zone for groundwater. Mixing models indicate that the amount of groundwater within the surface waters ranges from 50% in the spring-fen channels to 1% on the raised bog. (5) The development of the two peat mounds was reconstructed from peat cores collected near the crests of the spring-fen mound and raised bog. The raised bog developed over a depression, which was first filled in with fen peat before minerotrophic sphagna invaded the site around 2625 B.P. By 2200 B.P. all fen indicator species had disappeared and the site was dominated by assemblages similar to those in the present raised bog. (6) The spring-fen mound, however, developed over a rise in the mineral substrate, which was not covered by peat until 3000 B.P. The mound was quickly colonized by Sphagnum and no fen indicators appeared until 1160 B.P. At this time the bog vegetation was replaced by a sedge fen containing assemblages similar to the present spring-fen channel. (7) This sharp reversal from bog to fen under the spring-fen mound can best be explained by the discharge of alkaline groundwater at the peat surface in sufficient quantities to convert the bog vegetation to a spring-fen channel.