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1 Landsat imagery was used to map the distribution of 127 raised bogs in northwestern Minnesota. Bogs collectively cover 1236 [km.sup.2] (16% of the study area) despite the relatively dry regional climate and periodic droughts. 2 The physical, chemical, and biotic properties of these bogs have no apparent relationship to the westward climatic gradient indicating a high degree of buffering from changes in moisture stress. 3 Most bogs are located where groundwater discharge moderates moisture losses to the atmosphere and may decouple bogs from a direct climatic control. Bogs are also consistently related to physiographic features, such as drainage divides, interfluves of tributary streams, and beach ridges that constrain the surface and groundwater hydrology. 4 During droughts groundwater moves upward through the peat column toward the depressed water table, which is located 1-2 m below the peat surface. During moist periods, however, water-table mounds within these bogs drive surface water downward deflecting the deeper upwardly moving groundwater laterally to the bog margins. 5 Such short-term reversals in flow have little effect on the pore-water chemistry of major cations, which reflect the predominant downward flow over the past decade. Several chemical species, however, behave nonconservatively and respond more directly to climatic change. 6 Discharge zones for groundwater seem to be an essential prerequisite for bog formation in arid regions. Peat accumulation should be most rapid over discharge zones, which can maintain water table mounds, even during droughts Once a peat mound has formed, its higher water table will drive local recharge cells, which isolate the vegetation from groundwater and facilitate the development of a raised bog. Keywords: climatic gradients, groundwater, pore-water chemistry, raised bogs, species diversity

Reversals in hydraulic head and extensive methane losses were identified during a drought cycle from changes in hydraulic-head and dissolved-methane profiles in peat, Glacial Lake Agassiz Peatlands, northern Minnesota, United States. During the summer 1990 drought, regional ground water discharged from the mineral soil underlying the peat to raised bogs. Concentrations of dissolved CH4 in pore water showed that much of the peat column was supersaturated with respect to a reference standard of 1 atm partial pressure CH4. By the summer of 1991, the severity of the drought lessened, and the upper peat column was resaturated with precipitation-derived water. Ground-water flow was then controlled by local, precipitation-driven, recharge flow systems, and the regional ground water that was discharged affected only the peat immediately above the mineral soil-peat interface. In contrast, during the summer of 1991, concentrations of dissolved CH4 in pore water were all undersaturated with respect to 1 atm partial pressure CH4. The decrease in the amount of dissolved CH4 in the pore water from the summer of 1990 to the summer of 1991 probably was caused by changes in the CH4 flux and degassing to the unsaturated zone and to a lesser extent by changes in the partial pressure of gaseous CH4 in the peat as the peat volume increased with expansion as it resaturated.

1. The vascular flora of 65 raised bogs was determined across climatic gradients in eastern North America. The major geographic patterns in species richness and floristic composition were analysed using linear regression, detrended correspondence analysis (DCA), and canonical correspondence analysis (CCA). 2. The vascular bog flora consists of 81 species in this region. Four major floristic regions distinguished by DCA correspond to the geographic zonation of bog land-forms. Each of these floristic regions is characterized by a different mean value for species richness. 3. Raised bogs in the southern-continental floristic region (I) contain fewer than 20 species and have the most impoverished vascular floras in eastern North America. In the transitional-continental (II) and northern-continental (III) regions, the bog flora is slightly larger, although the mean number of species remains below 26. 4. The richest bog floras are found in the maritime region (IV), where species richness declines along a latitudinal gradient from 50 species in the south (subregion IV.1) to 32 in the north (subregion IV.2). 5. The relationship of species richness to various environmental factors was analysed using linear regression. The most important factors are mean annual precipitation (MAP) and annual freezing degree-days (FDD) with a base temperature of O [degrees] C. Factors that are also significantly related to species richness are mean annual temperature (MAT), the number of wet-to-dry habitats, and the concentration of magnesium and sodium in the surface water. No significant relationship was found for growing degree-days (base temperature = 5 [degrees] C.), mean annual snow depth, or spatial dimensions of bogs. 6. The regressions indicate that a climatic threshold of 1000 mm of annual precipitation and 1000 freezing degree-days separates the floristically rich maritime bogs from the floristically impoverished bogs of northern and continental regions. 7. The relative importance of these environmental factors is supported by CCA. In addition, the CCA ordination identifies two groups of species on the basis of their response to these environmental factors. Species that are generally restricted to the maritime region (IV) have weighted averages that are above average for MAP and MAT, but below average for FDD. Species that are common to all bogs or that are more continental in their distribution have the reverse relationship. 8. The tolerance of the regional bog flora to the acidic, anoxic rooting zone changes across this climatic gradient. One third of the species that grow on bogs in the southern maritime region are restricted to fens or mineral uplands in continental or northern regions. This shift in ecological tolerance may be related to (i) slower rates of peat accumulation on the maritime bogs, (ii) longer time available for the evolution of local tolerant populations on maritime bogs, or (iii) a physiological adaptation to the maritime climate.

An investigation of silicate weathering in a Minnesota mire indicates that quartz and aluminosilicates rapidly dissolve in anoxic, organic-rich, neutral-pH environments. Vertical profiles of pH, dissolved silicon, and major cations were obtained at a raised bog and a spring fen and compared. Profiles of readily extractable silicon, diatom abundance, ash mineralogy, and silicate surface texture were determined from peat cores collected at each site. In the bog, normally a recharge mound, dissolved silicon increases with depth as pH increases, exceeding the background silicon concentration by a factor of two. Silicate grain surfaces, including quartz, are chemically etched at this location, despite being in contact with pore water at neutral pH with dissolved silicon well above the equilibrium solubility of quartz. The increasing silica concentrations at circum-neutral pH are consistent with a system where silicate solubility is influenced by silica-organic-acid complexes. Silica-organic-acid complexes therefore may be the cause of the almost complete absence of diatoms in decomposed peat and contribute to the formation of silica-depleted underclays commonly found beneath coal.

SUMMARY (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 I % 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.

1 Groundwater-peatland interactions were assessed by a regional survey in the Hudson Bay Lowlands, where the rapid rate of isostatic uplift has perturbed hydrological flow systems across a 6000-year chronosequence. 2 A 24 000 km2 study area along the Albany River consists of 55% fen, 35% bog and 10% mineral soil. The peatland vegetation may be further subdivided into 11 noda, which are closely related to different water levels, ranges in water chemistry, and peat landform type. Species richness generally declines with increasing water level and acidity, whereas the gradient from bog to extremely rich fen is marked by the changing abundance and occurrence of fen-indicator species. 3 Bog landforms are restricted to physiographic settings where surface waters flow downwards and the bog vegetation is therefore isotated from the influence of geogenous waters. In contrast, fens are located in areas where mineral solutes are transported to the peat surface either by upwelling groundwater or by advective/dispersion along lateral flow paths. 4 Peatlands spread across the study area between 6000 and 3000 bp, coinciding with the emergence of new land from the sea. The release of organic acids from the nearly continuous peat cover acidified this calcareous landscape, leading to the convergence of the surface-water chemistry into four discrete groupings of pH vs. calcium. 5 Isostatic uplift, however, continues to alter the topography, fluvial geometry and groundwater flow systems of the lowlands, maintaining diverse peatland types on land surfaces of similar age. The formation of water-table mounds under the interfluvial divides and rising moraine system spurred the development of raised bogs, whereas the formation of regional seepage faces for goundwater on the margins of the moraine and rivers of the till plain maintains large areas of fen. 6 Although peatland succession seems to follow predictable pathways within a given hydrogeological setting, these pathways are locally altered by tectonic drivers that continually modify surface and groundwater flow systems. In this large peat basin the pace and pathway of peatland succession seems to be driven by tectonic rather than climatic forcing.

1 The Hudson Bay Lowlands have been rising isostatically for the past 7000 years, creating a regional chronosequence as new land emerges from the sea. Rates of uplift are most rapid in the eastern portion of the lowlands near the lower Albany River study area. 2 The stratigraphy of three raised bogs was investigated to determine rates and pathways of peatland development in the Albany River region. The bogs are distributed evenly along the regional chronosequence from the oldest site at Oldman (5980 [+ or -] 100 bp) to progressively younger sites at Albany River (4810 [+ or -] 70) and Belec Lake (3960 [+ or -] 60). 3 Each bog had the same stratigraphic sequence, beginning with a basal tidal marsh assemblage that was rapidly replaced by a Larix-dominated swamp forest, followed by a Picea-forested bog, and ultimately a non-forested bog. The bog-fen boundary is marked by the disappearance of fen indicators, dominance of bog-forming Sphagna, and a sharp decline in nitrogen. Each of these successional stages was associated with different rates of vertical growth. 4 The rate of successional change was more rapid at the younger sites, and their vertical growth curve was more curvilinear. The formation of a raised bog, for example, was 1.3 times more rapid at Albany River and 5.5 times more rapid at Belec Lake than at Oldman. Belec Lake reached its ultimate successional stage first, although it was the last site to emerge from the sea. 5 The differential rate of isostatic uplift across this region rather than climate was the principal environmental driver for peatland development. The faster rate of uplift on the lower reaches of the drainage basin continues to reduce the regional slope, impede drainage and shift river channels, continually altering the local hydrogeological setting. 6 Groundwater flow simulations based on the Dupuit equation show that the growth of these raised bogs was probably constrained by their local hydrogeological setting. Bog formation was first induced by the creation of interfluvial divides between headwardly eroding streams or shifting river channels, and further bog growth was ultimately constrained by the width of the interfluve and the depth of river incision. The Belec Lake bog was the first to approach its limiting height because its narrow interfluve could only support a low water-table mound. 7 Although peatland succession largely followed the same conservative pathway at each site, both the pace and direction of these pathways were set by geological processes, which are probably the decisive drivers for the evolution of this large peat basin.