Minnesota Peatlands Research Articles

Across-model spread and shrinking in predicting peatland carbon dynamics under global change.

Large across-model spread in simulating land carbon (C) dynamics has been ubiquitously demonstrated in model intercomparison projects (MIPs), and became a major impediment in advancing climate change prediction. Thus, it is imperative to identify underlying sources of the spread. Here, we used a novel matrix approach to analytically pin down the sources of across-model spread in transient peatland C dynamics in response to a factorial combination of two atmospheric CO2 levels and five temperature levels. We developed a matrix-based MIP by converting the C cycle module of eight land models (i.e., TEM, CENTURY4, DALEC2, TECO, FBDC, CASA, CLM4.5 and ORCHIDEE) into eight matrix models. While the model average of ecosystem C storage was comparable to the measurement, the simulation differed largely among models, mainly due to inter-model difference in baseline C residence time. Models generally overestimated net ecosystem production (NEP), with a large spread that was mainly attributed to inter-model difference in environmental scalar. Based on the sources of spreads identified, we sequentially standardized model parameters to shrink simulated ecosystem C storage and NEP to almost none. Models generally captured the observed negative response of NEP to warming, but differed largely in the magnitude of response, due to differences in baseline C residence time and temperature sensitivity of decomposition. While there was a lack of response of NEP to elevated CO2 (eCO2 ) concentrations in the measurements, simulated NEP responded positively to eCO2 concentrations in most models, due to the positive responses of simulated net primary production. Our study used one case study in Minnesota peatland to demonstrate that the sources of across-model spreads in simulating transient C dynamics can be precisely traced to model structures and parameters, regardless of their complexity, given the protocol that all the matrix models were driven by the same gross primary production and environmental variables.

Impacts of historical ditching on peat volume and carbon in northern Minnesota USA peatlands

Peatlands play a critical role in terrestrial carbon (C) storage, containing an estimated 30% of global soil C, despite occupying only 3% of global land area. Historic management of peatlands has led to widespread degradation and loss of important ecosystem services, including C sequestration. Legacy drainage features in the peatlands of northern Minnesota, USA were studied to assess the volume of peat and the amount of C lost in the ~100 years since drainage. Using high-resolution Light Detection and Ranging (LiDAR) data, we measured elevation changes adjacent to legacy ditches to model pre-ditch surface elevations, which were used to calculate peat volume loss. We established relationships between volume loss and site characteristics from existing geographic information systems datasets and used those relationships to scale volume loss to all mapped peatland ditches in northern Minnesota (USA). We estimated that 0.165 ± 0.009 km3 of peat have been lost along almost 4000 km of peatland ditches. Peat loss upslope of ditches was significantly less than downslope (P < 0.001). Mean width of the entire ditch-effect zone was 333 ± 8.32 m. Using our volume loss estimates, literature estimates of oxidation, and mean bulk density and peat C% values from Minnesota peatlands, we calculate a total historic loss 3.847 ± 0.364 Tg C. Assuming a constant oxidation rate during the 100 years since drainage, euic and dysic peatlands within the ditch effect zone have lost 0.26 ± 0.08 and 0.40 ± 0.13 Mg C ha−1 yr−1, respectively, comparable to IPCC estimates. Our spatially-explicit peat loss estimates could be incorporated into decision support tools to inform management decisions regarding peatland C and other ecosystem services.

The potential importance of methylated substrates in methane production within three northern Minnesota peatlands

Peatlands store close to half of the terrestrial soil carbon and release substantial amounts of the greenhouse gas methane (CH4) to the atmosphere. While CH4 is thought to be produced mainly by hydrogenotrophic and acetoclastic methanogenic pathways, methylotrophic methanogenesis may be a hidden, yet potentially important, source of CH4 in these globally significant ecosystems. To explore the importance of methylotrophic methanogenesis in northern peatlands, 13C-labeled methylated substrates (methanol, dimethylsulfide, and trimethylamine) were added to soils along a depth profile and the production of 13C–CH4 was measured. Additionally, the concentrations of methylated compounds were measured across a soil depth profile in situ.Results suggest that methylated substrates can be used by peatland microbes, resulting in CH4 production in all sites at all depths. Methanol was converted to CH4 in each site at all depths. In contrast, trimethylamine and dimethylsulfide were only converted to CH4 at the surface depth regardless of site. In situ concentrations of dimethylsulfide were below detection limit in all sites and depths. Monomethylamine and methanol were present in situ with concentrations ranging from 30 to 1225 nM.For the first time in peatlands, we have estimated contributions of methylated substrates to the total CH4 produced. Microbial processing of methylated substrates contributed a maximum of 11% of the total CH4 produced at depth in one peatland; but, in shallow soils with higher rates of CH4 production, the potential contribution of methylotrophic methanogenesis to overall CH4 production was minor.

Methylotrophic methanogenesis in Sphagnum-dominated peatland soils

Although it is generally assumed that CH4 production in peatlands is dominated by the acetoclastic and hydrogenotrophic methanogenic pathways, we found evidence that methylated substrates (including methanol, methylamines, and dimethylsulfide) support methanogenesis in these organic-rich environments. The role of methylotrophic methanogenesis was investigated in three northern Minnesota peatlands by amending surface soils with 13C-labeled substrates. Labeled methanol was converted to enriched δ 13C-CH4 at all three sites. Additionally, dimethylsulfide and monomethylamine were processed through methanogenic pathways, although this was not evident at every site. The addition of 13C labeled methanol and monomethylamine also resulted in the enrichment of δ 13C-CO2, further suggesting that either methanogens or non-methanogenic microbes were processing methylated substrates. Results demonstrate the potential for methylotrophic methanogenesis in northern peatlands and suggest that further studies are warranted to quantify the amount of CH4 produced by this process in comparison to traditional pathways of methanogenesis.

Data‐Constrained Projections of Methane Fluxes in a Northern Minnesota Peatland in Response to Elevated CO2 and Warming

AbstractLarge uncertainties exist in predicting responses of wetland methane (CH4) fluxes to future climate change. However, sources of the uncertainty have not been clearly identified despite the fact that methane production and emission processes have been extensively explored. In this study, we took advantage of manual CH4 flux measurements under ambient environment from 2011 to 2014 at the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experimental site and developed a data‐informed process‐based methane module. The module was incorporated into the Terrestrial ECOsystem (TECO) model before its parameters were constrained with multiple years of methane flux data for forecasting CH4 emission under five warming and two elevated CO2 treatments at SPRUCE. We found that 9°C warming treatments significantly increased methane emission by approximately 400%, and elevated CO2 treatments stimulated methane emission by 10.4%–23.6% in comparison with ambient conditions. The relative contribution of plant‐mediated transport to methane emission decreased from 96% at the control to 92% at the 9°C warming, largely to compensate for an increase in ebullition. The uncertainty in plant‐mediated transportation and ebullition increased with warming and contributed to the overall changes of emissions uncertainties. At the same time, our modeling results indicated a significant increase in the emitted CH4:CO2 ratio. This result, together with the larger warming potential of CH4, will lead to a strong positive feedback from terrestrial ecosystems to climate warming. The model‐data fusion approach used in this study enabled parameter estimation and uncertainty quantification for forecasting methane fluxes.

Intermediate-scale community-level flux of CO2 and CH4 in a Minnesota peatland: putting the SPRUCE project in a global context

Peatland measurements of CO2 and CH4 flux were obtained at scales appropriate to the in situ biological community below the tree layer to demonstrate representativeness of the spruce and peatland responses under climatic and environmental change (SPRUCE) experiment. Surface flux measurements were made using dual open-path analyzers over an area of 1.13 m2 in daylight and dark conditions along with associated peat temperatures, water table height, hummock moisture, atmospheric pressure and incident radiation data. Observations from August 2011 through December 2014 demonstrated seasonal trends correlated with temperature as the dominant apparent driving variable. The S1-Bog for the SPRUCE study was found to be representative of temperate peatlands in terms of CO2 and CH4 flux. Maximum net CO2 flux in midsummer showed similar rates of C uptake and loss: daytime surface uptake was −5 to −6 µmol m−2 s−1 and dark period loss rates were 4–5 µmol m−2 s−1 (positive values are carbon lost to the atmosphere). Maximum midsummer CH4-C flux ranged from 0.4 to 0.5 µmol m−2 s−1 and was a factor of 10 lower than dark CO2–C efflux rates. Midwinter conditions produced near-zero flux for both CO2 and CH4 with frozen surfaces. Integrating temperature-dependent models across annual periods showed dark CO2–C and CH4–C flux to be 894 ± 34 and 16 ± 2 gC m−2 y−1, respectively. Net ecosystem exchange of carbon from the shrub-forb-Sphagnum-microbial community (excluding tree contributions) ranged from −3.1 gCO2–C m−2 y−1 in 2013, to C losses from 21 to 65 gCO2–C m−2 y−1 for the other years.

Utilization of PARAFAC-Modeled Excitation-Emission Matrix (EEM) Fluorescence Spectroscopy to Identify Biogeochemical Processing of Dissolved Organic Matter in a Northern Peatland.

In this study, we contrast the fluorescent properties of dissolved organic matter (DOM) in fens and bogs in a Northern Minnesota peatland using excitation emission matrix fluorescence spectroscopy with parallel factor analysis (EEM-PARAFAC). EEM-PARAFAC identified four humic-like components and one protein-like component and the dynamics of each were evaluated based on their distribution with depth as well as across sites differing in hydrology and major biological species. The PARAFAC-EEM experiments were supported by dissolved organic carbon measurements (DOC), optical spectroscopy (UV-Vis), and compositional characterization by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS). The FT-ICR MS data indicate that metabolism in peatlands reduces the molecular weights of individual components of DOM, and oxygen-rich less aromatic molecules are selectively biodegraded. Our data suggest that different hydrologic and biological conditions within the larger peat ecosystem drive molecular changes in DOM, resulting in distinctly different chemical compositions and unique fluorescent fingerprints. PARAFAC modeling of EEM data coupled with ultrahigh resolution FT-ICR MS has the potential to provide significant molecular-based information on DOM composition that will support efforts to better understand the composition, sources, and diagenetic status of DOM from different terrestrial and aquatic systems.

Radiocarbon and stable carbon isotopic evidence for transport and transformation of dissolved organic carbon, dissolved inorganic carbon, and CH4 in a northern Minnesota peatland

To elucidate the roles of hydrology and vegetation in below ground carbon cycling within peatlands, radiocarbon values were obtained for pore water dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), CH4, and peat from the Glacial Lake Agassiz peatland. The major implication of this work is that the rate of microbial respiration within a peat column is greater than the peat decomposition rate. The radiocarbon content of DOC at both bog and fen was enriched relative to solid‐phase peat by ∼150‐300‰ consistent with the advection of recently photosynthesized DOC downward into the peat column. Fen Δ14C values for DIC and CH4 closely track the Δ14C of pore water DOC at depth, indicating that this recent plant production was the predominant substrate for microbial respiration. Aceticlastic methanogenesis apparently dominated the upper third of the peat column (α = 1.05), shifting toward CO2 reduction with depth (1.05 &lt; α &lt; 1.08). Upwelling groundwater contributed as much as 15% of the DIC to the bulk DIC pool at depth in the fen. The similarity of Δ14C values for DIC and CH4 suggests that methanogens utilized DIC from this source as well as DIC produced in situ. Bog Δ14C values for pore water DIC and CH4 differ by ≤ 15‰ at all depths and are depleted in 14C relative to DOC by ∼100‰, suggesting microbial utilization of a mixture of older and modern substrates. CO2 reduction was the primary pathway for methanogenesis at all depths in the bog (α = 1.08), and groundwater influence on bulk DIC was negligible. For both sites, Δ14C−DIC and Δ14C−CH4 are approximately equal at depths where stable isotope data indicate a predominance of CO2 reduction and dissimilar when acetate fermentation is indicated.

Micrometeorological measurements of methane flux in a Minnesota peatland during two growing seasons

Methane flux was measured, employing themicrometeorological eddy correlation technique,during two growing seasons (1991 and 1992) in apeatland in Minnesota. As compared to 1991, the1992 season was wetter and cooler. Here we examinethe seasonal distributions of CH4 flux and therelationship between concurrently measured CH4and CO2fluxes. Midday CH4 flux was low(1.5 mg m-2 h-1) during late May in bothseasons. Subsequently, the flux ranged from 2.5 to5.5 mg m-2 h-1 during early June to earlyJuly in both years. Methane flux peaked at6.5 mg m-2 h-1 during mid July in 1991. The peak flux (8.0 mg m-2 h-1) in 1992occurred 3 weeks later. A sustained drop in watertable during late July to late August in 1991 mayhave reduced the methane emission. During midAugust–mid October in 1992, the water table wasconsistently high and the flux ranged from 2.0 to3.0 mg m-2 h-1. As compared to 1991,CH4 flux during this time in 1992 was higher byabout 1.0 mg m-2 h-1 because of theoverriding influence of the water table. Integration over the growing season (late May to midOctober), indicated that this ecosystem releasedapproximately 10.4 and 11.5 g C m-2 of CH4in 1991 and 1992, respectively. We examined our concurrent measurements on methaneflux and canopy photosynthesis under a variety ofenvironmental conditions from different parts of thegrowing season. On a time scale of the entireseason, the overall patterns of methane flux andcanopy photosynthesis were similar in both years. Canopy photosynthesis, however, showed large day-to-daychanges in response to variations in temperatureand moisture. Corresponding changes in methane fluxduring these selected periods were relatively small. The slopes and correlation coefficients of linearregressions between methane flux and photosynthesisdata varied widely. Accordingly, a close coupling between short-term (day to day) variations inmethane flux and canopy photosynthesis was not evident.

Radiocarbon evidence for the substrates supporting methane formation within northern Minnesota peatlands

Bogs and fens from northern Minnesota produce large quantities of CH4, which may be either emitted to the atmosphere or stored in below-ground reservoirs. The identity of the organic materials that support CH4 production has been uncertain, but we present evidence that a significant fraction of surface emission and below-ground CH4 is derived from recently fixed organic compounds. First, the CH4 emitted from both bogs and fens has a '4C signature equivalent to contemporary values for atmospheric CO2. Second, in flooded fens rates of CH4 emission are linearly related to rates of CO2 exchange and to the δ13C of emitted CH4. Third, peat-porewaters as deep as several meters below the surface contain mixtures of CH4 derived from both modern and older organic substrates. The source of the modern organic substrates is most likely dissolved organic compounds produced from the decay of recently produced litter, roots and root exudation products and transported into deeper layers of the peat. These data indicate that CH4 emissions are closely linked to the living vegetation and hydrology of northern peatlands and less dependent on the lability and decomposition of peat within the deeper layers of the catotelm.

Seasonal distribution of methane flux in a Minnesota peatland measured by eddy correlation

Methane flux was measured using the micrometeorological eddy correlation technique during 62 days in mid‐May through mid‐October 1991 in a peatland ecosystem in north central Minnesota. Application of this technique allows measurement of spatially integrated fluxes. The distribution of methane flux consisted of a gradual pattern with several episodic emissions superimposed. The gradual (nonepisodic) pattern of methane flux (daytime average) exhibited an increase from 30–120 mg m−2 d−1 in late May to early July to 125–160 mg m−2 d−1 in mid‐July to mid‐August and then a decline to 100–35 mg m−2 d−1 in early September to mid‐October. Peat temperature (at 0.1 m depth) and water table depth accounted for about 70% of the variance in the methane flux data. The episodic emissions were associated with drops in atmospheric pressure and a declining water table. Generally, the hourly values of daytime methane flux were fairly stable, perhaps with a slight depression during the midday. However, on days with episodic emissions, the daytime methane flux had a peak during the midafternoon, and its pattern appeared to be similar to those of the standard deviation of atmospheric pressure fluctuations and mean horizontal wind velocity. The ratio of soil CO2 to methane fluxes (during nonepisodic emission periods) increased with increasing water table depth in a manner similar to that observed in a laboratory study of peat columns from different wetland types in Quebec. The annual methane emission for this ecosystem was estimated to be about 16–19.5 g m−2 yr−1.

Methane emission from Minnesota peatlands: Spatial and seasonal variability

The variability of methane flux with season, year, and habitat type was investigated in northern Minnesota peatlands from September 1988 through September 1990. Average daily fluxes calculated by integration of annual data for an open poor fen, an open bog, a forested bog hollow, a fen lagg in the forested bog and a forested bog hummock were 180,118, 38, 35, and 10 mg CH4 m−2 d−1, respectively. Fluxes among the five ecosystems were significantly different from one another, although emission from all sites was highest in July and lowest in March. Winter fluxes occurred in all sites but the fen lagg. There was no difference in fluxes measured from the same sites in the spring of 1986, 1989, or 1990, but summer fluxes were significantly higher in the wetter year of 1989 than in 1990, and a summer pulse in methane emission occurred in 1989 that was not seen the next year. Concentrations of methane in pore water, reflecting the seasonal balance of production, oxidation, and release, declined during the month of peak flux, then increased to levels of about 500 μM in December. Consistent spatial and temporal differences in flux could be ascribed to differences in water table, temperature, and peat nutrient status, although additional variability remained. Integration gave an annual average flux of 20 g CH4, m−2 ot; for the three bog ecosystems and 39 g CH4, m−2 for the two fen ecosystems. This gives an estimate of 1–2 Tg CH4, yr−1 from peatlands in the Great Lake states of Minnesota, Wisconsin, and Michigan.

Correction to “Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands” by Cheryl A. Kelley, Nancy B. Dise, and Christopher S. Martens

Global Biogeochemical CyclesVolume 6, Issue 4 p. 347-347 CorrectionFree Access Correction to “Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands” by Cheryl A. Kelley, Nancy B. Dise, and Christopher S. Martens First published: December 1992 https://doi.org/10.1029/92GB02548AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume6, Issue4December 1992Pages 347-347 RelatedInformation

Effects of Acidic Bog Habitats on Amphibian Reproduction in a Northern Minnesota Peatland

Breeding amphibians in the boreal peatlands of northern Minnesota encounter a mosaic of water quality: acidic bog sites (pH 5.0), and marginal fen sites (pH 4.5-5.0). Laboratory and field work demonstrated that peatland amphibians (Rana sylvatica, R. pipiens, Pseudacris triseriata, P. crucifer, Hyla versicolor, Bufo americanus, Ambystoma laterale) are differentially tolerant of bog water. Embryonic development is arrested or hatching is inhibited in bog water, and larval growth and survival are negatively affected

Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands

The stable carbon isotopic composition of methane (δ13C) emitted from two peatland sites in the Marcell Experimental Forest in northern Minnesota was investigated during the snow‐free season of 1989–1990. A seasonal range in δ13C values of 13‰ was seen for a forested bog with heavier (13C enriched) methane emitted during the wanner summer months. This shift was correlated with water table level suggesting control by microbial oxidation. Methane from a nearby poor fen transitional to bog dominated byCarex oligospermashowed a similar temporal trend but with a much smaller range of 5‰ during the same time period and with no water table level correlation. The methane emitted from the fen was consistently heavier than that emitted by the bog.

Soil surface CO2 flux in a Minnesota peatland

Soil surface CO2 flux was measured in hollow and hummock microhabitats in a peatland in north central Minnesota from June to October in 1991. We used a closed infrared gas exchange system to measure soil CO2 flux. The rates of CO2 evolution from hummocks (9.8 ± 3.5 g m−2 d−1, [mean ± SE]) were consistently higher than those from hollows (5.4 ± 2.9 g m−2 d−1) (the hummock values included the contribution of moss dark respiration, which may account for 10–20% of the total measured flux). The soil CO2 flux was strongly temperature-dependent (Q10 ≈ 3.7) and appeared to be linearly related to changes in water table depth. An empirical multiplicative model, using peat temperature and water table depth as independent variables, explained about 81% of the variance in the CO2 flux data. Using the empirical model with measurements of peat temperature and estimates of hollow/hummock microtopographic distribution (relative to water table elevation), daily rates of “site-averaged” CO2 evolution were calculated. For the six-month period (May–October), the total soil CO2 released from this ecosystem was estimated to be about 1340 g CO2 m−2.

Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands

Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands

Winter fluxes of methane from Minnesota peatlands

Winter fluxes of methane were investigated in northern Minnesota during 1988–89 and 1989–90. Two bogs and a fen emitted methane throughout the snow-covered season (November through March). Fluxes decreased to a low level of 3–16 mg CH4 m−2 d−1 in late March, reflecting decreasing peat temperatures and (in 1989–90) increasing depth of frost in the peat. Winter fluxes calculated by integration for an open poor fen, an open bog, a forested bog hollow, and a hummock site in the forested bog averaged 49, 12, 13, and 5 mg m−2 d−1, respectively, in 1989–1990 (the year most measurements were made). These comprised 11%, 4%, 15%, and 21% of total annual flux.

Methane flux from Minnesota Peatlands

Northern (&gt;40°N) wetlands have been suggested as the largest natural source of methane (CH4) to the troposphere. To refine our estimates of source strengths from this region and to investigate climatic controls on the process, fluxes were measured from a variety of Minnesota peatlands during May, June, and August 1986. Sites included forested and unforested ombrotrophic bogs and minerotrophic fens in and near the U.S. Department of Agriculture Marcell Experimental Forest and the Red Lake peatlands. Late spring and summer fluxes ranged from 11 to 866 mg CH4 m−2 d−1, averaging 207 mg CH4 m−2 d−1 overall. At Marcell Forest, forested bogs and fen sites had lower fluxes (averages of 77 ± 21 mg CH4 m−2 d−1 and 142 ± 19 mg CH4 m−2 d−1) than open bogs (average of 294 ± 30 mg CH4 m−2 d−1). In the Red Lake peatland, circumneutral fens, with standing water above the peat surface, produced more methane than acid bog sites in which the water table was beneath the moss surface (325 ± 31 and 102 ± 13 mg CH4 m−2 d−1, respectively). Peat temperature was an important control. Methane flux increased in response to increasing soil temperature. For example, the open bog in the Marcell Forest with the highest CH4 flux exhibited a 74‐fold increase in flux over a three‐fold increase in temperature. We estimate that the methane flux from all peatlands north of 40° may be on the order of 70 to 90 Tg/yr though estimates of this sort are plagued by uncertainties in the areal extent of peatlands, length of the CH4 producing season, and the spatial and temporal variability of the flux.

Bogs as Beaver Habitat in North-Central Minnesota

In a 100-km2 area surveyed for beaver activity, one-third of the active colonies were in bogs and two-thirds were in lakes and rivers. From 1979 to 1981, the percent of colonies in bogs increased from 29% to 36%. Of 481 bogs in the study area, 200 (42%) had current or previous history of beaver activity (colony sites and work areas). At 101 bogs (21 %), dam construction in seepage zones created flowages. Most impoundments were built at minerotrophic sites. Moats were used at 99 bogs (21 %), where beavers preferred sedge-moss cover and avoided tall shrub and wooded cover. The results show that bogs are suitable habitat for beavers. INTRODUCTION The suitability of lake and stream habitat for beavers (Castor canadensis Kuhl.) has been well-studied (Retzer et al., 1956; Northcott, 1964, Tufts, 1967). Although beavers frequently colonize lakes and streams that are adjacent to bog areas, few researchers have mentioned that beavers are capable of colonizing bogs that lack open water. Northcott (1964) reported a single instance of a colony in a closed bog, but reference by others to similar phenomena has been anecdotal (Schwintzer and Williams, 1974; Buell and Buell, 1975; Worley, 1981). Marshall and Miguelle (1978) in their review of wildlife on Minnesota peatlands mentioned only that beavers colonize bogs that have existing open water. Although there are few accounts of beavers colonizing bogs, colonies in bogs are locally common in N-central Minnesota, and in some areas they are more prevalent than in lakes or streams. Beavers are well-adapted to aquatic habitats, but their ability to create open water in closed bogs is an unusual situation that may involve specialized behavior. The purpose of this paper is to document the extent of bog use by beavers in a section of Cass Co., Minnesota, and to determine what biological and physical features of bogs are associated with use by beavers. METHODS AND PROCEDURE The 100-km2 study area, located E of Backus in Cass Co., included most of Deerfield Township (T139N R31W) and a few adjacent sections of Hiram, Bull Moose, Powers and Birch Lake townships. The topography is characterized by a rugged terminal moraine belt with ca. 250 small lakes and 500 bogs in depressions between hills. The upland forest is predominantly paper birch (Betula papyrifera) and trembling aspen (Populus trernuloides). I used aerial photographs to detect the 512 bogs larger than 0.12 ha in the study area. Bog areas adjacent to lakes and streams were not included unless the connection was minimal and the bog occupied a separate basin. In the field, 481 bogs were evaluated for the presence of beaver activity and classified according to predominant cover type as sedge-moss, tall shrub or wooded bog. Sedge-moss bogs were characterized by floating mats of sedges (Carex) and/or Spagnum moss and ericaceous shrubs covering most of the basin. Tall shrub bogs were dominated by alder (Alnus rugosa) (the brushy fen of Boelter and Verry, 1977), or by Betula pumila, Larix laricina and Picea mariana -the early seral stages of bog forest on ombrotrophic sites. Wooded bogs were dominated by L. laricina, P mariana and occasionally by trees from later stages. Most bogs in the study area were less than 3 ha and had either monotypic vegetation or predominant cover types that were easily recognizable. Aerial and ground surveys were used to detect both colony sites and work areas at 'Present address: Department of Botany, Louisiana State University, Baton Rouge 70803.