Patterned Fens of Western Labrador and Adjacent Quebec: Phytosociology, Water Chemistry, Landform Features, and Dynamics of Surface Patterns1

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Patterned fens of western Labrador and adjacent Quebec: phytosociology, water chemistry, landform features, and dynamics of surface patterns1

DAVIDR. FOSTER Harvard Forest, Harvard University, Petersham, MA 01366, U.S.A. GEORGEA. KING U.S. Environrnental Protection Agency, Corvallis Environmental Research Laboratory, 200 SW 35th St., Corvallis, OR 97333, U.S.A.

AND MARYV. SANTELMANN Department of General Science, Oregon State University, 355 Weniger Hall, Corvallis, OR 97331, U.S.A. Received June 30, 1987

FOSTER,D. R., KING,G. A,, and SANTELMANN,M. V. 1988. Patterned fens of western Labrador and adjacent Quebec: phytosociology, water chemistry, landform features, and dynamics of surface patterns. Can. J. Bot. 66: 2402-2418. The landforms, vegetation, water chemistry, and stratigraphy of four patterned fens (aapamires) in western Labrador and adjacent Quebec are described in a study investigating the origin and characteristics of surface patterns on northern peatlands. Phytosociological analysis by the relev6 approach, in conjunction with analysis by TWINSPAN,is used to describe 11 floristic noda. The vegetational patterns are largely controlled by depth to the water table. Mire landforms discussed in detail include ice-push ridges, flarks and pools, peat ridges, and mire-margin hummocks. Water chemistry is typical of minerotrophic conditions, with pH ranging from 4.4 to 6.7 and calcium concentrations from 20 to 430 pequiv. L-'. The water chemistry, vegetation, and landforms on the mires are compared with other studies from Labrador and circumboreal regions. Stratigraphic results and field observations support the theory that surface patterns on the mire develop slowly through the interplay of biological and hydrological processes, specifically differential rate of peat accumulation controlled by vegetation type and depth to water table. Pool formation apparently involves four steps: (i)gradual differentiation of shallow flarks on previously undifferentiated mire surface; (ii)expansion and deepening of flarks and development of ridges due to differential peat accumulation; (iii)degradation of flark vegetation into mud bottoms and open-water pools; and (iv)coalescence, continued expansion, and deepening of open-water areas. Hydrological controls over the rate and extent of pool formation are discussed as a probable explanation of the geographical distribution of patterned mires.

FOSTER,D. R., KING,G. A., et SANTELMANN,M. V. 1988. Patterned fens of western Labrador and adjacent Quebec:

.phytosociology, . water chemistry, landform features, and dynamics of surface patterns. Can. J. Bot. 66 : 2402-2418. La topographie, la vkgitation, la chimie de l'eau et la stratigraphie de quatre plaines marCcageuses B reliefs dans l'ouest du Labrador et les rkgions frontalikres du Quebec sont dCcrites dans une Ctude dont le but Ctait d'Ctablir l'origine et les caractiris- For personal use only. tiques des motifs superficiels des tourbikres septentrionales. Des analyses phytosociologiques B l'aide de relevCs, conjointe- ment avec des analyses par TWINSPAN,ont kt6 utiliskes pour dCcrire 11 nodus floristiques. Les modkles de vCgCtation sont en grande partie dCterminCs par la profondeur du niveau hydrostatique. Le relief de certains bourbiers est discutC en dktail, dont des sillons formks par des banquises, des flaques et des mares, des sillons de tourbes et des buttes du pourtour des bourbiers. La chimie de l'eau est typiquement celle des conditions minCrotrophes, le pH allant de 4,6 B 6,7 et les concentrations de calcium de 20-430 pequiv. L-I. La chimie de l'eau, la vCgCtation et le relief des bourbiers sont cornparks B d'autres Ctudes faites au Labrador et dans les rkgions circumborCales. Les donnkes stratigraphiques et les observations faltes sur le terrain soutiennent la thCorie que les motifs superficiels des bourbiers sont ClaborCs lentement B la suite d'effets d'accumulation de tourbe qui est dCpendante du type de vigCtation et de la profondeur du niveau hydrostatique. La formation de mares semble indiquer quatre Ctapes: (i)la diffkrenciation graduelle de flaques peu profondes B la surface, jusqu'alors indiffCrenciCe, des bourbiers; (ii) l'expansion et une augmentation de la profondeur des flaques et le dCveloppement de billons B la suite d'accumulation prkfkrentielle de tourbe; (iii)la dkgradation de la vCgktation des flaques en fonds du boue et en Ctangs; et (iv)la coalescence, l'expansion continue et l'augmentation de la profondeur des secteurs oh skjourne l'eau. Les effets hydrologiques sur le taux et 1'Ctendue de la formation des mares sont proposCs comme une explication probable de la distribution gCogra- phique des bourbiers B reliefs. [Traduit par la revue]

Introduction south. Surface features such as hummocks, hollows, mud Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 Boreal and subarctic patterned mires are quite distinct from bottoms, and open-water pools develop as secondary features the kettlehole "bogs" and swamps found to the south in the on many northern mires and form intricate and prominent temperate zone, both in their developmental history and in patterns (Sjors 1948). In areas of great moisture surplus, pools their surface landforms. As a result of moisture surplus and may develop to cover broad areas on both ombrotrophic raised bogs and minerotrophic fens (Sjors 1963; Foster and Glaser cool temperatures in northern regions, peat accumulation may be initiated across broad upland regions (paludification; cf. 1986). Furthermore, the developmental sequence of northern Malmstrom 1932) in addition to mire formation through lake- patterned mires is opposite of that described for most filling or terrestrialization, processes that predominate in the temperate wetlands: peatland formation is followed by gradual development and expansion of open-water pools in contrast to the infilling of water-bodies and conversionif open-water sites 'This paper is contribution No. 342 from the Limnological to wetland (Auer 1930; Heinselman 1963). However, despite Research Center, University of Minnesota, Minneapolis, MN. the large areal coverage of boreal and subarctic peatlands,

Printed in Canada i Imprim6 au Canada FOSTER ET AL.

LABRADOR SEA

QUEBEC

FIG. 1. Map of southern Labrador and adjacent Quebec showing the location of the study sites. Inset shows the location of the study area in relationship to eastern Canada. Sch., Schefferville, Quebec.

For personal use only. there are few studies that critically investigate the possible TABLE1. Location of study sites mechanisms by which the distinctive surface patterns develop (Moore 1982; Gorham 1982). Name (unofficial) Location Elevation (m) Patterned mires are especially prominent landforms in Contrast fen 55"20f50"N, 67"26'54"W 457 Labrador (Allington 1961) and recently a description of the Rose fen 55"14'00"N, 67"15'16"W 472 vegetation, landforms, and stratigraphy of a patterned fen Shovel fen 52"40121"N, 65"55'00"W 549 complex in southeastern Labrador was undertaken (Foster and T-Bone fen 54"33'21 "N, 65"49'39"W 503 King 1984). This study resulted in the formulation of a hypoth- Leech fen 53"09'50"N, 58'26'08"W 370 esis concerning the development of surface patterns on north- NOTE: em mires (Foster et al. 1983). Since then, analyses of the Data for Leech fen an: from Foster and King (1984). zonation, floristics, and recent history of raised bogs in southeastern Labrador have been completed (Glaser and Foster of the predictions of the proposed hypothesis of pattern for- 1984; Foster and Glaser 1986; Foster et al. 1988) and studies mation. of long stratigraphic sections (P. Glaser, unpublished data) and near surface peat chemistry (P. Pakarinen and E. Gorham, Study area unpublished data) have been initiated. Four fens were chosen for study (Fig. 1, Table 1). Shovel and With the exception of a photo-reconnaissance study by T-Bone fens are located on the Lake Plateau that forms a crescent-

Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 Allington (1961) and preliminary descriptions by Thom shaped, drift-covered belt in the south central portion of the (1972) and E. Thompson (unpublished data) in Schefferville, Labrador-Ungava peninsula (Hare 1959; Greene 1974). Highly Quebec, little attention has been directed towards the mires in metamorphosed rocks, primarily quartzofeldspathic gneisses, under- western Labrador along the Labrador-Quebec border. To lie both Shovel and T-Bone fens (Greene 1974). Contrast and Rose address this gap in the literature and to examine the applica- fens are located in the Labrador Hills, a region with ridge and valley bility of hypotheses concerning mire formation and surface relief extending south from Ungava Bay to Schefferville and va~ying in width from 25 to 80 krn. The ridges trend north-northwest, and pattern dynamics developed for southeastern Labrador fens, relief averages about 90 m. The bedrock underlying Rose and four fens in western Labrador and adjacent Quebec were Contrast fens is distinctly different from that underlying Shovel and chosen for study. Our objectives were to describe in detail the T-Bone fens. Rose fen overlies dolomite with sandstone and silicate- landforms, water chemistry, and phytosociology of the fens, carbonate ironstone abundant locally. Half of Contrast fen is under- and to examine stratigraphically the development of surface lain by dolomite, the other half by slate (Dimroth 1978). patterns. The results of this study are then compared with some Patterned fens dominate the extensive lowlands surrounding Shovel 2404 CAN. 1. BOT. VOL. 66, 1988

fen. The uplands are predominantly covered by closed forests of from the other noda by differential species, i.e., individual species or Picea mariana with a ground cover of Pleurozium schreberi. The groups of species that have an affinity to particular vegetation types other three fens, which are farther north, are surrounded primarily by (Hill 1979). Differential species generally have an intermediate fre- mineral uplands forested with open lichen woodlands of Picea mari- quency of occurrence in the table and are present in greater than 50% ana and Cladonia stellaris. hrix laricina is also common in these of the relevCs that they characterize and less than 10% of the relevCs woodlands and Picea glauca is present. in other groups (Mueller-Dombois and Ellenberg 1974). The climate of the study region is cold and moist. The three north- Water samples for chemical analysis were collected from flarks and em sites have a mean July temperature of about 12.6"C, a mean pools where surface water was present; only relevCs from these land- annual temperature of about -4.8"C (Environment Canada 1982a), forms (noda V, VII-XI) have water chemistry data. The pH of and an annual precipitation between 650 and 770 mm (Environment surface water was measured in the field using a Radiometer M29b Canada 1984). Shovel fen is slightly warmer and wetter, with a mean field pH meter (standardized to buffers of pH 4.0 and 7.0). July temperature of 13S°C, a mean annual temperature of -3.g°C, Samples for cation analysis were collected in acid-washed 60-mL and 895 mm of precipitation per year (Environment Canada 1982b). polyethylene bottles (rinsed three times with water from the sampling At all four sites summer precipitation is slightly higher than winter site), and refrigerated at 4OC upon return from the field. One week precipitation and the growing season is less than 140 days long before filtration through acid-washed Gelman AIE glass-fiber filters, (Department of Energy, Mines and Resources 1973). samples were acidified with Ultrex ultrapure nitric acid (1% by volume). Filters were presoaked for 1 h in 10% HC1; then each was rinsed with 250 mL of 10% HCl followed by 1 L of distilled deion- Methods ized water and two rinses with 5- 10 mL of sample. Each filter was Sites were selected from aerial photographs and topographic maps used once. A blank of distilled deionized water was filtered after based on their geographic distribution, the prominence and pattern of every 12 samples; blank values (uniformly low) were subtracted from surface features, and accessibility by float plane via nearby lakes. values obtained for samples. Each fen was mapped from aerial photographs taken at 1000-m alti- Samples were analyzed for Ca, Mg, and Na on a Varian Techtron tude with scale provided by ground truthing. Cross-sectional profiles 1475 atomic absorption spectrophotometer with an air-acetylene of the peat deposits were made for selected sites using a surveyor's flame. Because K concentrations were below the detection limit in 10 transit and thin metal rods. Measurements were taken at 20-m or of a subset of 12 samples and <4 pequiv. L-' in the other 2, the closer intervals along designated transects. remaining samples were not analyzed for K. Lanthanum was added A piston corer 10 cm in diameter, with serrated teeth for cutting for Ca and Mg analysis to reduce interference. Although 30 of 37 through wood and rhizomes, was used for taking long peat cores samples were not visibly colored, the method of standard additions (Wright et al. 1983). Monoliths of surface peats (approximately 10 x was used (Varion Techtron user manual) to minimize interference 10 x 50 cm) were cut with a serrated knife and loose sediments were from the organic matrix. Each set of major cation analyses was cali- sampled with a standard 5-cm piston corer. All cores and monoliths brated with EPA standards; results for the standards were accurate were described in the field, wrapped in plastic wrap and plastic pipe, within limits of instrument precision. and transported to the laboratory for further analysis. Radiocarbon dates were provided by the Radiocarbon Laboratory, University of Results Wisconsin, Madison, WI. The vegetation was sampled using the relevC method to produce a Water chemistry

For personal use only. descriptive phytosociological classification (Mueller-Dombois and The chemistry of surface water from T-Bone, Contrast, and Ellenberg 1974). This semiquantitative approach is best suited for an part of Shovel fens resembles that of poor fens (Sjors 1963) in initial survey of a little-known vegetation as it (i) provides a descrip- cation concentrations, although these waters are slightly less tion and classification of the common vegetation; (ii) enables compar- acid than those of the classical poor fens. Rose fen and the ison with vegetational data from other regions; (iii) establishes an central pool area of Shovel fen are more cation rich (Table 2, initial, albeit general, framework for considering the ecological and Fig. 2). Fourteen samples from Shovel fen were analyzed to spatial relationships of species and vegetation types; (iv) reveals effi- compare water chemistry in different parts of the mire. The ciently the principal direction of variation within the vegetational water flowing into Shovel fen from adjoining mires is acid (pH complex; and (v) permits the generation of working hypotheses 4.4-4.7, Fig. 3) and low in Ca (about 20 pequiv. L-I). In concerning the role of environmental factors influencing the vegetation (Birks 1973). pools along transect A, pH rises to 5.1 -5.5 and Ca to 35- All species were recorded in plots subjectively placed in homo- 80 pequiv. L-l; in the large flarks and pools along the central genous stands. Standard plots were 3 X 3 m but this was modified as axis of the mire, pH is about 6.0-6.7, with Ca concentrations necessary in size and shape to accommodate irregular or small land- of 75-85 pequiv. L-I (transect B, Fig. 3). These central pools forms. No attempt was made to assess a minimal area curve as this are less acid and have higher Ca concentrations than adjacent has been considered by Poore (1955b), Dahl(1956), and Birks (1973) Shovel lake (pH 5.7, Ca = 70 pequiv. L-I). The larger pools to be of less value than experience and general assessment of the scale such as Trout pool, Push pool, and perhaps locations in Lac of vegetation. Visual estimates of cover were assigned on the 10-point d'Isles (Fig. 3) have mineral bottoms of primarily silt and sand Domin scale (Mueller-Dombois and Ellenberg 1974). Voucher (see Surface features). From the orientation of the pool Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 samples of lichens and vascular plants are deposited in the herbarium patterns (cf. Ivanov 1981) and changes in the chemistry of sur- of the University of Minnesota (MIN). Duplicates of all bryophyte collections are stored in the National Herbarium, National Museum of face water, we infer that relatively dilute water flowing onto Canada (CAN). Nomenclature follows Femald (1970) for vascular Shovel fen from adjoining peatlands to the west flows into species, Stotler and Crandall-Stotler (1977) for liverworts, Ireland Trout pond and Lac d'Isles, where it mixes with mineral- et al. (1980) for mosses, and Hale and Culberson (1970) for lichens enriched soligenous water before flowing down the axis of the except Cladonia, which follows Ahti (1961). mire (cf. Sjijrs 1948; Du Rietz 1950). A phytosociological table was compiled using the FORTRAN program TWINSPAN (Hill 1979; Gauch 1982). The final groupings of Vegetation relevCs, characterized solely on similarities in floristic composition, Based on the TWINSPAN classification, the relevt data form abstract plant communities or noda (sensu Poore 1955~).Each has been separated into 11 noda in a phytosociological table nodum is characterized by dominant species, which have the greatest (Table 3; cf. Birks 1973; Mueller-Dombois and Ellenberg cover-abundance within a layer of vegetation, and is distinguished 1974). The steps in the hierarchical dichotomization of the FOSTER ET AL. 2405

Dicranum undulatum, Trientalis borealis, and Vaccinium myrtilloides. Stunted Picea and Larix are present in low abundance with the shrubs Ledum groenlandicum, Chamaedaphne calyculata, and Vaccinium myrtilloides. Carex trisperma is the prominent herb and Sphagnum russowii dominates the ground layer with Pleurozium schreberi. This assemblage occurs at Rose and Shovel fens on forested peat ridges 20 -40 cm above the water table. Nodum IV: Carex exilis - Epilobium palustre assemblage Juniperus communis, Sarracenia purpurea, Epilobium palustre, and Carex exilis are differential species of this assemblage. Chamaedaphne calyculata and Juniperus form an interrupted shrub cover over Carex exilis, Scirpus cespitosus, and Potentilla palustris in the herbaceous layer and the Sphag- num russowii ground cover. This assemblage dominates low sedge ridges on all of the fens studied and occurs on lawns (sensu Sjors 1948) with nodum V. The ridges are generally quite firm and elevated 15-25 cm above the water in the adjoining mud bottoms and 200 400 pools. Ca concn. (pequiv. c') Nodum V: Scirpus cespitosus - Sphagnum angustifolium - Smilacina trifolia assemblage FIG. 2. The relationship between Ca concentrations (pequiv. L-') This nodum is characterized by a continuous ground cover of and pH for Contrast (0),Rose (A), T-Bone (@), and Shovel Carex exilis, Scirpus cespitosus, and Sphagnum angustifolium, (A) fens. scattered Smilacina trifolia, Aulacomnium palustre, and Carex aquatilis, and low shrub cover of Chamaedaphne calyculata. stands are illustrated in the dendrogram from the TWINSPAN Differential species include Rubus acaulis, Mitella nuda, and output (Fig. 4). Each nodum is described below in terms of Viola renifolia. This assemblage has the greatest areal extent differential and dominant species and general habitat char- on the mires and generally occurs on firm sedge peat 10- acteristics. 20 cm above the water table. In particular, it occupies low Nodum I: Picea mariana - Abies balsamea - Ptilium crista- ridges and extensive lawns and blankets unpattemed mire castrensis assemblage slopes. This assemblage is differentiated by Abies balsamea, Ptil- Nodum VI: Carex oligosperma - Scirpus cespitosus

For personal use only. ium crista-castrensis, and Ptilidium ciliare. Picea mariana assemblage forms a continuous overstory, Ledum groenlandicum predomi- This assemblage occupies very low ridges, which, as a result nates in the discontinuous shrub layer, Rubus chamaemorus in of their undulating surface near the water table, present a the field layer, and Pleurozium schreberi and Sphagnum rus- diversity of microsites that accommodate a variety of species sowii form the ground cover. Paludified forests composed of with different moisture requirements. Carex oligosperma is the this assemblage bordered all the fens. The soil organic layer only differential species and it occupies low spots between ranges from 20 to 40 cm in depth. small hummocks of Scirpus cespitosus. Drepanocladus exan- Nodum II: Sphagnum fuscum - Cladonia assemblage nulatus and Carex limosa also occupy moist depressions, This assemblage is differentiated by a group of oligotrophic whereas Betula michauxii, Chamadaphne caly culata , and cryptogams including Sphagnumfuscum, Mylia anomala, and Andromeda glaucophylla are scattered throughout. Cetraria Cladonia stellaris. The vascular species Empetrum nigrum and islandica, Picea mariana, and Larix laricina are generally Myrica gale are most common in this assemblage. Picea situated on elevated hummocks. mariana forms a stunted and discontinuous canopy and Nodum VII: Betula michauxii - Chamaedaphne calyculata Chamaedaphe calyculata and Vaccinium uliginosum are the assemblage predominant shrubs. The ground cover is composed of Sphag- Along the upslope margins of ridges, this assemblage occu- num nemoreum, Sphagnum fuscum, Sphagnum russowii, pies a fringe that extends from the peat bank 30-50 cm into Pleurozium schreberi, and Cladonia rangiferina. the open water pool. Shrubs include the dominant species

Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 This assemblage occurs most frequently on mire margin Betula michauxii and the more-ubiquitous Chamadaphne caly- hummocks, elevated ridges, and isolated hummocks in the culata, Andromeda glaucophylla, and Kalmia polifolia. Herbs mire center. On these sites the peat surface is raised 30-45 cm include Carex limosa and Menyanthes trifoliata. Most of the above the water table in the adjoining pools and flarks and plants are rooted in the firm peat banks of ridges and extend by thereby is largely isolated from soligenous water flow and con- rhizomes or shoots out into the water. Menyanthes, however, sequently is nutrient poor (Sjors 1946, 1948). Myrica gale, a is occasionally rooted in the bottom of pools. minerotrophic indicator (Sjors 1948), is restricted to the lower Noda VIII-XI: Flark and mud bottom assemblages flanks of the ridges. These four noda share many of the same species (e.g., Nodum III: Vaccinium myrtilloides - Carex pauciflora Scirpus cespitosus, Carex oligosperma, Carex limosa, Drep- assemblage anocladus exannulatus, and Menyanthes trifoliata) and occupy Differential species in this nodum include Carexpauciflora, progressively moister phases of flark vegetation (sensu Sjors Cladonia comuta, Clintonia borealis, Dicranum fuscescens, 1948). 2406 CAN. 1. BOT. VOL. 66. 1988

FIG. 3. Map of Shovel fen. Open-water pools are black, flarks are enclosed, minor patterning is indicated by wavy lines, and uplands are shaded. Landforms are labelled as TP, Trout pool; LI, Lac d'lsles; and PP, Push pool. The ends of cross-sectional transects are labelled by their corresponding letter and the pH of sampled flarks and pools is indicated. For personal use only. TABLE2. Concentrations of Ca, Mg, and Na (pequiv. L") and pH from surface waters of fens in western Labrador and adjacent Quebec

Rose fen (n = 12) 5.9 183 162 7.0 (5.5 -6.4) (65-430) (58-345) (3- 10) 54 Contrast fen (n = 6) 5.5 52 40 5.0 (5.3-5.7) (35-75) (25-74) (1-12) 51 Shovel fen (n = 14) RelevCs (n = 10) 4.9 56 23 6.5 (4.4-5.9) (20-195) (8-58) (1-18) 51 Central pools (n = 4) 6.3 82 39 16.4 (6.0-6.7) (75-85) (25-58) (15-19) 52 T-Bone fen (n = 4) 5.8 65 37 15 (5.5-6.0) (45-85) (25-49) (14-17) 51 Leech fen (n = 14) 4.7-5.5 16 16 nd nd

NOTE: Values are avenges of all samples analyzed and the nnge is given in parentheses. Analytical precisions of the data

Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 are as follows: Ca, k 10 pequiv. L-I; Mg, f8 pequiv. L-I; Na, +5 pequiv. L-'; pH, k0.1 unit. Data from Leech fen in southeastern Labndor (Foster and King 1984) are shown for comparison. nd, not determined.

In nodum VIII, Carex limosa, Carex oligosperma, and Scir- trifoliata form a continuous surface mat that was seen to fluc- pus cespitosus occur in very shallow depressions and incipient tuate in elevation with changes in the local water table after a flarks. The scattered hummocks of Scirpus cespitosus repre- rainfall. Widely scattered hummocks of Scirpus are the only sent the degradation of a previously continuous Scirpus lawn remnants of the previous Scirpus lawn represented by under- as indicated by the firm, underlying sedge peat. Menyanthes lying Scirpus peat. and Drepanocladus occupy depressions with Carex limosa. On progressively moister sites (i.e., sites lower relative to Carex limosa has a continuous cover in relevis from nodum the water table and with a greater extent of open water) the IX and is associated with Drepanocladus exannulatus and Carex limosa mat is locally moribund and the exposed mud Menyanthes. The rhizomes of carex limosa and Menyanthes bottoms that occur in the holes in the mat are covered with Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 For personal use only. FOSTER ET AL. 2407

Relev6 Nodum ridge forms the present lake shore. The elevational difference between the inner and outer ridges is slight, suggesting that only a minor change in water level was required for their respective formation. The ice-push ridges impede the flow of water off the mire slopes and into the lake as documented by the deep water that ponds up after rainstorms. This restricted drainage may be an important triggering mechanism for the initiation of peat accumulation and mire formation at the base of a slope. In addition, the elevated ridges and especially the stand of con- ifers that they support (cf. Fig. 5) are an effective windbreak and cause the accumulation of deep snow at the base of the mire (D. R. Foster, personal observation). This persistent snow cover delays spring leaf-out of vascular species and increases the waterlogging of the site. Flarks and pools Flarks and pools are moist depressions that develop as secondary features on the peat surface (Sjors 1948). Flark depressions are filled with a generally continuous lawn or carpet vegetation. On the mires studied the most common flark vegetation includes, in order of decreasing height above the water table, assemblages dominated by Scirpus cespitosus and Sphagnum angustifolium (nodum VI, Table 3), Carex limosa (nodum VIII), Carex limosa and Drepanocladus exannulatus (nodum IX), and Cladopodiella fluitans (nodum X). FIG.4. Dendrogram from the TWINSPAN output showing the divi- In contrast, pools are deeper depressions that are generally sions resulting in the assignment of stands to particular noda, which water-filled throughout the year and lack a continuous vegeta- are identified by roman numerals. tion cover except a bottom film of algae, scattered Menyanthes trifoliata, Sparganium angustifolium, and Carex rostrata, or Cladopodiellafluitans, forming nodum X. Menyanthes trifoli- occasional sedge or Sphagnum islets. The pools on Shovel fen ata occurs in all plots and Carex oligosperma and Carex ros- range from 10 to 175 cm in depth and generally are completely trata are locally important in deep water. On Rose fen, T-Bone contained in peat (Fig. 6). However, in larger pools the under- fen, and Contrast fen the mud bottoms also supported Carex lying peat may be quite thin or even absent, as in Trout and Push pools (Fig. 6). In Trout pool, peat detritus, silt, and sand For personal use only. livida, Drosera anglica, and Juncus stygius (nodum XI). In form the bottom with only a narrow band of peat pinching off broad, open areas in which the Carex limosa mat has broken - - and pools of open water occur, the very sparse vegetation con- from the pool edges. sists of ~arexrostrata,C. oligosper~a,~~ar~anium angusti- The orientation and shape of flarks and pools is controlled by folium, and Menyanthes trifoliata. the topography of the peat surface and local hydrology (Ivanov 1981) and this is apparent along the central axis of Shovel fen Surface features (Fig. 3). Broad, open-water pools occur at the top of the The following descriptions are compiled from observations north-south water divide, which receives drainage from east on all of the mires investigated. The more detailed descriptions and west slopes, and at the southern base of the mire; both are are from Shovel fen (Fig. 3), which was studied most inten- locations where the slope levels off and lateral water move- sively. The major landforms characteristic of all the mires are ment, therefore, would be slow. On the intervening steep slope described: ice-push ridges, strings, flarks, pools, and mire the elongate flarks are shallow and are arranged in a gentle margin hummocks. Prominent noda on each landform are also stair-step pattern. Narrow and elongate depressions also occur identified. on steeply sloping surfaces such as along transect A (Figs. 3 and 7). This stepwise pattern is independent of the underlying Ice-push ridges substratum (Fig. 7). Although ice rampart ridges, which separate mires from an The morphology of flarks and pools has been shown to be adjoining lake, are not a feature of all Labrador mires, they relatively dynamic (cf. Masing 1982; Mets 1982; Boatman

Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 occur frequently. The ice-push ridges are narrow, generally 1983). For example, the shape of these many features may continuous, and range from 0.5 to 3.0 m in height. They are change as adjoining flarks or pools coalesce (Sakaguchi 1976; composed of glacial till, sand, and incorporated organic Kuznetsov 1986). On the western Labrador fens such a process matter. The formation of the ridges is evidently a result of ice is suggested by the broad pools along the north -south axis of expansion in the late winter and spring and ice-rafting during Shovel fen (Fig. 3), where the shape of flarks indicate that they the breakup of lake ice (Raup 1931). have merged both along and up the slope and small pools have Shovel fen, for example, is bounded by one northern and joined in the large central flark (also cf. Fig. 8). two southern ice-push ridges (Fig. 5) that are separated by a 200-m wide strip of wet forest overlying 50- 100 cm of woody Stratigraphy of flarks and pools peat (Fig. 3 and 6). The inner and more southern ridge (Fig. 3) The stratigraphy beneath flarks and pools varies with water -is evidently a relict, probably formed when the lake level was depth and is described in part by a series of 12 cores and 6 slightly higher than it is now, and then stranded. The outer monoliths taken along a transect from a robust ridge across a CAN. J. BOT. VOL. 66, 1988 For personal use only.

FIG.5. Oblique photograph of the north end of Shovel fen, including Trout pool. Note the forested ice-push ridge along the lake shore and the remnant sedge mat fringing Trout pool. See Fig. 3 for scale.

firm lawn and mud bottom to an open pool (Fig. 9). Ridges mud bottoms covered by Cladopodiella puitans, Gymnocolea adjoining flarks are underlain by shrub peat that grades into inpata, and scattered vascular species (nodum X, Table 3). sedge peat below 20-50 cm, thereby suggesting differen- The open areas are underlain by Carex limosa and Menyanthes

Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 tiation from a sedge lawn. The shallowest flarks are covered rhizomes and other remains of plants previously forming the with a lawn vegetation (nodum VIII, Table 3, Fig. 8) under- mat. In open water the residual mat breaks up and the flark sur- lain by firm sedge (Scirpus cespitosus, Carex exilis) or face is composed of discontinuous mat, mud bottoms, and sedge-shrub peat. In moister portions of the same flark, the open holes that extend 15-35 cm below the surface through surface vegetation forms a rhizomatous mat of Carex limosa peat detritus to the firm peat below (Fig. 10). Breaks in the mat and Menyanthes (nodum IX, Fig. 10) that floats on a layer of begin to dominate the flarks until open-water pools gradually water-saturated peat detritus overlying firm peat (Fig. 9). The develop (Fig. 11). The open-water portions of the depressions buoyancy of the mat and the flocculent detritus allows the mat support little vegetation except scattered Menyanthes and to fluctuate in elevation with small changes in water table Carex rostrata (nodum XI, Table 3), unlike the adjoining height as documented following rainstorms. flarks. In deeper depressions the sedge or Sphagnum vegetation of Long cores (a minimum of six from each pool) taken from a the mat becomes discontinuous and is separated by bare peat or small rubber raft show that deep pools have a bottom corn- FOSTER ET AL.

Lac d'lsles Trout Pool

50cd Peat Frozen peat Mineral soil 50 rn FIG.6. Cross-section of Shovel fen along transect B. For personal use only.

u peat Minerol soil 50 m FIG.7. Cross-section of Shovel fen along transect A. The width of the ice-push ridge is slightly exaggerated. Location of cores taken from a ridge and adjacent flark to describe pattern formation is indicated by an arrow.

posed of peat detritus overlying firm peat, or in the case of the it must be concluded that the dates provide a maximum age for largest pools, silt and sand (Figs. 6 and 12). Radiocarbon dates the origin of the features. obtained from wood embedded in the bottom of Trout pool and peat detritus from the bottom of a large pool in Rose fen (here- Ridges and mire margin hummocks

Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 after called Rose fen pool) are 3210 & 80 years BP and Ridges, which are elevated -sublinear landforms, are elon- 3430 k 70 years BP, respectively (Table 4). The peat 80 cm gated along the slope contours and dam the lower margin of below the peat detritus - water interface in Rose fen pool is pools and flarks. On the mires investigated, the ridges range dated at 4510 + 80 years BP. from <1 to 5 m in width, from 10 to 50 cm in height The radiocarbon dates document the absence of accumula- above the surrounding mire surface, and extend up to 35 m in tion or sedimentation in the pools over the last few thousand length. The ridges frequently interconnect across the slope years but do not exclude the possibility of active removal of but may be broken in spots where two adjacent depressions peat or peat detritus through degradation and erosion (cf. have coalesced. Prominent plant communities, in order of Kuznetsov 1986; Foster and Fritz 1987). In addition, the dates increasing height above the water table include the Carex document that the flarks and pools are old and quite persistent oligosperma - Scirpus cespitosus assemblage (nodum VI), landforms (cf. Lundqvist 1951; Masing 1982). Recognizing the Scirpus cespitosus - Sphagnum angustifolium - Smila- the possibility of actual removal of peat from within the basin, cina trifolia assemblage (nodum V), the Carex exilis - Epi- 2410 CAN. J. BOT. VOL. 66. 1988

FIG. 8. Coalescence of two adjacent flarks through the degradation of the intervening ridge has left a string of Scirpus cespitosus islands mark- ing the former position of the ridge. Photograph from the southwest portion of Shovel fen. For personal use only.

lawn on firm peat stranded hummock mud bottom open pool

Woody peat Sedge peal Peat detritus Water

FIG. 9. Cross-section of a flark-pool complex based on shallow cores and peat monoliths showing the stratigraphy and approximate distribution of the species. Water depth increases from left to right. Location of cores and monoliths is indicated by arrowheads. Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 lobium palustris assemblage (nodum IV), the Vaccinium Fritz 1987). The vegetation is dominated by oligotrophic myrtilloides - Carex paucifora assemblage (nodum 111), and species (Sphagnum fuscum - Cladonia assemblage) and the the Sphagnum fuscum - Cladonia assemblage (nodum 11). hummocks are largely composed of Sphagnum fuscum and Mire margin hummocks (sensu Sjors 1948) are substantially S. nemoreum peat. elevated peat accumulations that are generally oval to elongate in outline and range from a few meters to over 10 m in length. Stratigraphy of ridges Hummock height ranges from 25 to 75 cm above the surround- On Shovel fen two cores taken in a robust ridge occupied ing mire surface. Although generally located towards the by dense black spruce 5-m tall and the adjacent mat-covered periphery of mires where the influence of soligenous water is flark (Fig. 7) show that the stratigraphy below both features, lowest (Sjors 1948), the hummocks may extend onto the mire although identical over the lower 1.5 m, diverges at a point expanse in locations receiving little water flow (Foster and 60 cm from the peat surface (Fig. 13). Whereas the flark FOSTER ET AL. 2411

FIG. 10. Degradation of the Carex limosa mat results in the loss of the underlying floculent material and the formation of a 25 cm deep pool. Photograph taken at Shovel fen.

For personal use only. TABLE4. Radiocarbon dates from fens in western Labrador and adjacent Quebec

Radiocarbon date Location Depth Material (years BP) Laboratory No. Basal dates Shovel fen 136- 139 cm Sedge peat 5940 + 80 WIS-1527 Rose fen 143 - 148 cm Shrub peat 4120+80 WIS-1529 Rose fen 190 - 195 cm Sedge peat 4700 + 70 WIS-1531 Pool dates Shovel fen (Trout pool) Sediment Wood 3210+80 WIS-1526 surface Rose fen pool Sediment Peat detritus 3430 +70 WIS-1528 surface Rose fen pool 80 cm below Sedge peat 4510+80 WIS 1530 surface

Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 stratigraphy contains a sequence of sedge peat to peat detritus, of climate and local hydrology upon water residence time and the ridge stratigraphy progresses from sedge peat at 60 cm to rates of peat decomposition (and thus the concentrations of dis- Sphagnum and shrub peat, and then to an abrupt transition at solved organic acids). In the study area, cation concentrations 15 cm to an uppermost peat dominated by wood and macro- in precipitation are low (Munger and Eisenreich 1983). fossils of black spruce. Because most of these fen waters are not visibly colored, we infer that organic acid concentrations are probably low. The chemistry of surface water in the fens appears to be controlled Discussion by two factors: the composition of groundwater flowing into Water chemistry of western Labrador and Quebec fens the mire and the proportion of groundwater to more dilute The chemistry of surface water in the fens is influenced by waters from precipitation or surface runoff from adjacent inputs from groundwater and surface flow, the amount and mires. chemical composition of local precipitation, and by the effects The relative importance of groundwater contributions of 2412 CAN. I. BOT. VOL. 66, 1988 For personal use only.

FIG. 11. Lac d'Isles at Shovel fen. The lighter areas of Carex limosa carpet and small islands are remnants of the process of flark conversion to pool. Various stages in ridge degradation can be seen in the intricately patterned area above. See Fig. 3 for location and scale.

cations to the surface waters of the mire can be inferred by physiographic regions (T-Bone and Shovel) have a lower ratio examining the Ca:Mg ratio of waters in the four fens. At each than those in the Labrador Hills (Contrast and Rose). Regional of these fens, the Ca:Mg ratio, which is thought to be char- differences in Ca:Mg ratio would be expected, because the

Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 acteristic of the specific groundwater source, remains rela- groundwater in the various-regions is in contact with different tively constant despite changes in absolute concentration of Ca types of bedrock. Bedrock in the Labrador Hills consists of and Mg. The constant ratio indicates that changes in concentra- sedimentary and volcanic rocks (all of Rose fen and parts of tion are caused primarily by dilution with cation-poor water Contrast fen lie on dolomite, a calcium -magnesium carbonate (cf. Siege1 and Glaser 1987). For example, at Shovel fen the (Dimroth 1978)), whereas the bedrock in the Lake Plateau Ca:Mg ratio remains approximately 3: 1 despite a 10-fold range region (underlying Shovel and T-Bone fens) is primarily in element concentration (Fig. 14), as expected if soligenous quartzofeldspathic gneiss (Greene 1974) and contains less water rich in dissolved base cations mixes with varying propor- soluble magnesium. tions of more dilute water from precipitation or from surrounding mires. Vegetation and water chemistry This same pattern of constant Ca:Mg ratio is shown by the Despite differences in the chemistry of surface water, there other fens (Fig. 14), although mires in the Lake Plateau is much overlap in pH and element concentration in all the FOSTER ET AL.

Peat detritus/ 25 m [$;1 Peat silt (++I Mineral soil Peat detritus FIG. 12. Cross-section of Trout pool at Shovel fen.

0 crn

woody peat

Sedge mat

Peat detris

Sedge peat

Sphagnum peat

Mineral soil

For personal use only. Ca concn. (yequiv. L-I) *-87years BP FIG. 14. Plot of Mg concentration vs. Ca concentration (pequiv. L-') in the surficial waters of Contrast (O),Rose (A), FIG. 13. Peat stratigraphies from a shallow flark and adjacent T-Bone (a),and Shovel (A) fens. wooded ridge at Shovel fen. Location of cores is indicated on Fig. 7.

fens, especially in the pH ranges 5.0-6.0 and the Ca range of 1962). Cation sums (Ca + Mg + Na + K + H) for T-Bone 50-100 pequiv. L-' (Fig. 2). The only distinctly separate and Contrast fens and all but one sample from Shovel fen range nodum is nodum VIII whose waters are the least acid and from 60 to 160 pequiv. L-', whereas these sums range from richest in cations (average pH 6.2, mean Ca and Mg concen- 340 to 707 pequiv. L-' in the fens studied by Malmer and trations 256 and 222 pequiv. L-', respectively (Table 5)). Witting. RelevCs in nodum VIII have the highest cover-abundance of Much of the difference in surface water chemistry between Drepanocladus and lack Sphagnum cover. Noda IX-XI do Swedish and western Labrador and Quebec mires may be not show trends in water chemistry associated with cover- explained by differences in cations supplied by precipitation. abundance of particular species. Instead, water-chemistry data The Swedish mires are mostly within 50- 100 krn of the sea are similar for relevCs in these noda, which share many of the and receive precipitation enriched in Na and Mg by sea spray. Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 same species. Similar cation enrichment of surface waters by precipitation has been reported for Atlantic coastal bogs in North America Comparison with other mires (Gorham et al. 1985). Acidity and Ca concentrations in the surface waters of Calcium concentration in mire waters is negatively corre- T-Bone, Contrast, Shovel, and parts of Rose fens are similar to lated with acidity (Witting 1947, 1948; Gorham 1956). How- those reported for fens with Sphagnum papillosum and Sphag- ever, organic acids can influence this relationship (Gorham num recurvum in the English Lake District (Gorham and Pear- et al. 1985). For example, the surface waters of the poor fen sall 1956; Gorham 1956). All four western fens have higher Ca areas described by Glaser et al. (1981) in the Red Lake Peat- concentrations and higher pH than Leech fen in southeastern land of Minnesota (pH range 4.0-5.1, Ca concentrations Labrador (Table 2; Foster and King 1984). With the exception 2.2-4.3 mg/L) are more acid than those of Labrador and of Rose fen, these fen waters are generally more dilute than Quebec fens, although their Ca concentrations are higher, poor-fen waters in Sweden (Witting 1947, 1948; Malmer because colored organic acids in the waters of midcontinental 2414 CAN. J. BOT. 1

TABLE5. Chemistry of the subset of water samples that Pattern development were taken within relevis Based in part on earlier studies in Labrador (Foster and King Concn. (~equiv.L-') 1984; Foster and Glaser 1986; Foster et al. 1988) and northern Minnesota (Glaser et al. 1981 ; Glaser and Wheeler 1980) a RelevC No. Nodurn Fen pH Ca Mg Na hypothesis has been presented to explain the development of surface patterns on boreal peatlands (Foster et al. 1983). In 3 3 v this hypothesis pool formation is thought to involve three basic 29 v 28 v steps: (i) the gradual differentiation of depressions on a 54 VII previously undifferentiated mire surface; (ii) expansion and 55 VIII deepening of these depressions into flarks or shallow pools as 5 8 VIII a result of differential peat accumulation within and adjacent 56 VIII to the depressions and on the adjoining raised surfaces; and 57 VIII (iii) formation of large open-water pools through coalescence 62 IX of depressions, continued differences in peat accumulation, 61 IX and degradation of peat through decomposition and possible 60 IX erosion. 64 X Subsequent stratigraphic studies and field observations have 63 X sought to test this hypothesis by examining criteria consistent 66 X 7 1 XI with this proposed developmental pattern. Expected observa- 70 XI tions would include (i) secondary development of hollows and 68 XI pools on mires surfaces after considerable peat accumulation; (ii) stratigraphic sequences suggesting an upward change to NOTE: R, Rose; C, Contnst; S, Shovel; and T, T-Bone. To convert pequiv. L-' to rng L", divide the values given by the following numbers: progressively more mesic vegetation in peat cores taken in Ca, 49.90; Mg, 82.29: Na, 43.50. hollows or pools and to progressively more xeric assemblages in cores from adjacent ridges, with both types of landforms peatlands contribute significantly to their acidity. The waters originating from a similar vegetation type, (iii) field observa- of these Labrador fens are not highly colored and are thus less tions of modem analogues for this process that are in agree- acid. This trend of increasing acidity with increasing yellow- ment with the stratigraphic data; (iv) old radiocarbon dates of brown color (as a proxy for organic acid content) has been peat forming the bottom of pools, indicating both that pools documented for bog waters by Gorham et al. (1985) and for develop secondarily yet continually over a long period and Nova Scotian lake waters by Gorham et al. (1986). the absence of sedimentation; and (v) modem observations, including morphometric analyses, suggesting an active process Comparison with other phytosociological studies of fusion of adjoining pools and flarks into enlarged areas. The vegetation units distinguished in the present study have The ideas presented in the hypothesis of Foster et al. (1983) For personal use only. close analogues in phytosociological investigations of eastern are new only in their current organization and formulation. The Labrador mires (Foster and King 1984; Glaser and Foster basic observations and many elements of the proposal have 1984; and Foster and Glaser 1986). Minor floristic differences been recognized for many years by mire ecologists includ- may result from the more nutrient-rich groundwater of the ing Nichols, Ganong, Lundqvist, Sjors, Gorham, Boatman, western Labrador sites. Ivanov, Mets, and Masing. However, the proposed hypothesis Fens in southeastern Labrador support ridge and hummock attempts to organize many observations into a coherent vegetation similar to that observed in this study. Tall explanation and differs from other hypotheses by (i) emphasiz- hummocks, especially along the mire margins, support a ing biological over physical processes (e.g., Washburn 1985); Sphagnum fiscum nodum (nodum I1 in this study), whereas (ii) emphasizing the expansion of flarks and pools in contradis- progressively lower ridges support vegetation dominated by tinction to infilling (e.g., Vitt et al. 1975; Horton et al. 1979); Scilpus cespitosus and Scirpus-Carex exilis assemblages and (iii) arguing for similar processes on ombrotrophic bogs (Foster and King 1984). Additional species found on ridges in and minerotrophic fens. A brief review of this supporting evi- the present study but not on ridges in southeastern Labrador dence will put the present study into perspective. include Juniperus communis, Xhalictrum pubescens, Salix pedicellaris, Potentilla palustris, Equisetum fluviatile, Mitella Supporting evidence nuda, and Selaginella selaginoides. There is abundant support from studies on bogs and fens The major difference in vegetation on the western fens is the (Aartolahti 1967) for the first element of the hypothesis, i.e.,

Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 much greater importance of flark vegetation dominated by that pools develop secondarily on the mire surface from Carex limosa and mud bottoms. In contrast, fens from south- hollows. Sjors (1946, 1948) makes the very clear distinction eastern Labrador support broader areas of open-water pools. between secondary water bodies (pools) and primary water The carpet and mud-bottom assemblages, although less impor- bodies (tams) on mires. In subsequent papers Sjors (1961, tant in the east, occur most frequently there as broad mats or 1983) cites the evidence from pollen analysis of Lundqvist quagmires (sensu Vorren 1983) adjoining tams rather than (1951) to demonstrate that fen pools in central wede en-are across flarks (Foster and King 1984). A major floristic differ- secondary landforms that persist over thousands of years. ence in the mesic vegetation is the replacement of Sphagnum Radiocarbon datings and stratigraphy from fen pools in Soviet lindbergii by Sphagnum angustifolium and Sphagnum papil- Karelia extend this conclusion to other sites (Kuznetsov 1986). losum on western mires. Sphagnum lindbergii is prominent on Studies from minerotrophic and ombrotrophic mires in central both fens and ombrotrophic bogs in the east (Foster and King Sweden provide additional confirmation and lend credence to 1984; Foster and Glaser 1986). the view that similar processes may operate on fens and bogs FOSTER ET AL. 2415

(Foster and Fritz 1987; Foster et al. 1988). Boatman and his of potential great longevity. Sediment of an algal gyttja type, coworkers (Boatman 1983) come to similar conclusions con- which accumulates on lake bottoms, is lacking in pools. The cerning the origin of pool patterns on the large blanket bog absence of sediment and the old dates suggest that the pool "flowes" in southwestern Scotland. To date, however, no basins are either stable or erosional. comparable evidence of old radiocarbon dates from pools have Comparison of the peat stratigraphy below a flark and the been reported from North American mires to provide unequi- adjacent ridge provide further information on this develop- vocal evidence of the age of these features and their duration mental process. Both features have a similar basal stratigraphy on the mire surface. Similarly, there is considerable uncer- and then differentiate in opposing directions consistent with tainty concerning the vegetation assemblages involved in the the predictions of the hypothesis. The flark develops through hollow to pool transition or possible geographic variations in progressively more mesic assemblages whereas the vegetation this developmental sequence. below the strings become more woody. A transect of cores The second element of the mire formation hypothesis, from a ridge across a flark and into a pool describes the namely that pool formation results from reduced peat accumu- sequence of communities involved in the flark to pool develop- lation in depressions relative to the surrounding raised areas, is ment in more detail. From the highest to lowest elevation along also supported by experimental and quantitative evidence from the transect the vegetation changes from a ridge assemblage numerous studies. Boatman and his coworkers have studied the across a sharp transition to Carex lirnosa - Scirpus cespitosus growth and accumulation of hollow and pool bryophytes, espe- lawns, then gradually changes towards open water through cially Sphagnum cuspidatum (Boatman 1977; Boatman et al. carpets of Drepanocladus and discontinuous mud bottoms. 1981). Their studies suggest that the growth of Sphagnum and The stratigraphy at depth (>80 cm) underlying each assem- the net accumulation of peat in wet hollows and pools is insuf- blage on this gradient is similar and composed of sedge- ficient to keep up with the corresponding accumulation of Sphagnum peat with some wood. Below the firm lawns there is organic matter on hummocks. Consequently the hollows a transition to more mesic and more decomposed peat, whereas become progressively lower relative to the water table, thereby a loose unconsolidated detrital material underlies the weak providing a positive feedback system for increasing hollow carpets, mud bottoms, and shallow pool margins. Away from depth and for generating pools. Evidence that the peat accumu- the margin of the pool this detrital layer is reduced and gener- lation rates in carpets or lawns of flarks and hollows is less ally is comprised of firm and decay-resistant material such as than that of the hummock communities is provided by some twigs, rhizomes, and wood. comparative studies (Sonesson 1972; Damman 1979a; Clymo The stratigraphic evidence and zonation of vegetation, as and Hayward 1982; Ilomets 1982). In general, many studies well as extensive observations of the disintegration and show a positive relationship between height above the water break-up of the flark vegetation, suggest a pattern of gradual table and peat accumulation. Additional evidence has been formation of pools from sedge fen through progressively more provided by Boatman (Armstrong and Boatman 1967; Boat- mesic stages of flark vegetation. This conclusion from minero- man 1972, 1977) that depressed rates of plant growth (and trophic mires in western Labrador would be in essential agree- therefore peat accumulation) adjacent to depressions lead to ment with the ideas proposed for ombrotrophic bogs (Boatman For personal use only. gradual expansion of the depressions. 1983; Ivanov 1981) and recently for minerotrophic fens Coalescence of pools, the third element of hypothesis, also (Kuznetsov 1986). has support in the literature. Consider first that if pools on The vegetation and stratigraphic data correspond to a northern mires behaved like primary water bodies they would sequence in which small depressions, once initiated on the be progressively filled in by sediment and overgrown margin- mire surface, become gradually lower relative to the general ally by mats of vegetation (e.g., Vitt et al. 1975; Horton et al. mire surface and the water table. The vegetation adjusts to 1979). Although stratigraphic studies document that this may these changes and becomes a progressively more mesic assem- occur following a pronounced shift to drier conditions (Walker blage. In the final stages of carpet vegetation the predominant and Walker 1961), many studies provide evidence that under species, such as Carex limosa, decline in vigor as the site prevailing moist and cool conditions pools expand through becomes wetter. This leads to the gradual breakup of the Carex flooding of adjacent hollows and coalescence with adjoining mat and replacement by Cladopodiella, Drepanocladus, algae, pools (Boatman et al. 1981; Ivanov 1981; Mets 1982; and Drosera anglica mud bottoms on the bare spots that Kuznetsov 1986). Sjors (1963) provides photographic and develop. As the mat degrades further, larger mud bottoms observational support of this process in the Hudson Bay Low- appear and the supporting mat, composed of rhizomes of land. Similar evidence and supporting morphometric data have Carex and Menyanthes, breaks open, exposing the underlying been provided from raised bogs in eastern Labrador (Foster flocculent detritus. This material is readily decomposed or et al. 1988). Uncertainty concerning the extent to which these flushed away with runoff and small open-water pools develop.

Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 observations pertain to other regions of Canada provided par- Because the delicate surface patterning originates from tial motivation for the present study. similar assemblages in an apparently relatively homogeneous mire, substratum effects on this process should be relatively Evidence from the present study minimal. This conclusion is supported by the cross-sectional The observations and stratigraphic data from this study profiles of the mires. There is no apparent correspondence broadly support the work of other researchers and the hypothe- between the topography of the substratum and the patterning of sis for pattern formation. Relevant data are summarized below. the mire. The large pools occur on the broad, level peat sur- The stratigraphic and radiometric evidence from Shovel and faces where water collects and outflow would be expected to Rose fens confirms that pattern formation commenced at least be low. These observations support the general conclusion that 1000-3000 years after mire formation on fens in eastern pool formation is largely a function of hydrology, surface char- Canada. Dates from peat and wood on the pool bottoms 3210 acteristics, and peat hydraulic conductivity rather than purely and 3430 years BP document that pools are secondary features physical forces, e.g., gravity, relating primarily to substratum 2416 CAN. 1. BOT. \IOL. 66, 1988 characteristics (Ivanov 1981; Boatman 1983). common than pools and the process of flark degeneration to The process of pool expansion and coalescence controls the pools is important under these conditions. In areas marked by specific orientation of pools on a mire surface. Our observa- still lower summer moisture availability, sedge flarks predomi- tions suggest that, on level surfaces, hollow and pool extension nate, as in central Canada and northern United States at the occurs in a nonoriented direction. Equiformal expansion southern limit of patterned fens (Auer 1930; Damman 19796). results in the broad, subcircular hollows and pools at the base For example, in northern Minnesota, the flarks and ridges are of Shovel fen (Push pond) and at the top of the slope (Trout distinguishable by only very slight microtopographic and pond and Lac d'Isles). Similar broad pools develop on the flat floristic differences (Glaser and Wheeler 1980; Glaser et al. plain of plateau bogs (Foster and Glaser 1986; Glaser and 1981). This probably results from the low summer moisture Janssens 1986). In contrast, on sloping surfaces the elongation levels on the mire. The water table drops so low that the entire and coalescence generally occurs across the slope. The result- surface, including ridges and minor depressions, is roughly ing sublinear patterns are seen on the sloping fens in western similar in moisture status and accumulates peat at approxi- Labrador and on sloping bogs (Sjors 1948; Ruuhijarvi 1983). mately equal rates (Heinselman 1963). A probable explanation for this difference in pattern shape can be suggested. If slow rates of peat accumulation are associ- Acknowledgements ated with closer proximity to the water table, then expansion of hollows or pools will occur preferentially along low areas and We thank H. E. Wright, Jr., and A. Loiselle for help with over vegetation situated at the water table. On a generally level the fieldwork; C. Wetmore and R. Ireland for assistance with surface, such as the plateau of a raised bog or the level portion cryptogam identifications; T. R. Moore and the staff of the of a fen, such low sites will be distributed randomly around a McGill University Subarctic Research Station for logistical flark or pool. Expansion will therefore be random and hollows help; and F. Phillips, B. Flye, and D. Smith for assistance and pools will exhibit broad shapes and haphazard orientation. with the manuscript. H. E. Wright, Jr., and D. H. Vitt pro- On a sloping surface the water table around a hollow is not vided valuable comments on the manucript. Research was sup- level and low spots are not randomly distributed. Sites immedi- ported by National Science Foundation grants DPP-8400638 ately above a depression or pool will necessarily be drier as the and DEB-7922142 and the Graduate Society of Harvard Uni- mire slopes upward. Immediately downslope the water table versity. must drop below and away from the peat surface. The lateral AARTOLAHTI,A. 1967. On dating the genesis of peat banks and margins may be any elevation above the hollow or pool. There hollows in the raised bogs of southwestern Finland. C. R. Soc. is no hydrological constraint that these lateral areas must be as Geol. Finl. 39: 71 -86. elevated as those areas above or below. Low areas will be AHTI,T. 1961. Taxonomic studies on reindeer lichens Cladonia, sub- swamped preferentially and therefore the depressions may genus Cladina. Ann. Bot. Soc. Zool. Bot. Fenn. Vanamo, 32: extend laterally. 1 - 160. ALLINGTON,K. R. 1961. The bogs of central Labrador - Ungava: an Implications for geographical trends in patterned mires examination of their physical characteristics. Geogr. Ann. 43: 401 -417. For personal use only. The development of pools in the western Labrador fens, as described above, differs principally from that observed in ARMSTRONG,W., and BOATMAN,D. J. 1967. Some field observations southeastern Labrador in that there-is a prolonged period of relating the growth of bog plants to conditions of soil aeration. J. flark dominance in western mires before the formation of Ecol. 55: 101 - 110. AUER,V. 1930. Peat bogs in southeastern Canada. Can. Dep. Mines pools. In the west, there is a gradual differentiation of flarks Geol. Surv. Mem. No. 62. pp. 1-32. into deeper flarks, then mud bottoms, and ultimately pools. In BIRKS,H. J. B. 1973. Past and present vegetation of the Isle of Skye. the southeast, pools form directly through a degradation of the Cambridge University Press, Cambridge, England. surface vegetation cover and the flark stage has a shorter dura- BOATMAN,D. J. 1972. Pools on blanket mires in Scotland. In tion (Foster and King 1984). Proceedings of the 4th International Peat Congress, Otaniemi, Thus, it appears that there may be geographic variation in Finland, June 25-30, 1972. pp. 111-119. the length of time and process involved in the conversion of 1977. Observations on the growth of Sphagnum cuspidaturn flarks or depressions to open-water pools. Because the ultimate in a bog pool on the Silver Flowe National Nature Reserve. J. Ecol. formation of pools depends on inundation, it appears that the 65: 119- 126. rate and extent of conversion should be controlled by the mois- 1983. The Silver Flowe National Nature Reserve, Galloway, Scotland. J. Biogeogr. 8: 163-274. ture balance of the mire and the local hydrology within the BOATMAN,D. J., GOODE,D. A., and HULME,P. D. 1981. The Silver vicinity of the pool. Under very moist regimes and in moist Flowe. 111. Pattern development on Long Loch B and Craigeazle portions of mires, the conversion would proceed rapidly and Mires. J. Ecol. 69: 897-918. the mires would contain a large proportion of open-water CLYMO,R. S., and HAYWARD,P. M. 1982. The ecology of Sphag- Can. J. Bot. Downloaded from www.nrcresearchpress.com by HARVARD UNIVERSITY HERBARIA on 08/19/11 pools. Under progressively drier conditions this process would num. In Bryophyte ecology. Edited by A. J. E. Smith. Chapman take place relatively slowly (and may, in fact, halt in the flark and Hall, London. pp. 229 -289. stage) and sedge lawn and Sphagnum carpet flarks should pre- DAHL,E. 1956. Rondane. Mountain vegetation in south Norway and dominate in the depressions. its relation to the environment. Kornrnisjon Hos H. Aschehoug and This prediction may explain some well-documented geo- Co., Oslo. graphic trends in the extent of surface patterns on mires. Open- DAMMAN,A. W. H. 1979a. Geographic patterns in peatland development in eastern North America. In Classification of Peat and Peat- water pools predominate under moister maritime or northern lands: Proceedings of the International Symposium, Hyytiala, climatic conditions, as in southeastern Labrador (cf. Eurola Finland, September 17-21, 1979. International Peat Society, 1962; Eurola and Vorren 1980; Ruuhijarvi 1983; Sjors 1959, Helsinki. pp. 42 -57. 1961; Foster and King 1984). Under somewhat drier and more 1979b. Ecological and floristic trends in the ombrotrophic continental conditions, as in western Labrador, flarks are more peat bogs of eastern North America. In La vCgCtation des sols tour- FOSTER ET AL. 2417

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