Full-glacial upland tundra vegetation preserved under tephra in the Beringia National Park, Seward Peninsula, Alaska

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1 Quaternary Science Reviews 20 (2001) 135}147 Full-glacial upland tundra vegetation preserved under tephra in the Beringia National Park, Seward Peninsula, Alaska Victoria G. Goetcheus *, Hilary H. Birks Department of Geology and Geophysics, University of Alaska-Fairbanks, Fairbanks, AK 99775, USA Botanical Institute, University of Bergen, Alle& gaten 41, N}5007 Bergen, Norway Abstract The nature of the full-glacial vegetation of Beringia has been the subject of a great deal of investigation and debate. Here we present a reconstruction of an intact example of the full-glacial upland vegetation of part of the northern Seward Peninsula at one point in time. The area was blanketed by more than 1 m of tephra ca. 18,000 C BP (ca. 21,500 cal. BP), and the former land-surface was preserved in the permafrost. The discovery of the land-surface provides a unique opportunity to study a fossil ecosystem preserved in situ. Macrofossils were used to reconstruct the vegetation growing at several sites on the buried land-surface. The macrofossil assemblages indicate a vegetation characterized by graminoids and forbs, with the occasional occurrence of Salix arctica. The vegetation was dominated by Kobresia myosuroides, other sedges (Carex), and grasses, with a "ne-scale mosaic related to snow accumulation and moisture availability. Overall, the vegetation was a closed, dry, herb-rich tundra-grassland with a continuous moss layer, growing on calcareous soil that was continuously supplied with loess. Nutrient renewal by loess deposition was probably responsible for the relatively fertile vegetation, and the occurrence of a continuous mat of acrocarpous mosses. Good physiognomic analogues can be suggested, but no exact modern vegetational analogues have been found, probably because the full-glacial environment and climate with loess deposition do not occur today Elsevier Science Ltd. All rights reserved. 1. The Beringian background Knowledge of the Beringian environment during the last glacial maximum (LGM), 28,000}14,000 BP, provides a context for understanding species exchange between Asia and America, megafaunal extinctions at the end of the Pleistocene, development of plant communities in the Arctic, life at high latitudes during glacial stages, and the migration of humans from Asia to the Americas (see Hopkins et al., 1982). Exploration of these topics helps us understand how climate change a!ected the arctic environment and in particular how vegetation change can a!ect human and animal populations. The nature of the full-glacial vegetation of Beringia has been controversial (e.g. Cwynar and Ritchie, 1980; Ritchie, 1984; Guthrie, 1990) due to disagreements about reconstructions of the composition and productivity of the vegetation and its ability to support the Pleistocene megafauna. Schweger and Habgood, as early as 1976, proposed the occurrence of a vegetational mosaic over * Corresponding author. Tel.: address: vwolf@mail1.gi.alaska.edu (V.G. Goetcheus). Beringia that was in#uenced by local and regional factors such as altitude, aspect, and soil moisture. This idea was developed further by Schweger (1982, 1992) and others (e.g. Anderson, 1985, 1988; Anderson and Brubaker, 1996; Elias et al., 1997), supported by evidence of regional variation from pollen analysis and other fossils (e.g. Young, 1982; Ritchie, 1984; Anderson et al., 1994; Elias et al., 1996, 1997). Anderson and Brubaker (1994) summarized available LGM pollen data across central Alaska, and proposed an increase in mesic conditions from east to west. This inferred gradient is supported by records of late-glacial vegetational development following climatic warming ca. 14,000 BP, in which dwarf-birch expanded eastward from central Beringia into the Yukon (Lamb and Edwards, 1988; Elias et al., 1997). Local vegetation reconstructions and insect evidence from the now-submerged part of the Bering Land Bridge indicate the presence of mesic tundra with fens and pools during the full glacial (Elias et al., 1996, 1997). In contrast, conditions appear to have been much drier in eastern Beringia (western Yukon) (e.g. Cwynar, 1982; Cwynar and Ritchie, 1980; Ritchie, 1984) and western Beringia (eastern Siberia) (Lozhkin et al., 1993), where herb-dominated rather than shrub-dominated tundra covered /01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S ( 0 0 ) X

2 136 V.G. Goetcheus, H.H. Birks / Quaternary Science Reviews 20 (2001) 135}147 Fig. 1. Map of Beringia. The full-glacial Bering Land Bridge is bounded by Russia on the west, Alaska on the east, and the 200 m isobath to the north and south. much of the upland landscape. Drier conditions on the continental parts of Beringia may have resulted from the regional in#uence of large icesheets, particularly the Laurentide ice sheet over Canada (Barry, 1982; Bartlein et al., 1991), on the atmospheric circulation, with the southward displacement of the jetstream (Edwards and Barker, 1994). In this paper, we present palaeobotanical results from the northern Seward Peninsula (Fig. 1), consisting of plant macrofossil analyses from a land-surface buried in situ by a deep layer of tephra at ca. 18,000 C BP. The vegetation reconstructions and their environmental implications are considered in the context of previous fullglacial results and reconstructions from elsewhere in Beringia. 2. Modern geology, vegetation, and climate of the Northern Seward Peninsula The Seward Peninsula forms the eastern side of the Bering Strait, with the westernmost tip of the peninsula only 75 km from Siberia. When sea level was lowered during glacial stages, the Peninsula adjoined the central section of the Bering Land Bridge (Fig. 1). The Peninsula consists of lowlands in the north, rolling uplands with some mountains in the centre, and lowlands in the south (Wahrhaftig, 1965). Continuous permafrost '100 m thick underlies the north and central parts of the peninsula (Hopkins, 1988; Beget et al., 1996). Lowland regions are covered with thermokarst lakes and basins of drained lakes, intermixed with yedoma hills and plateaux. Four maar lakes and the shield volcano, Devil Mountain, and its associated cinder cones, and three other volcanic structures add to the diversity of the landscape (Fig. 2). The tephra that buried the surface studied in this paper erupted from the youngest of the maar vents, Devil Mountain Lake, and is called the Devil Mountain Lake tephra (Beget et al., 1996). The modern vegetation of the northern Seward Peninsula coastal plain consists mostly of lowland tundra, with communities ranging from cottongrass-dominated tussock tundra with scattered dwarf shrubs and mosses, through Betula and Salix shrub-dominated vegetation with fewer tussocks, to Betula and heath-dominated tundra communities in drier parts of the landscape. The most common species in this vegetation complex are Eriophorum vaginatum, Carex bigelowii, Betula nana and B. glandulosa, and various shrub Salix spp. (Viereck et al., 1992). Eriophorum and Carex spp. also dominate extensive fens. Dry upland-tundra communities occur on Devil Mountain and other cinder cones, while birch and

3 V.G. Goetcheus, H.H. Birks / Quaternary Science Reviews 20 (2001) 135} Fig. 2. The northeastern Seward Peninsula, showing the Cape Espenberg * Devil Mountain area. The area contains abundant lakes, not all of which can be shown here. The informally named lakes with sites where the buried surface was sampled are indicated. Tern, Plane, and Point sites are located on Tempest Lake. willow shrub-tundra occupies #oodplains and swales. The dominant mosses across much of the region are Sphagnum spp. Kotzebue, 60 km to the northeast of the study area, has a mean temperature of!6.03c (US Weather Bureau). January temperatures average!20.23c and July temperatures average 11.93C. Summers in Kotzebue tend to be slightly warmer than those near the north shore of the Seward Peninsula. Average precipitation is approximately 230 mm yr. More than half the precipitation falls during July}September. 3. The buried surface In 1968, David M. Hopkins discovered an ancient buried vegetated land-surface on the Seward Peninsula, within an area that later became part of the Bering Land Bridge National Park and Preserve. Approximately 2500 km of the original land-surface had been buried by the Devil Mountain Lake tephra, which over approximately 1200 km was more than 1 m thick. Where the tephra was thicker than the summer-thawed active surface layer, it preserved the plant and animal remains present on the surface by in situ freezing. Subsequent loess deposition buried the tephra to depths of 6}300 cm. Holocene thermokarst lakes have incised themselves into the sediments by the melting of large, syngenetic ice wedges. The resulting steep banks along the lake shores expose the tephra as a distinct dark horizon separating two lighter coloured loess units. A typical site is located on the eroding bank of a thermokarst lake and many are underlain by ice wedges that were actively growing when the tephra fell. The occurrence of a subnivian rodent nest (Goetcheus et al., 1994) and large ice lenses that appear to be compressed snowbeds suggest that the tephra fell either during the late winter or early spring. The buried surface lies near the mid-point of the north-south axis of the Bering Land Bridge (Figs. 1 and 2) and preserves the vegetation near the time of the thermal minimum of the last glacial period, ca. 18,000 radiocarbon years BP. A macrofossil record of the plant community has been preserved intact by the burial of the land-surface. The fact that the tephra fell during the late winter or early spring limits the quality of the plant tissues preserved on the surface. Plant material either deteriorated or was eaten during the winter, leaving only remains that were protected, sturdy, or very abundant. But all the remains found on the surface are strictly local, and can be interpreted directly in terms of the past in situ #ora and vegetation.

4 138 V.G. Goetcheus, H.H. Birks / Quaternary Science Reviews 20 (2001) 135}147 The chance to sample fossil in situ upland vegetation is unique as far as we know. Water-transported Laacher see tephra buried late-glacial river-bank vegetation in Germany, where well-preserved fossils give a good representation of the community (Waldmann, 1995, 1997). Other macrofossil assemblages have been preserved in #uvial and glacio#uvial sediments (e.g. Miller, 1993, see Jackson et al., 1997) or in glacial sediments (e.g. Rosendahl, 1948, Miller, 1976). Often these are lowland peaty deposits laid down in shallow pools or poorly drained areas (e.g. the Two Creeks Forest Bed; Miller, 1976), and do not represent actual upland vegetation. However, their often good preservation, together with well-preserved mosses, allows a detailed reconstruction of the past vegetation to be made (e.g. Miller, 1976). Fens and bogs are other examples where lowland vegetation is preserved in situ, but here the timelines are obscured by peat growth, root penetration, and compaction, so that it is very di$cult to reconstruct actual vegetation other than by assemblage analogies. 4. Radiocarbon dates Eighteen samples from the buried surface have been radiocarbon dated during the last 21 years (Table 1). Conventional dates on 12 samples collected between 1968 and 1994 produced a surprisingly large scatter of age estimates, 16,880 $ 120 to 18,140 $ 200 C years BP (Table 1a), given that the tephra from the Devil Mountain eruption would have buried the entire landscape within a period of a few hours to a few days (Beget et al., 1996). Several possible explanations exist for this large scatter: (1) signi"cant modern contamination may have been introduced during sampling and subsequent handling; (2) large fossil willow roots included in the earlier samples may have been as much as several hundred years old when buried by the tephra; (3) undetected inaccuracies in counting in conventional radiocarbon dating techniques may have caused the wide scatter of dates (Baillie, 1990; Lowell, 1995), and (4) inclusion of old carbon from calcium carbonate in the loess. Consequently, a more rigorous protocol was adopted when six new samples were collected for AMS dating in 1995 (Table 1b). Latex surgical gloves were worn while collecting and the trowel was rinsed in "ltered water and #amed with alcohol before each sample was collected. Twigs and roots were avoided. Plant material was scraped o! the ground surface, avoiding the underlying soil. Each sample was placed in a ziplock bag that was immediately sealed. The AMS radiocarbon dates are much more tightly clustered. The weighted mean of the six uncalibrated AMS dates was calculated using the CALIB 3.0 program of Stuiver and Reimer (1993), and is 18,070 $ 60 C years BP, with a range of 17,770}18,260 at 2σ (Table 1b). Using the calibration curve in CALIB 3.0 the Table 1 (a) Bulk dates on material collected using older protocols, 1968}1994 (The older dates are from Hopkins, 1988) Site Laboratory number Age Material Egg lake Beta ,880$120 Wood on surface Eh'cho lake Beta ,980$170 Roots from surface Nuglungnugtuk Beta ,990$150 Plant remains Fritz lake Beta ,360$130 Plant remains Lake Rhonda 1 Beta ,420$260 Woody root White"sh thaw pond W ,630$800 Plant remains Lumpy drained Lake Beta ,740$110 Woody stems Tempest lake Beta ,740$220 Woody stems Ulu lake Beta ,850$100 Plant remains Ulu lake Beta ,880$110 Subnivian microtine nest Kiliwooligoruk creek Beta ,910$110 Detrital twigs in tephra-rich alluvium Eh'cho Lake Beta ,980$110 Plant remains White"sh thaw pond Beta ,140$200 Plant remains (b) AMS dates on plant remains collected using the rigorous sampling protocol of 1995 (see text) Site Laboratory number Age Calibrated age Tempest lake Beta ,090$70 21,780 (21,600) 21,400 White"sh thaw pond Beta ,880$110 21,550 (21,330) 21,100 Lake Rhonda 3 Beta ,950$70 21,610 (21,420) 21,230 Eh'cho lake Beta ,170$90 21,900 (21,700) 21,500 Egg lake Beta ,090$110 21,810 (21,600) 21,380 Lake Rhonda 2 Beta ,190$70 21,900 (21,725) 21,390 Mean age 18,070$69 21,570

5 V.G. Goetcheus, H.H. Birks / Quaternary Science Reviews 20 (2001) 135} Table 2 Properties of the buried-surface soil. Data from Hoe#e (1995). OC"organic carbon Site ph OC (%) C/N CO (%) Palaeo-active layer depth (cm) Rhonda Ulu Eh'cho n.d. Egg Tempest Fritz Foot calibrated age of the weighted mean centres on 21,570 cal BP. The new AMS dates place the age of the buried surface more precisely within the time of the Last Glacial Maximum. 5. Soil The physical and chemical properties of a soil have a large in#uence on the plants that are rooted in it, and thus on the composition of the vegetation. The buried soils developed on massive Pleistocene loess with a silty loam texture. According to Hoe#e (1995) and Hoe#e and Ping (1996), the ph of the 21,500 year-old soil ranges from 7.4 to 8.6 with a mean of 8.0, and the presence of carbonate along with other chemical properties of the soil indicate that it formed under dry conditions (Table 2). Fine roots were found throughout the soil pro"le, within both the palaeo-active layer and the palaeo-permafrost layer. These indicate the former presence of abundant herbaceous vegetation with a large sub-surface biomass. A dense network of "ne roots in calcareous soil under an open, graminoid-dominated vegetation is characteristic of northern steppe communities (Yurtsev, 1982). The high organic carbon content and the narrow C/N ratios (8}11) (Table 2) suggest a high nutrient, or at least, a high nitrogen availability that may have been maintained by the constant input of loess with new nutrients into the system (see Jacobson and Birks, 1980), and the recycling of plant material by annual turnover and by grazing by small mammals (Schweger, 1992, 1997). Modern analogues on the Arctic Coastal Plain where loess is currently being deposited, tend to remain alkaline and places without loess input normally become acidic as peat accumulates (Walker and Everitt, 1991). Studies in the Kluane Lake Region of the Yukon Territory support the idea that loess input increases plant productivity; such areas might have supported grazing during the LGM (Laxton et al., 1996; Smith, 1997). It seems probable that the continuing input of loess and manuring by animals were important for maintaining the carbonaterich substrate and the productivity of the vegetation (Schweger, 1992, 1997; Schweger et al., 1982). 6. Methods 6.1. Sampling During the summers of 1993, 1994, and 1995, the buried surface was excavated at 18 sites located on the banks of nine thermokarst lakes and beneath the bottom of one recently drained thermokarst lake. None of these lakes have formal names, so informal names were assigned in the "eld, and are used when referring to them in this paper (Fig. 2). Material from 11 sites has been examined so far for macrofossils, and material from three sites was studied in considerable detail. The tephra was carefully removed from the surface using trowels and paintbrushes. Each site was excavated along the blu! face to expose 2}10 m 30}40 cm of the original ground surface. At each site the percentage cover of the di!erent vegetation components was estimated by a point transect. The presence or absence of moss, woody material, graminoids, and herbaceous material was determined for each point. Cover estimates commonly exceed 100% due to the presence of several plant layers at a point. From each site on the buried surface numerous areas of 5 10 cm were sampled with a trowel to a depth of 2 cm (&100 cm ) Macrofossil analysis Subsamples were taken in the laboratory. Their volumes were measured by water displacement and they were sieved through 125 μm mesh with water to disaggregate the material and remove soil and tephra. The retained material was sorted using a binocular dissecting microscope at 12 magni"cation into moss, insect remains, seeds, and organic debris. The organic debris included an abundance of faeces of small mammals, such as voles, etc. The few lichen remains found were classed with the organic debris due to their decayed and fragmentary state. For convenience, both fruits and seeds will be referred to here as &seeds'. By comparison with reference collections located in the Herbarium of the Museum of the University of Alaska-Fairbanks, and at the Botanical Institute, University of Bergen, seeds were identi"ed to the lowest taxonomic unit possible (several to species level) based on their morphological characteristics (Table 4) and counted. Although the concentration data are available, they are not used here, as we are considering the relatively homogeneous vegetation as a whole. The spatial variation over a few cm within the vegetation of a site, or over a few km between sites, will be considered in a future paper. Therefore, we have estimated

6 140 V.G. Goetcheus, H.H. Birks / Quaternary Science Reviews 20 (2001) 135}147 Table 3 Moss taxa identi"ed from the buried surface. Habitat information is from Steere (1978) and personal observations. Nomenclature follows Steere (1978) Moss taxa Habitat Abietinella abietina Well}drained calcareous soils, xerophytic Aloina cf. brevirostris On open calcareous soil Amblystegium serpens var. juratzkana Moist soil cf. Amblystegium varium Wet-to-moist calcareous substrates Brachythecium groenlandicum On moist soil and among other mosses Brachythecium cf. nelsonii On moist or wet soil, among litter cf. Bryoerythrophyllum recurvirostrum On calcareous soil, often with other mosses in moss turf Bryum cf. pseudotriquetrum On moist calcareous soil Bryum neodamense On moist calcareous soil Campylium hispidulum On moist calcareous soil Campylium stellatum On moist to wet calcareous soil Desmatodon leucostoma On open calcareous soil Dichodontium pellucidum On wet calcareous soil, #ushes Dicranum bonjeanii On moist soil Didymodon rigidulus var. icmadophila On open calcareous soil, e.g. frost boils, gravel, and in moss turf Distichium cf. capillaceum On calcareous soil and in moss turf Ditrichum yexicaule On calcareous soil Drepanocladus brevifolius Calcareous fens and tundra depressions Encalypta alpina On dry, open calcareous soil Eurhynchium pulchellum On well}drained calcareous soil Fissidens arcticus On "ne}grained silt on e.g. frost boils and disturbed soil Grimmia sp. Dry calcareous or acidic substrates Hypnum bambergeri On damp calcareous soil Hypnum vaucheri On calcareous soil Jungermanniales Various Myurella julacea On calcareous soil and other mosses Orthothecium strictum On moist calcareous soil Plagiomnium ellipticum Mesic fens mixed with other mosses Pohlia cf. wahlenbergii On moist to wet soil, e.g. snow Melt}water #ushes Pottiaceae Various Pseudocalliergon turgescens On seasonally wet calcareous soil cf. Sanionia uncinata Rich fens and damp calcareous soil Stegonia pilifera On calcareous silt on frost boils Timmia austriaca On calcareous soil Timmia norvegica var. excurrens On calcareous soil Tomenthypnum nitens Mesic calcareous fens and damp soil Tortula norvegica On calcareous soil in late snow}melt areas the abundance of the taxa on a 4-point scale related to their percentage occurrence in the samples (see Table 4) through the total data set. Most of the mosses were identi"ed by Jan Janssens, and are listed in Table Macrofossil results Macrobotanical remains found on the buried surface can be classed in four groups: mosses, prostrate shrubs, graminoids, and other herbaceous plants (forbs). Little or no bare ground was observed during excavations of the buried surface. Mosses covered at least 60}70% of the ground at most sites and often formed an understorey to the other plant types. All the identi"ed mosses are listed in Table 3. No lichens were seen on the buried surface in the "eld, but some small remains were found when sorting the samples under a microscope, growing on the surface of the moss mat. They are too small to be identi"ed further than thallose and foliose types. The mosses formed a carpet across the land-surface with many di!erent species mixed together. One small piece of the carpet (2 2cm ) consisted of a dense acrocarpous mat of Distichium cf. capillaceum, Encalypta alpina, and Didymodon rigidulus var. icmadophila, with Drepanocladus brevifolius and Myurella julacea creeping over it. D. rigidulus var. icmadophila, Hypnum vaucheri, and Bryum spp. are the most common moss taxa, occurring abundantly in most samples. Abietinella abietina, Fissidens arcticus, and Stegonia pilifera are the least common taxa, occurring sparsely in a few samples. The vascular plants identi"ed are listed in Table 4. Prostrate shrubs are the least abundant component of the buried surface vegetation. Salix arctica was identi"ed at three sites where it covered approximately 30}50% of the area sampled. Leaves and a catkin belonging to Salix arctica were discovered at two sites located at Tempest Lake; more leaves were found at the Egg Lake site. The buried surface at the two Tempest Lake sites was covered by large, horizontal ice bodies, evidently buried snow, indicating a snow-bed environment for the shrubs. No estimates of willow groundcover are possible at any other sites due to a lack of shrub bases or leaves. Salix remains such as buds and capsules have been found at other sites but cannot be identi"ed to species level. The buds and capsules indicate the transport of Salix parts by wind from plants growing nearby but not in sampled sections of the buried surface. Willow shrubs appear to have been con"ned to patches on the landscape leaving most of the surface without shrub cover. Graminoid remains are abundant on the buried surface, but the dried bases and pieces of leaf and stem have not been identi"ed to family. Gramineae and Carex seeds have been identi"ed, but it has only been possible to identify one seed type, Kobresia myosuroides, to species

7 V.G. Goetcheus, H.H. Birks / Quaternary Science Reviews 20 (2001) 135} Table 4 Plant taxa identi"ed from macrofossil remains. &seed' is used to indicate reproductive propagules, including fruits and seeds. Nomenclature follows HulteH n (1968) Plant taxa Macrofossil Sites present Abundance in sample Dwarf shrubs: Salix arctica Leaves TT, PT, EL Uncommon Salix undi!. Capsules and Buds All but EL and PL Uncommon Graminoids: Carex bigelowii type Seeds EH, UL and RH Uncommon Carex nardina type Seeds All but EL and RL Common Cyperaceae undi!. Seeds All but PL, R3 and RH Uncommon Kobresia myosuroides Seeds All Abundant Herbs: Bupleurum triradiatum Seeds All but R3 and RH Uncommon Campanula uniyora type Seeds TT, UL, EH, RH Uncommon Caryophyllaceae undi!. Seeds TT, PT, EL, UL, EH, R3 Uncommon Cerastium beeringianum Seeds All but RL, RH and R1 Uncommon Compositae undi!. Seeds TT, PT, RL, EH Rare cf. Artemisia Seed PL Rare Cruciferae undi!. Seeds R2, R3, TT, PT, EL Rare Draba Seeds All Abundant Eutrema edwardsii type Seeds TT, R3, EH, UL Uncommon Juncus Seeds TT, PT, RL, EL, UL, EH Uncommon Luzula Seeds UL, EH Rare Melandrium azne Seeds All but PL, R1 and RH Uncommon Melandrium apetalum Seeds TT, EH, R3 Rare Minuartia arctica Seeds TT, PT, EL, UL, EH, R1, R3 Uncommon Minuartia obtusiloba Seeds All but PL and RL Common Oxyria digyna Seed UL Rare Papaver sect. Scapiyora Seeds All but PL Common Papaver walpolei Leaves TT Rare Polygonum viviparum Bulbils TT, RL, EH, UL Common Potentilla hookeriana Seeds All Common Potentilla hyparctica type Seeds, whole Plant TT, PT, R1, R2, EH, UL, RH Common Potentilla nivea type Seeds R2, PT, UL, EH Uncommon Primulaceae Seeds All but R3 Uncommon Ranunculus undi!. Seeds All but PL Uncommon Saxifraga oppositifolia Seeds, leaves UL, EH, R3 Uncommon Taraxacum Seeds PL, EL, EH, R1, R3 Rare Valeriana Seeds TT, UL Rare &Undi!.'"undi!erentiated. The site abbreviations are as follows: TT"Tern, PT"Plane, PL"Point, R1"Reindeer 1, R2"Reindeer 2, R3"Reindeer 3, RL"Rhonda 1, RH"Rhonda 2, EL"Egg, EH"Eh'cho, and UL"Ulu. Abundance rating is based on the percentage of samples the taxon is found in: abundant '80%, common 50}80%, infrequent 16}49%, rare (16%. (this taxon was misidenti"ed as Trichophorum caespitosum by Goetcheus et al., 1994). Kobresia myosuroides is often abundant, and comprises the majority of the Cyperaceae seeds (Table 4). It occurs in 98% of the samples and averages more than 30 seeds per 100 cm. Most of the other Cyperaceae seeds are Carex belonging to two di!erent groups: Carex nardina type and Carex bigelowii type (see Berggren, 1969). Carex nardina often occurs with Kobresia myosuroides in the drier part of its habitat range (Walker, 1990), although other species with the C. nardina seed type occur in other habitats. Graminoids covered approximately 30}50% of the ground at each site. The forb #ora on the buried surface is diverse. The most commonly found seeds were Potentilla and Draba, occurring in 90 and 98% of the samples, respectively. Potentilla hookeriana, the most common of the three Potentilla species found in samples from the buried surface, is locally common in subarctic steppe vegetation of eastern and western Beringia today (Murray et al., 1983; Eriksen, 1995). Papaver walpolei leaves were found at only two sites, Tern and Plane, located on opposite sides of Tempest Lake, but Papaver sect. Scapiyora (that includes P. walpolei) seeds were found at 10 sites including Tern and Plane. Saxifraga oppositifolia is most common at the Eh'cho site though it was found occasionally in

8 142 V.G. Goetcheus, H.H. Birks / Quaternary Science Reviews 20 (2001) 135}147 samples from other sites. The forb vegetation covered approximately 25}40% of the ground at each site. We have small samples from relatively few sites scattered in a small area (Fig. 2), so we are unlikely to have sampled the full diversity of the regional upland vegetation. Macrofossils, as opposed to pollen, are locally deposited (Birks, 1973), especially so in this situation, where the upland vegetation at a site is being sampled and there is no sorting or blurring of the assemblage by taphonomic processes. This local deposition means that we have the potential to take adjacent fossil samples to describe the vegetation variability, like sampling the variation in modern vegetation with quadrats. In the present study, however, we have spot samples, and thus may have missed recording species that occurred in the local vegetation, but not at our sampling points. However, there is a large degree of homogeneity between our fossil assemblages, suggesting that a fairly uniform vegetation may have occupied the landscape, with small variations related to environmental factors, such as moisture availability. 8. Reconstruction of vegetation and environment The plant remains provide important information about the environment of the northern Seward Peninsula during the LGM. The vegetation was predominantly dry tundra, rich in graminoids and forbs and lacking erect mesic shrubs. The physiognomy of the vegetation resembled arctic steppe, a rare vegetation type today (Yurtsev, 1982; Guthrie, 1990, photo in Fig. 8.12; Walker, 1990; Walker et al., 1991). Most of the identi"ed taxa from the buried surface share two characteristics: they grow on mineral soil and they prefer calcareous substrates (HulteH n, 1968; Steere, 1976). The taxa di!er in their degree of drought tolerance (Tables 3 and 5), but all the vascular plants and most of the mosses have some tolerance for drought (HulteH n, 1968; Steere, 1976). The well-drained loess soil would have dried quickly in the windy conditions, and some of the vegetation may have been blown free of snow in the winter (Schweger et al., 1982). Many of the moss taxa require moist conditions, at least seasonally, but several others are indicators of dry and/or disturbed soil (Table 3). The study sites fall into two main groups, those with drought-tolerant plants and those with plants with higher moisture requirements (Tables 3 and 5), suggesting some di!erentiation in moisture availability at di!erent places. The latter group includes two sites where the ice masses that are interpreted to be "rni"ed, late-lying snow banks are found. Several of the moss taxa presently grow in snow-bed habitats or areas with snow meltwater, and it is probable that Salix arctica was also con"ned to areas of snow protection in winter. Another possible source of Table 5 Vascular plant taxa identi"ed from the buried surface and their habitats. The hatbitat information is from HulteH n (1968) and other references in this paper, supplemented by personal observations Plant taxa Bupleurum triradiatum Carex nardina type Cerastium beeringianum Draba spp. Eutrema edwardsii Kobresia myosuroides Luzula spp. Melandrium azne Melandrium apetalum Minuartia arctica Minuartia obtusiloba Oxyria digyna Papaver walpolei Polygonum viviparum Potentilla hookeriana Salix arctica Saxifraga oppositifolia Valeriana (capitata) Modern habitat Dry grassy and open tundra, fell}"eld Wind}exposed open grassy tundra on calcareous soils Gravel and cli!s, soil disturbed by animals Open, often calcareous, soil Open damp soil on tundra, soli#uction areas Dry, calcareous tundra with stable soils and little snow}cover in winter Open soils, fell}"eld, screes and gravel Dry places, open rocks and disturbed soil Dry grassy slopes, open gravel and screes Dry ridges and rocky slopes, on open soils in grassy tundra Dry slopes and snow beds Snow beds on tundra, open gravel, screes Dry calcareous soil Dry and damp meadows and open grassland Dry calcareous open grassland Dry tundra with winter snow protection Dry to moist exposed calcareous soils Mesic grassland diversity of moisture requirements is irregularity in the micro-topography, with 2}10 cm changes in the height of the surface from the lowest point at some sites. Small hollows would collect snow, and act as channels for surface run-o!. The diversity and widespread distribution of the taxa present on the surface that require moist conditions suggest that a moderate amount of water was available during the growing season. At other sites, lack of snow cover may have led to some periglacial soil disturbance, as suggested by the occurrence of Fissidens arcticus and Stegonia pilifera. Mammal grazing, trampling, and burrowing may also have resulted in open disturbed areas of soil. Many of the vascular taxa also tolerate soil disturbance, e.g. Draba spp., Luzula spp., Cerastium beeringianum, Minuartia obtusiloba, M. arctica, Melandrium apetalum, Campanula uniyora, and Papaver spp., and they can occur today where animal disturbance by grazing, manuring, and burrowing is strong (e.g. Walker, 1990). The abundance of Kobresia myosuroides in the fossil assemblages suggests that it was dominant in many places. In several samples, the seeds had been germinating, suggesting good regeneration of Kobresia. It was usually associated with herbs of open habitats, such as Potentilla spp., Draba, Eutrema, Papaver walpolei, the Caryophyllaceae in Table 4, Bupleurum triradiatum, Campanula uniyora, and Luzula spp. Kobresia myosuroides is a circumpolar species (HulteH n, 1968) that characteristically

9 V.G. Goetcheus, H.H. Birks / Quaternary Science Reviews 20 (2001) 135} grows on xeric to mesic wind-blown ridges, and in meadows and heaths (Cooper and Sanderson, 1997), on dry, usually calcareous slopes, gravel bars, and lichen tundra (HulteH n, 1968) that are neither extremely windexposed nor covered by deep snow in winter (Gjvrevoll, 1954; Bell and Bliss, 1979). It is associated with many of the above-mentioned taxa in its present-day communities. On the buried surface, the almost continuous moss cover in the Kobresia-dominated communities was mostly a carpet of acrocarpous species, typically Didymodon rigidulus var. icmadophila, Encalypta alpina, Distichium cf. capillaceum, and rarely Desmatodon leucostoma and Aloina cf. brevirostris, supporting creeping stems of Myurella julacea and pleurocarps such as Campylium spp., Pseudocalliergon turgescens, Tomenthypnum nitens, and Drepanocladus spp. Small thallose and foliose lichens also grew over the moss carpet. Such moss communities are not recorded in modern Kobresia-dominated communities (as cited above) and may have no modern analogue, as loess deposition is a relatively rare process today. The acrocarpous mosses were able to grow through the accumulating loess, and may have e!ectively blocked competition from larger pleurocarps in dry habitats. Many taxa from the buried surface have a requirement for calcareous soil today (Tables 3 and 5) (HulteH n, 1968; Steere, 1976). The soil analyses (Table 2) (Hoe#e, 1995) give direct evidence for a high carbonate content and a high ph, resulting from loess deposition. In modern tundra, the key environmental parameters in#uencing #oristic composition are slope, aspect, substrate ph, and moisture availability (Murray and Murray, 1975; Schweger, 1982; Ritchie, 1984; Murray, 1992, 1997). Unfortunately, it was not possible to determine the palaeoslopes and aspects for the buried-surface sites, so their in#uence on the vegetation of the buried surface is unknown. However, the majority of sites supported an open, Kobresia-dominated community on calcareous soil with high ph, maintained by the continuous addition of small amounts of loess. Moisture conditions varied with local topography and micro-topography, resulting locally in the occurrence of di!erent plant assemblages. A number of taxa are conspicuous by their absence from the buried surface. Ericads, Betula, Selaginella sibirica, and Dryas are all missing from the assemblages recorded from the buried surface. These taxa are deciduous or semi-evergreen with leathery leaves. If they were present on the landscape we would expect to "nd their remains preserved and easily seen upon examination of the surface. The absence of Ericads and Betula may suggest that environmental conditions were beyond their tolerance (i.e. dry, windswept, and cold). The absence of Ericads can also be explained by the preference of the majority of these taxa for acidic soils. Other taxa common in arctic vegetation today are also not recorded, such as Fabaceae and Saxifraga other than S. oppositifolia. Potential fossils of many of these taxa are often sparsely produced and poorly preserved. The moss Rhytidium rugosum is also surprisingly not recorded from the buried surface. It is very commonly associated today with Dryas-dominated and open grass-herb communities on calcareous soil in Alaska and the Yukon. Artemisia is a prominent pollen type in full-glacial Beringian pollen spectra, but we only found one doubtfully identi"ed seed of Artemisia. Macrofossil remains of Artemisia are rare, and may not be well preserved. However, Artemisia may have been genuinely rare in our vegetation, and the contemporaneous pollen recorded in upland Beringia may be largely long-distance transported (cf. Birks and Mathewes, 1978). The absence of Dryas is one of the main di!erences between modern calcareous tundra communities and our fossil vegetation. Its apparent absence is di$cult to explain, as it is calciphilous and drought tolerant. Dryas macrofossils were also rare or absent in full-glacial samples from Eastern Beringia examined by Matthews (1982) though some Dryas remains have been found in older deposits (Matthews and Ovenden, 1990). Dryas spp. are widespread in modern montane and arctic communities and on the Arctic Coastal Plain but are absent or are minor components in steppe-like vegetation remnants (e.g. Yurtsev, 1982; Cooper, 1986) and the graminoiddominated vegetation on pingos in northern Alaska (Walker, 1990; Walker et al., 1991). Dryas spp. are typically pioneers on calcareous soils, but they need soil disturbance by frost or soil erosion to regenerate. On stable soils their abundance is reduced in time due to competition with grasses and sedges (Elkington, 1971). Dryas octopetala is a poor competitor in conditions of high available phosphorus and nitrogen that encourage the growth of graminoids (Je!rey and Pigott, 1973). In the 21,500 year old ecosystem, phosphate may have been continuously supplied by loess deposition (e.g. Jacobson and Birks, 1980) and rapid turnover of dead plant material and mineralization of nitrogen resulted in the low C/N ratios (Table 2). The successful growth of graminoids over thousands of years, combined with grazing of the #owers and seed heads of Dryas by the abundant small mammals, may have excluded Dryas from the vegetation. Another explanation for the absence of the above taxa should be considered. We only have a few small samples from a relatively large area, so we cannot exclude the possibility that Dryas and the other species were locally present but not sampled or recorded by us. 9. Discussion The vegetation of the buried surface appears to have no precise modern counterpart that has been described in

10 144 V.G. Goetcheus, H.H. Birks / Quaternary Science Reviews 20 (2001) 135}147 the literature. We can "nd analogues for the physiognomic vegetation type, but although the compositions of the modern communities overlap in part, they do not contain exactly the same species assemblages, and usually contain Dryas. The dry-soil vegetation of the buried surface matches best with the dry meadow and Kobresia meadow plant community-types described by Yurtsev (1982, unpublished manuscript). The species compositions do not, however, exactly match that of the buried surface, but similarities in structure and dominant plant genera suggest that some analogy can be drawn between them. The individualistic behaviour of steppe and tundra species was proposed by Murray et al. (1983) and Walker et al. (1991) in connection with the composition of the full-glacial steppe-tundra of Beringia. The individual responses of tundra plants on the modern landscape to changing conditions were demonstrated by Edwards and Armbruster (1989) and Lloyd et al. (1994) along an altitudinal gradient in eastern Alaska. They showed how species have the potential to combine in di!erent assemblages under di!erent environmental conditions. Present-day steppe-tundra communities are Holocene combinations of species in situations where environmental conditions resemble those of the full-glacial, namely open, calcareous, well-drained soils, and are the current version of a continually changing community (Murray et al., 1983). However, the full-glacial environment of strong continentality and aridity (Barry, 1982), combined with strong winds (Harrison et al., 1992) and loess deposition in certain areas of Beringia, including the Seward Peninsula (Hopkins, 1982), does not occur today. Following their individual tolerances, plants combine into di!erent communities under di!erent environmental regimes, as proposed by Gleason (1926) and Griggs (1934), and demonstrated experimentally by Chapin and Shaver (1985) and Chapin (1995) Guthrie (1990), considered that no exact analogue exists at the biome level for the environment of fullglacial Beringia, and proposed the occurrence of an extinct biome, the &mammoth steppe'. His reconstruction is based on pollen evidence alone and the need of large mammal populations for adequate grazing. If a biome is de"ned as a community of plants and animals, then Guthrie is correct that no analogue can exist because so many of the mammals known to have inhabited Beringia during the LGM are extinct. However, looking solely at the vegetation, analogues for the structure and composition of the vegetation are possible. Our macrofossil evidence, including the important evidence from the mosses, together with the macrofossil and Coleopteran evidence of Elias et al. (1996, 1997), and the careful, taxonomically re"ned pollen analyses of Cwynar (1982) demonstrate that the full-glacial vegetation can be satisfactorily reconstructed, at least at a landscape level (Schweger, 1982; Ritchie, 1984) using broad and partial modern analogues, as found in diverse parts of Beringia today. The Kobresia-dominated vegetation on the Seward Peninsula would have provided forage for grazing mammals, whose presence in Beringia during the full-glacial is attested by the "nds of bones (e.g. Guthrie, 1990). Direct evidence that large mammals ate grasses and sedges, including Kobresia, is provided by the analyses of stomach contents of frozen mummies (Ukraintseva, 1993). The ages of the animal fossils range through the full-glacial period, and it is probable that the open steppe-like vegetation provided food for them and for smaller mammals throughout this time. The relative roles of small and large mammals in maintaining the Kobresia communities cannot be estimated, but the direct evidence of abundant small-mammal droppings on the soil surface and the discovery of the Microtus nest demonstrate that there was a long-term balance between plant growth and grazing in this ecosystem. Similar modern communities are able to provide grazing for large animals such as Dall sheep (Cooper, 1986), and for an often great density of small mammals (Walker, 1990). The full-glacial vegetation of the loess area of the Seward Peninsula reconstructed here, almost in the centre of Beringia, contrasts with the lowland mesic shrub tundra reported by Elias et al. (1996, 1997) for parts of the central Land Bridge, some of which are only about 100 km away. The important environmental e!ects of loess deposition may largely be responsible for the di!erences, and point to the presence of a mosaic of landscapes and environments across this part of Beringia at this time, dependent on areas of loess deposition and ground moisture. 10. Conclusions The results of the macrofossil analyses of the samples of in situ vegetation from a landscape that was buried by deep tephra ca. 21,500 years ago show that the northern Seward Peninsula was covered by dry, meadow and herb-rich tundra, often dominated by Kobresia myosuroides with a mixture of grasses, sedges, and herbs and a continuous understorey of mosses. Prostrate shrubs were a minor, though locally important, part of the vegetation. Snowbeds and shallow hollows provided damper habitats for more moisture-demanding species assemblages. Acrocarpous mosses were an important element of the vegetation. The vegetation seems to have no exact modern analogue, although it shares similarities with presently restricted vegetation types of dry graminoid and herb tundra of open, arctic, and very dry habitats in Siberia and Alaska today. The vegetation di!erences are probably caused by di!erences in the full-glacial environment.

11 V.G. Goetcheus, H.H. Birks / Quaternary Science Reviews 20 (2001) 135} Deposition of loess rich in basic cations (e.g. calcium) played an important role in in#uencing the vegetation composition. The occurrence of dry Kobresia- and graminoid-dominated tundra in central upland Beringia compared with the occurrence of mesic shrub-tundra on the nearby lowland Land Bridge demonstrates the presence of a mosaic of landscapes and environments in Beringia during the full-glacial period. Acknowledgements David Hopkins discovered the buried surface on the Seward Peninsula, and recognized its potential value for the reconstruction of the palaeoenvironment of fullglacial Beringia. To him we owe the greatest thanks for originating and stimulating the work reported here. We also thank the National Park Service and the Centre for Global Change and Arctic System Research for funding this research, and Jeanne Schaaf and Rich Harris of the Park Service who provided support for this project. 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