1 1 The Holocene 12,2 (2002) pp. 177–186 2

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27 Holocene vegetation history from the

28 Salym-Yugan Mire Area, West Siberia

1 2,3 2 29 Aki Pitka¨nen, * Jukka Turunen, Teemu Tahvanainen and 2 30 Kimmo Tolonen

1 31 ( Karelian Institute, Section of Ecology, University of Joensuu, PO Box 111, 2 32 FIN-80101 Joensuu, Finland; Department of Biology, University of Joensuu, 3 33 PO Box 111, FIN-80101 Joensuu, Finland; Department of Geography and the

34 Centre for Climate and Global Change Research, McGill University, 805

35 Sherbrooke Street West, Montreal, Quebec, H3A 2K6, Canada)

36 Received 21 November 2000; revised manuscript accepted 25 July 2001

37 Abstract: The pollen stratigraphy of an ombrotrophic patterned ridge-hollow raised bog in the Salym-Yugan Mire Area in boreal West Siberia (60°10ЈN, 72°50ЈE) covers the entire Holocene period. Pollen data from three parallel peat cores suggest that, contrary to previous assumptions, Betula forests did not spread into until the Boreal period (9000–10000 cal. BP). After 9000 cal. BP, Pinus sylvestris and Picea abies forests displaced Betula forests in the area and dominated until 4100–4300 cal. BP, when Picea decreased considerably due to a climatic change and Pinus sylvestris became the most abundant tree species. Average pollen influx estimates during the wooded period, from about 9000 cal. BP onwards, were 5600–6350 grains cm–2 yr–1, similar to pollen-trap estimates from boreal coniferous forests.

Key words: Paleoecology, palaeoclimate, vegetation history, boreal region, raised bog, mire, West Siberia, 38 Holocene.

49 50

51 Introduction part of West Siberia, (Figure 1). Salym-Yugan Mire is part 71 of the Great Vasyugan Mire, which covers an area of 5.4 million 72 52 West Siberia refers to the area between the Ural mountains and hectares (Botch and Masing, 1983). The climate is continental, 73 53 the river, largely occupied by the - basin, lying with mean January and July temperatures of –22°C and +16°C. 74 54 25–95 m above sea level. Mires of the West Siberian lowland The mean annual temperature is –3.2°C and the annual precipi- 75 55 cover 50–75% of the land (Romanova, 1967) and provide about tation 400–500 mm (Russia’s Weather Server, 1999). The study 76 56 60% of all the peat deposits of the former Soviet Union and 40% area is situated about 200 km south of the permafrost limit (Baulin 77 57 of the global peat deposits (Walter, 1977). Knowledge of the veg- and Danilova, 1984). 78 58 etation history of this vast West Siberian area is important for a The study area is covered with wooded or open mires with 79 59 better understanding of climatic changes in the Northern Hemi- islands of dry uplands. The wooded islands and dry soils on river 80 60 sphere during the Holocene. Present knowledge is based mostly banks comprise about 20% of the land area. The landscape is 81 61 on data collected from sites along the Ob and Irtysh rivers almost flat, and the absolute heights in the area range between 58 82 62 (Khotinskiy, 1984), and the palaeoclimatic reconstructions are and 65 m a.s.l. Latin names of the tree species are according to 83 63 based on only a few dated pollen diagrams (Velichko et al., 1997; Ha¨met-Ahti et al. (1998). The wooded mires are dominated 84 64 Peteet et al., 1998; Khotinskiy, 1984; Blyakharchuk and Sulerzhit- mainly by Scots pine (Pinus sylvestris L.) with scattered Siberian 85 65 sky, 1999). Our aim is to reconstruct the Holocene vegetation pines (Pinus cembra L. ssp. sibirica (Du Tour) Krylov). The for- 86 66 history in the middle boreal vegetation belt from peat cores from ests on dry uplands consist of pure young Pinus sylvestris stands. 87 67 the Salym-Yugan Mire. Birch (Betula pubescens Ehrh.) forests and a few large Siberian 88 pines are found along riverbanks. Siberian fir (Abies sibirica 89 Ledeb) larch (Larix sibirica Ledeb) and spruce (Picea abies L. 90 68 Study area Karst. ssp. obovata (Ledeb) Hulten) are completely absent on the 91

69 The study area is situated in the Khanty-Mansi Autonomous Dis- dry uplands in the study area. However, Picea abies and Abies 92 70 trict of the Tyumen Region (60°10ЈN, 72°50ЈE), in the central sibirica are relatively abundant about 10–20 km north of the study 93 area, where dry uplands are more prevalent in the landscape. 94 17 *Author for correspondence (e-mail: aki.pitkanenȰjoensuu.fi) Paludification has been a strong process since the end of the last 95

12  Arnold 2002 10.1191/0959683602hl533rp

1 1 1 178 The Holocene 12 (2002) 2

534 1312

14 15 Figure 2 Aerial view of the study site and the coring points (A, B and 16 6 7 C). The point studied by Liss and Berezina (1981) is indicated by L. 17 8 Figure 1 Location of the study area (indicated with an asterisk). Sites (Photograph provided by D.L. Varlyguin.) 18 9 referred to in text indicated by crosses (x). (1) Pur-Taz site. (2) Niznevar- 10 tovsk site. (3) Bugristoye bog. (4) Sverdrup island. all identified pollen taxa and unidentified pollen (Varia). Spores 125 were excluded from the total pollen sum. Spore values were based 126 96 glaciation (Walter, 1977). However, the mire has expanded on the sum of spores (excluding Sphagnum) and total pollen. 127 97 slightly during recent millennia and was nearly to its present width Sphagnum values were calculated from the sum of Sphagnum 128 98 7000–8000 years ago (Turunen et al., 2001). spores and total pollen. For absolute pollen values, the volume of 129 99 According to Grosswald (1980; 1998), the study area was pollen samples was based on the volume-mass samples of the peat 130 100 covered during the last glaciation by an ice-dammed lake, which cores (Turunen et al., 2001). Pollen influx values were calculated 131 101 was formed by an ice sheet centred in the Kara Sea area. The according to the formula (C*L)/T, in which C = average pollen 132 –3 102 latest studies (Mangerud et al., 1999; Svendsen et al., 1999) indi- concentration of samples (pollen grains cm ) between two 133 103 cate that the maximum extent of the Eurasian ice sheet in the Kara radiocarbon datings, L = length (cm) of the sequence between 134 104 Sea region during the Late Weichselian has been overestimated datings, and T = time period (cal. BP yr) in the peat sequence. 135 105 and that the great rivers of West Siberia had free outlets to the The treeless vegetation phase is evident in the pollen assem- 136 106 Kara Sea. However, the existence of ice-dammed lakes during blages. However, the long-distance transport of arboreal pollen 137 107 earlier phases of Weichselian glaciation is possible. (AP) and the weak pollen production of non-arboreal pollen 138 (NAP) may obscure the interpretation of the data (Birks and Birks, 139 1980). Studies from tundra areas indicate that about 60–25% of 140 108 Material and methods the pollen consists of NAP taxa (including Cyperaceae and 141 Poaceae as major taxa), reflecting a treeless vegetation (Aario, 142 109 Three peat cores were taken from the central plateau along a tran- 1940; 1943; Ritchie, 1974; Ritchie and Lichti-Federovich, 1967; 143 110 sect at 800 m intervals in September 1998 with a Russian side- Lichti-Federovich and Ritchie, 1968). In forest tundra areas, 144 111 cutting peat sampler 50 × 500 mm (Jowsey, 1966). Two peat cores which are open wooded landscapes, the proportion of NAP is 145 112 (4.54 and 5 m long) with radiocarbon datings from several depths lower (18–12%) (Ritchie, 1974; 1977; Lichti-Federovich and Rit- 146 113 were chosen for pollen analyses (cores A and B). In addition, chie, 1968). In this study, AP values less than 25% of the total 147 114 pollen samples were analysed from the basal peat of core C, which pollen are interpreted to indicate treeless vegetation. 148 14 115 had the oldest radiocarbon age (Figure 2). Peat subsamples of 1 Peat samples taken for C dating (Table 1) were analysed at 149 3 116 cm were taken from the basal 10 cm at continuous 1 cm intervals, the University of Helsinki Dating Laboratory. All results were 150 13 117 at 2 cm intervals between 10 and 20 cm from the base and at 10 corrected for isotopic fractionation based on the C values given 151 118 cm intervals for the rest of the cores. The pollen samples were in Table 1. Radiocarbon ages were converted to calendar years 152 119 weighed and treated according to procedures described by Ber- using CALIB 3.0.3 (Stuiver and Reimer, 1993). Two additional 153 120 glund and Ralska-Jasiewiczowa (1986), excluding sieving of datings (Table 1) from the marker level of an abrupt increase of 154 121 samples. A minimum of 200 arboreal pollen grains were counted, pine and birch pollen in the study area were made by Liss and 155 122 but in a few samples poor in pollen the minimum number of tree Berezina (1981). 156 123 pollen counted was 50 pollen grains. The relative pollen values The names of chronozones and their ages (converted to cali- 157 124 of trees and herbs were based upon the total pollen sum including brated radiocarbon years) used in this paper follow Mangerud 158

1 1 1 Aki Pitka¨nen et al.: Holocene vegetation history from a mire in West Siberia 179 2

74373 75 Table 1 Peat radiocarbon datings from the studied points. Two additional an open treeless landscape. If Cyperaceae is excluded from the 189 76 datings (TA-933 and TA-934) are based on marker levels of abrupt pollen sum, the AP percentage value is 63–65%. Pinus pollen was 190 77 increases in birch (1) and pine (2) pollen (Liss and Berezina, 1981). Peat almost absent in this zone except in core A, where a small amount 191 = = = = 78 constituents: C Carex;Er Eriophorum;Eq Equisetum;L wood; of pine pollen was found in all samples. In core A, mineral par- 192 79 Mn = Menyanthes;N= shrub; S = Sphagnum; Sch = Scheuchzeria;H= ticles were abundant in the bottom 30 cm of peat, but in cores B 193 80 degree of decomposition in von Post’s (1922) 10-grade scale. Calendar and C only in the 3 cm layer above the base. Mineral matter and 194 81 date refers to the calibrated calendar years (Stuiver and Reimer, 1993) 82 relatively abundant Pinus pollen suggests a mixing of peat in core 195 8384 196 13 14 A that is not seen in cores B and C. The mixing may have been 85 Site Sample Peat ␦ C C date Calibrated caused by cryogenic processes in permafrost soil (see Velichko 197 depth material (yr BP) age (cm) (yr BP) and Nechyev, 1984). While the values of Picea and Betula were 198 97 between 20 and 40%, relative AP values calculated versus NAP 199 9899 indicate significant long-distance transport of AP taxa (Figure 6). 200 100 A95–100 Sch-C-S, H3 –23.1 850 ± 80 780 195–200 N-Er-S-C, H4 –25.9 3690 ± 120 4110 Larix pollen values up to 3.5% were found. The average Larix 201 295–300 N-Sch-S-C, H3 –24.9 5530 ± 140 6350 values in cores A and B were 0.7 and 0.9%, respectively (Figures 202 410 9000 2) 3, 4 and 5). 203 420 9910 1) Ephedra pollen was present in a few samples in this zone, but 204 ± 490–495 Mn-Eq-C,H6 –28.5 9420 110 10430 was absent in higher peat layers. Filipendula, Rubiaceae, Apia- 205 ± 495–500 Mn-Eq-C, H6 –28.6 8940 130 10000 206 136 ceae and Lamiaceae pollen were found in this zone, mostly in the 137 B95–100 S-C, H6 –26.1 1760 ± 80 1740 lower samples. Values of Equisetum and Polypodiaceae spores 207 195–200 N-Er-C-S, H5 –26.7 3860 ± 100 4310 were considerably higher than in the pollen zones above. How- 208 245–250 Sch-C-S, H5 –27.2 5550 ± 100 6360 ever, in relation to NAP, Equisetum and Polypodiaceae in the 209 ± 295–300 S, H4 –26.6 6220 130 7210 NAP zone are equally abundant as in other zones, suggesting that 210 ± 345–350 Er-C-S, H4 –26.2 6980 110 7805 the higher values are caused by the low representation of AP taxa. 211 393 9000 2) Bryales spores were not quantified but were very abundant in 212 413 9910 1) several samples. In higher zones, the Bryales spores were gener- 213 444–449 N-S-C, H4 –28.4 9640 ± 120 10945 ally much less abundant. 214 449–454 N-S-C, H4 –28.7 9250 ± 110 10330 183

184 C 310 9000 2) AP-NAP zone 215 330 9910 1) This zone is defined between the stratigraphic level at which AP 216 330–335 N-C, H6 –27.8 10370 ± 130 12305 percentages reach an average level of 90%. The lower limit is 217 335–340 N-C, H6 –27.7 9530 ± 100 10835 199 defined by an abrupt increase in Betula and Pinus pollen (Figures 218 200 3, 4 and 5). The amount of Picea pollen apparently begins its 219 increase prior to Betula and Pinus pollen as indicated by the 220 159 et al. (1974). The limits of chronozones sensu Mangerud et al. Picea/NAP pollen ratio (Figure 6). There is a decline in Picea 221 160 (1974) are not fully equivalent with those of biozones used in pollen values between 413 and 393 cm in core B, and correspond- 222 161 Russian literature (Khotinskiy, 1984). ing declines are also found in cores A and C (Figures 3, 4 and 223 5). The continuous presence of Ericaceae pollen begins in this 224 zone at the level where Betula pollen increases. The relative abun- 225 162 Results and interpretation dance of NAP pollen and spores declines due to an increase in 226 AP taxa (see Figure 6). 227 163 The pollen stratigraphy of cores A and B is presented in Figures 164 3 and 4. The pollen diagram from the base of core C is shown in AP1 and AP2 zones 228 165 Figure 5. According to the main components and changes in The AP1 zone in core A (380–310 cm) and core B (383–273 cm) 229 166 pollen assemblages, the stratigraphy can be divided into 5 zones. is similar to the AP2 zone in both cores and differs only in the 230 virtual absence of Abies pollen. Abies pollen is found continuously 231 167 NAP zone in core A at 310 cm and in core B at 273 cm. The establishment 232 168 NAP taxa dominate between the base of peat and a major increase of the Abies pollen curve in core A occurred about 7500 cal. BP, 233 169 in Betula pollen (Figures 3 and 4). In core C, the Betula pollen as extrapolated from the radiocarbon age at the 300 cm level. The 234 170 increases 10 cm from the base of peat, suggesting a considerably corresponding age estimate for the Abies curve in core B is 7700 235 171 lower peat-accumulation rate in the basal level compared with cal. BP (Figures 3 and 4). The age of the lower limit of the AP1 236 172 cores A and B (Figure 5). The increase in Betula pollen at another zone is about 8700 cal. BP. The abundance of Picea pollen, with 237 173 site is dated to 9910 cal. BP (Table 1). values up to 20%, is characteristic of this zone, indicating that 238 174 The most abundant taxon is Cyperaceae, which increases in Picea was abundant in the study area. 239 175 cores A and B from about 20% in the basal peat to 60–80% near 176 the upper limit of the NAP zone. In core B, the ratio of Cypera- AP3 zone 240 177 ceae to other NAP taxa indicates that Cyperaceae pollen was twice Decreasing Picea pollen values begin at the 2 m level in cores A 241 178 as abundant in the NAP and AP-NAP zones compared to the and B (4100–4300 cal. BP) and decline to less than 5%. At the 242 179 zones above. Also, the values of other NAP taxa are higher in same time, Pinus sylvestris and P. cembra pollen values increased 243 180 this zone, including Salix, Artemisia, Chenopodiaceae, and Betula pollen decreased. The decrease in Betula pollen values 244 181 Asteraceae, Cichoriaceae, Poaceae, Rosaceae, Caryophyllaceae is probably caused by an increase in Pinus pollen, since the 245 182 and undefined herbs (Varia). They are most abundant in the basal Betula/NAP ratio showed no decrease in Betula pollen (Figure 6). 246 183 10 cm of cores A and B (Figures 3 and 4). The ratio of Poaceae In the AP3 zone the average AP percentage increased slightly 247 184 to AP pollen indicates that there are on average 40 Poaceae pollen in core B, from the previous 90.7% (NAP2 and NAP1 zones) to 248 185 grains to 100 AP pollen in this zone (Figure 6). That is about 30 94.3%. The difference is highly significant on Student’stest(t= 4, 249 186 times higher in value than in zones above. The AP percentage of df:28, significance level 0.01). The change may be due to a 250 187 total pollen ranges between 10 and 48%, and the averages in cores decrease in Cyperaceae pollen from an average 3.5% in the AP2 251 188 A, B and C were 37%, 29% and 28%, respectively, suggesting and AP1 zones, to 1.1% at the limit of the AP3 zone. Since the 252

1 1 1 180 The Holocene 12 (2002) 2

22321 Core A

isia Depth C ages (yr BP) cm Lithology 14 Picea Pinus sylvestris Pinus AbiescembraLarix Betula AlnusEricaceaeSalixArtemChenopodiaceaeAsteraceaeCichoriaceaeCyperaceae PoaceaeApiaceaeVaria Arboreal pollenLycopodiumPolypodiaceaeEquisetum Spores Sphagnum Zone

40 ErS, H12

60 ErCS, H3 80 100 780 SchCS, H3 120 AP3 140 SchSC, H4-5 160 180 200 4110 220 240 NSchSC, H3-5 260 AP2 280 300 6350 320

340 AP1 360 NSC, H4-5 380 400 AP-NAP MnC, H4 420 440 MnEqCB, H3-4 460 NAP

480 NEqC, H5-6 10430 500 % 23 0 20 0 20 40 60 10 5 5 0 20 40 60 5 5 10 5 5 5 5 0 20 40 60 10 5 10 0 40 80 5 10 0 20 0 20 40 0 20 40 60 80 24 25 Figure 3 Relative pollen diagram of selected taxa from core A. Peat constituents: C = Carex;Er= Eriophorum;Eq= Equisetum;L= wood; Mn = 26 Menyanthes;N= shrub; S = Sphagnum; Sch = Scheuchzeria;B= Bryales;H= degree of decomposition in von Post’s (1922) 10-grade scale.

3029 Core B

yophyllaceae Depth C ages (yr BP) boreal pollen Spores cm Lithology 14 Picea Pinus sylvestris Pinus cembraAbiesLarix Betula AlnusEricaceaeSalixArtemisiaChenopodiaceaeAsteraceaeCichoriaceaeCar Cyperaceae PoaceaeVaria Ar LycopodiaceaePolypodiaceaeEquisetum Sphagnum Zone 20

40 ErS, H1-3

60 ErSchCS, H3-4 80 SchSC, H4-6 100 1740 AP3 120 NCS, H4-5

140 ErSC, H6 160 NSchSC, H5 180 CS, H3-4 NErCS, H5 200 4310 ErSC, H7-8 220 SchSC, H5 240 AP2 6360 260 NS, H4-5 280 7210 300 S, H3-4 320 NErS, H3-4 AP1 340 ErCS, H4 7805 360 NErC, H4 380

400 NMnC, H4-5 AP-NAP

420 SC, H3-4 NAP 440 NSC, H4 10945

31 % 0 20 0 20 40 60 0 20 5 5 0 20 40 60 5 10 5 5 5 5 5 5 0 20 40 60 80 10 10 0 40 80 5 10 0 20 0 10 20 30 0 20 40 60 80 32 33 Figure 4 Relative pollen diagram of selected taxa from core B. See Figure 3 for explanation of peat type symbols.

1 1 1 Aki Pitka¨nen et al.: Holocene vegetation history from a mire in West Siberia 181 2

37336 Core C

BP) is yr r en ( aceaeaceae vest pol l a yll aceaeum ages sy l si iaceae aceae al s og y s a aceae podi set Depth ol C ul ix em i yop h er x ia th 14 cea nu t cho r ricaceae bore cm Li Pi Pi Bet Lar Ar Car C henopodiCi Ast Cyper E PoaceaeSali Var Ar Poly Equi Spore Zone 280

C, H4 290 AP1 NC, H6 30 0

310 C, H8

32 0 AP-NAP NC, H7

330 12305 NAP 340

0 20 40 60 80 38 % 0 10 0 20 40 0 20 40 60 80 5 5 5 5 5 5 0 20 40 60 0 10 10 5 5 5 0 20 0 10 20 30 39 40 Figure 5 Relative pollen diagram of selected taxa from core C (280 cm to bottom). See Figure 3 for explanation of peat type symbols.

4443

45 46 47 Figure 6 Ratio of Picea/NAP (a), Abies/NAP (b), Betula/NAP (c) Pinus/NAP (d), Picea/spores (Equisetum, Polypodiaceae and Lycopodiaceae) (e), Poaceae/AP 48 (f), AP/NAP (Cyperaceae excluded) (g), AP/NAP (h) and NAP/spores (excluding Sphagnum) (i). The average Picea/NAP value between 1740 cal. BP 49 and present was 0.6 (s.e. Ϯ0.1, s.d. 0.4), between 4310 and 9000 cal. BP 1.4 (s.e. Ϯ0.2, s.d. 0.8), prior to 9000 cal. BP 0.14 (s.e. Ϯ0.02, s.d. 0.08). 50 Although values after 1740 cal. BP probably indicate transport of spruce pollen from sources at least several kilometres from the site, Picea/NAP values 51 prior to 9000 cal. BP are considerably lower, suggesting the absence of spruce in the landscape. Increasing values of Picea/NAP ratio from 450 to 383 52 cm without any fluctuations suggest that spruce was possibly approching the study area during the Boreal period. The data are from core B.

253 Cyperaceae/NAP ratio indicates that Cyperaceae pollen decreased Pollen concentration and influx 260 254 to half from the previous level in relation to other NAP taxa, The pollen concentration values are highly variable between 261 255 the decrease in Cyperaceae percentage values indicates an actual samples (Figure 7). During the wooded phase (after 9000 cal. BP) 262 256 decline in its abundance. However, in core A there is no corre- pollen concentrations ranged between 13410 and 908510 263 3 257 sponding change in the average AP and Cyperaceae percentages grains/cm . In the NAP zone, the concentrations were between 264 3 258 at the AP3 zone limit. The absence of this change in core A indi- 4660 and 760550 grains/cm . Pollen influx estimates are shown 265 259 cates that the change in core B must be local. in Figure 8. The average influx values from the present to 9000 266

1 1 1 182 The Holocene 12 (2002) 2

55354 Discussion 271

Age of the basal peat and absolute pollen values 272 The NAP zone covers the Preboreal (about 10000–11000 cal. BP) 273 period and the zone’s upper limit is in the early Boreal period 274 (about 10 000–9500 cal. BP). The basal peat datings in cores A 275 and B are only slightly different from the dating for the strati- 276 graphic upper level of the NAP zone from another site (Table 1). 277 Because there is 40–80 cm peat between the upper level of the 278 NAP zone and the base of peat in cores A and B, the radiocarbon 279 ages for the basal peat of cores A and B may be too young. Peat 280 radiocarbon ages that are too young are fairly common and are 281 caused by the downward transport of younger carbon by roots or 282 water flow (Tolonen et al., 1992). Since the pollen stratigraphy of 283 the basal peat in all cores suggests corresponding ages, possibly 284 the basal peat in cores A and B are as old as the base of core C. 285 The basal peat layers of core C date back to Lateglacial times, 286 to the Younger Dryas period (about 11000–13000 cal. BP). The 287 relatively abundant NAP taxa in the NAP zones of cores A, B 288 and C correlate well with pollen assemblages between 8600 and 289 10600 radiocarbon years BP at a site near Niznevartovsk, about 290 200 km northeast of our study site (Khotinskiy, 1984). Pollen 291 –2 influx estimates based on radiocarbon datings are 6300 grains cm 292 –1 –2 –1 yr for the core A and 3900 grains cm yr for the core B NAP 293 zone. One could assume that cores A and B are of the same age 294 as suggested by the pollen stratigraphy. If the pollen influx esti- 295 mate for core A is calculated according to the radiocarbon age of 296 the base of core B, the pollen influx estimate for the NAP zone 297 –2 –1 of core A is about 3000 grains cm yr . The age of the basal 298 peat in cores A, B and C may also be considerably older than 299

56 radiocarbon ages suggest. High NAP percentage values and 300 57 AP/NAP ratio (Figures 3, 4, 5 and 6) indicate open treeless veg- 301 58 Figure 7 Pollen concentration curves of cores A (2) and B (1). Abrupt etation, with most of the NAP consisting of dominating elements 302 59 increase in Betula pollen (about 9900 cal. BP, the upper limit of the NAP of open tundra pollen assemblages (Cyperaceae, Salix and 303 60 zone) is indicated with a dotted line. Poaceae) (Gajewski, 1995; Gajewski et al., 2000; Prentice et al., 304 1996). Pollen trap studies indicate very low total pollen influx 305 –2 –1 (less than 500 grains cm yr ) under open tundra conditions 306 (Hicks, 1986). Therefore, the high pollen concentration values in 307 6362 the NAP zone would suggest a much higher age for the basal peat 308 than suggested by the radiocarbon ages. However, peat initiation 309 data from other sites of West Siberia suggest that peat accumu- 310 lation started not earlier than 9000–10600 radiocarbon years BP 311 in this region (Peteet et al., 1998). 312 Absolute pollen values are not determined in Siberian mires. 313 The maximum pollen concentration values reported by Jasinski 314 et al. (1998) at a site in the Lena river valley (open larch forest 315 tundra) are at the same magnitude as values from our study site, 316 –3 and the minimum values (3400–12000 grains cm ) are of same 317 magnitude as the minimum values in the NAP zone. 318

Interpretation of tree pollen percentage values 319 The AP values ranging 20–60% in cores A, B and C in the NAP 320

321 64 zone (Figures 3, 4 and 5) correspond to values found at tundra 65 areas (Aario, 1940; 1943; Ritchie, 1974; Ritchie and Lichti-Fed- 322 66 Figure 8 Average pollen influx values calculated from cores A and B. erovich, 1967; Lichti-Federovich and Ritchie, 1968) and thus indi- 323 67 Estimates calculated from core B are indicated in the grey columns. The cate treeless vegetation. In treeless regions, relative AP values can 324 68 estimates are calculated for the following time periods. (a) Present to 780 325 69 cal. BP. (b) 780–4110 cal. BP. (c) 4110–6350 cal. BP. (d) 6350–9000 cal. be high due to long-distance transport of tree pollen and to poor 70 BP. (e) 9000–9910 cal. BP. (f) prior to 9900 cal. BP. (g) Present to 9000 pollen production of local NAP taxa. For example, in Arctic 326 71 cal. BP. Sverdrup Island, 100 km north of the coast of the Kara Sea, the 327 highest Picea values are about 20% and highest tree Betula values 328 20–30% of total pollen in peat samples (Andreev et al., 1997). At 329 Somerset Island, Canada (about 1000 km north from the forest 330 limit), long-distance pollen comprises 20–40% of total pollen 331 –2 –1 267 cal. BP in cores A and B were 5600 grains cm yr and 6350 (Picea 5–25% and Betula 5–20%) in surface lake-sediment 332 –2 –1 268 grains cm yr , respectively. The average influx estimate for samples (Gajewski, 1995). The most important local taxon in 333 269 NAP taxa between the present and 9000 cal. BP is only about Somerset Island is Cyperaceae, which comprises 10–60% of pol- 334 –2 –1 270 500 grains cm yr . len in surface-sediment samples. Poaceae and Oxyria digyna pol- 335

1 1 1 Aki Pitka¨nen et al.: Holocene vegetation history from a mire in West Siberia 183 2

3

336 len are also important local taxa in surface-sediment samples, site in Taimyr peninsula during the past 6000 years. The data of 403 337 comprising 5–40% of pollen. At Banks Island (western Arctic Clayden et al. (1997) suggest a Larix/NAP ratio between 0.2 and 404 338 Canada), about 600 km from the northern timber limit, there is 0.8. At our study site, the values of Larix/NAP ratio (mean = 405 339 up to 50% long-distance pollen (Alnus 0–20%, Picea Ͻ5–30% 0.017, SD = 0.019) in the NAP zone were 10 times lower than 406 340 and Pinus 0–10%) in lake-sediment cores covering the Holocene the lowest Larix/NAP values estimated from the data of Clayden 407 341 period. In Banks Island pollen, shrub birches (5–40%), Cypera- et al. (1997), suggesting long-distance transport of Larix pollen 408 342 ceae (10–60%) and Poaceae (5–60%) are the most important local to our study site. However, only macrofossil studies can prove 409 343 taxa (Gajewski et al., 2000). The lake-sediment pollen values are with certainty whether larch trees were present in the study area. 410 344 similar to pollen trap values from tundra areas (Ritchie and Lichti- 345 Federovich, 1967). However, one could suspect that low AP 346 values in our data are result of high deposition of in situ Cypera- Treeless vegetation prior to about 9900 cal. BP in 411 347 ceae pollen, because remains of sedges are important, or even a large regions of West Siberia? 412 348 major constituent of the peat matrix in the NAP zones of cores Contrary to our results, Russian literature (Blyakharchuk and 413 349 A, B and C. The lake-sediment data quoted above indicate that Sulerzhitsky, 1999) suggests that middle West Siberia was 414 350 very high Cyperaceae percentage values are common at tundra covered with Picea-Larix-Betula forest tundra, and in the southern 415 351 sites. In eastern Finland, Cyperaceae percentage values up to 90% forest zone there were open Betula and Picea-Betula forests dur- 416 352 dating back prior to 10000 cal. BP were reported from lake ing the Preboreal period. Also, Betula forest-steppe vegetation is 417 353 sediments (Bondestam et al., 1994). Thus, the high Cyperaceae assumed to have been an important part of the present middle 418 354 percentages in the NAP zone are necessarily not of in situ origin area. According to earlier Russian vegetation estimates 419 355 or due to the mire environment. Furthermore, the AP/NAP ratio (Grichuk, 1984), trees were present during the last glacial 420 356 calculated without Cyperaceae, and the Poaceae/AP ratio (Figure maximum (LGM) about 16000–20000 radiocarbon yr BP in the 421 357 6), clearly indicate that AP is scarce in relation to dry-land NAP middle Ob river region, forming a complex of tundra, steppe and 422 358 taxa prior to 9900 cal. BP. forest vegetation. The mean summer temperatures during the 423 359 AP percentage values in the NAP zone of cores A, B and C LGM may have been similar to present, and the coldest average 424 360 are similar to those found at Banks Island and Somerset Island winter temperatures were 5°C lower than today (Velichko and 425 361 (Gajewski, 1995; Gajewski et al., 2000), suggesting that the dis- Nechyev, 1984). However, more recent climate estimates 426 362 tance to the tree-line may have been very great. The very low (Tarasov et al., 1999) suggest tundra or cool steppe vegetation 427 363 values of Betula/NAP ratio (Figure 6) prior to 9900 cal. BP sug- and summer temperatures lower than present in West Siberia dur- 428 364 gest that the peaks in Betula values as high as 40% are also due ing the LGM. 429 365 to long-distance transport (Figure 6), although a considerable pro- The contradiction between recent data and the earlier view 430 366 portion of Betula pollen in the NAP zone may originate from local regarding vegetation history during the Preboreal period in West 431 367 shrub birches (Betula nana). In Sverdrup Island (Andreev et al., Siberia may result from interpretation of the pollen data. In Rus- 432 368 1997), situated 300–400 km north from the present pine limit, sian studies, pollen-data interpretations are based only on pollen 433 369 there is about 20% Pinus pollen in the surface soil, but in samples percentages of individual taxa, and AP/NAP ratios are not used. 434 370 dating back to 9770–11640 radiocarbon yr BP Pinus pollen is The coexisting AP and NAP taxa during the early Holocene are 435 371 almost absent. Therefore the virtual absence of Pinus pollen and interpreted as representing boreal forest, steppe and tundra 436 372 low Betula/NAP values suggest that those species may have been elements, and the relatively high AP percentage values are 437 373 present at a very long distance from our study site prior to 9900 believed to indicate the presence of trees in the region (Neustadt, 438 374 cal. BP. Macrofossil data from 400 km north of our study site 1957; Khotinskiy, 1984; Kremenetski et al., 1997; Velichko et al., 439 375 indicates that tree birches were present there about 10000 cal. BP 1997; Khotinskiy and Klimanov, 1997). 440 376 (Peteet et al., 1998). At a site 1000 km south, Pinus pollen was The NAP zone in Figures 3, 4 and 5 shows the same main 441 377 virtually absent prior to 8100 cal. BP (Kremenetski et al., 1997). features as a pollen diagrams from (120 km north of our 442 378 As well, at Bugristoye bog, 600 km southeast of our study site, study site) (Neustadt, 1974) and a site near Niznevartovsk 443 379 Pinus pollen values were low prior to 9500 cal. BP (Blyakharchuk (Khotinskiy, 1984; Neustadt and Selikson, 1985), about 200 km 444 380 and Sulerzhitsky, 1999). The data above suggest that Pinus syl- northeast of our study site. Khotinskiy (1984) believed that the 445 381 vestris and P. cembra forests were absent in large parts of West Picea and Betula pollen values from the Niznevartovsk site indi- 446 382 Siberia during the Preboreal and early Boreal period and were cate the presence of forests during the Preboreal period, although 447 383 only beginning to expand across the area. the high NAP percentages of Poaceae, Cyperaceae and Artemisia 448 384 Despite the high Picea pollen percentages (10–20% in most suggest treeless vegetation. A highly humified layer 20 cm thick 449 385 samples) in cores A, B and C, the low values of the Picea/NAP containing tree stems (dated to 12500 cal. BP) in a basal peat 450 386 ratio suggest long-distance transport of Picea pollen at least deposit below a thick layer of loam was used as evidence of for- 451 387 prior to 9900 cal. BP (Figure 6). Also, the ratio of spores ests at the Niznevartovsk site (Neustadt and Selikson, 1985). 452 388 (excluding Sphagnum)toPicea pollen suggests long-distance However, the wood was possibly carried to the spot by flooding 453 389 transport of spruce pollen (Figure 6). Since the spores (excluding over long distances, since the site was situated at the shore of the 454 390 Sphagnum)/NAP ratio suggests that the influx of spores has Ob river. However, in the Pur-Taz region (about 400 km north 455 391 remained at a low, relatively constant level during the Holocene, from our study site), the macrofossil data (Peteet et al., 1998) indi- 456 392 the spores/Picea ratio may reliably indicate low influx of Picea cate that Larix-Betula pubescens forests were present from at least 457 393 pollen in the NAP zone. Picea/NAP ratio values of the same mag- 10300 cal. BP onwards. The relatively low NAP values in the 458 394 nitude as prior to 9900 cal. BP (Figure 6) can be calculated from Pur-Taz region, in most samples less than 20% (Peteet et al., 459 395 pollen data several tens of kilometres north of the Picea limit in 1998), correspond with values from forest-tundra sites (Hicks, 460 396 northern Finland (Hyva¨rinen, 1975), but also from pollen data 1986). At our study site, the NAP values were considerably higher 461 397 from Somerset Island (Gajewski, 1995), hundreds of kilometres prior to 9900 cal. BP, ranging from 52 to 90%. 462 398 north of the spruce forests. Blyakharchuk and Sulerzhitsky (1999) suggested that open 463 399 Larix pollen percentages are not proportionate to its real abun- Larix, Larix-Picea and Betula steppe forests were present around 464 400 dance, and even very low values may indicate its presence the Bugristoye bog prior to 9200 cal. BP, about 700 km to 465 401 (Clayden et al., 1997). Clayden et al. (1997) estimated a Larix southeast of our study site. The Bugristoye bog data agrees with 466 –2 –1 402 pollen influx of 100–600 grains cm yr at a larch forest tundra data from our study site, since NAP (including Cyperaceae) was 467

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468 so abundant at the site, possibly indicating instead completely in core C (Figure 5), indicates that Picea was not present around 533 469 treeless vegetation. 9900 cal. BP, but became established slightly later (Figure 6). 534 The sharp increase in Picea pollen (Figure 6) correlates well with 535 470 Preboreal climate Picea macrofossil and pollen data for 9300 cal. BP from the Pur- 536 471 Based on the statistical interpretation of pollen assemblages Taz site (Peteet et al., 1998). The increase in tree pollen between 537 472 (Klimanov, 1984), the average summer temperatures in West 9900 and 9000 cal. BP is also contemporary with the development 538 473 Siberia are estimated to be 2–3°C lower than present during the of forests beyond the present tree-line due to climatic warming 539 474 Preboreal period (Velichko et al., 1997). Despite the fact that the that started around 9500 cal. BP (MacDonald et al., 2000). 540 475 pollen data from our study site indicates treeless vegetation, Larix The sudden increase in tree pollen at our study site beginning 541 476 wood dated to Lateglacial times is found north of the present tree- with Betula pollen is simultaneous at the Niznevartovsk site 542 477 line (Velichko et al., 1997; Peteet et al., 1998). Wood macrofossil (Khotinskiy, 1984; Neustadt and Selikson, 1985) and at Bugris- 543 478 data collected by MacDonald et al. (2000) indicate presence of toye bog (Blyakharchuk and Sulerzhitsky, 1999), indicating that 544 479 spruce and birch trees beyond the present northern tree-line about large areas of West Siberia were covered with Betula after 9900 545 480 10000 cal. BP and beginning of forest development there at that cal. BP. At the Niznevartovsk site (Khotinskiy, 1984; Neustadt 546 481 time. The date corresponds well with the beginning of increase in and Selikson, 1985) the increase in Betula pollen is dated at 9500– 547 482 arboreal pollen in our data (Figures 3, 4, 5 and 6). 9100 cal. BP. The younger date is also the level at which Pinus 548 483 The presence of forests north of our study site at least 10300 pollen starts to increase. Considering the long-distance transport 549 484 cal. BP (Peteet et al., 1998) indicates that a cool climate has not of Betula and Pinus pollen, the increase of both taxa in data from 550 485 been the main cause of treeless vegetation in the study area. Esti- Niznevartovsk may be contemporary with the changes at our 551 486 mates by Velichko et al. (1997) suggest that precipitation was study site. The values of NAP taxa and spores (especially 552 487 near to present values in West Siberia during the Preboreal period, Polypodiaceae) were also high prior to 9100 cal. BP at the Nizne- 553 488 but pollen data from our study site suggest a drier climate. Abun- vartovsk site (Khotinskiy, 1984; Neustadt and Selikson, 1985), 554 489 dance of Poaceae along with Asteraceae, Cichoriaceae, Filipend- suggesting a small influx of arboreal pollen. 555 490 ula, Rubiaceae, Caryophyllaceae and Ephedra found in the basal A significant indicator of climatic change may be the increase 556 491 peat layers may indicate cool steppe conditions (Tarasov et al., in Ericaceae pollen in 9900 cal. BP at our study site, found also 557 492 1999) but Artemisia and Chenopodiaceae, also typical of steppe in other pollen diagrams based on peat samples (Neustadt and 558 493 pollen assemblages, were not particularly abundant. However, the Selikson, 1985; Liss and Berezina, 1981; Blyakharchuk and 559 494 abundance of Cyperaceae, Salix and Betula (possibly partly B. Sulerzhitsky, 1999). Ericaceae pollen indicates sufficient precipi- 560 495 nana) pollen in the NAP zone suggests tundra vegetation. The tation for forest vegetation (Prentice et al., 1996). 561 496 shrinking continental ice sheet in Scandinavia possibly caused a 497 stable anticyclone in northern Europe that reduced the invasion Forests between 9000 and 4100 cal. BP 562 498 of moist air masses from the Atlantic Ocean to Siberia Corrected pollen values (Donner, 1972) and the AP/NAP ratio 563 499 (Khotinskiy, 1984). There is evidence of low annual precipitation indicate that Betula forests of the late Boreal period changed to 564 500 in Fennoscandia (Digerfeldt, 1988; Donner et al., 1978; Huttunen coniferous (Pinus and Picea) forests during the early Atlantic per- 565 501 et al., 1978) and the Russian Plain (Chepalyga, 1984) during the iod (Figure 6). The use of corrected pollen values for data 566 502 early Holocene. interpretation must be analysed critically, because different land- 567 scapes and climatic conditions affect pollen production. Thus, the 568 503 Establishment of forests after 9900 cal. BP estimates given here must be taken as magnitudes of abundance. 569 504 The last remnants of the Scandinavian ice sheet disappeared about Corrected relative pollen values suggest that Picea was the domi- 570 505 9500 cal. BP (Eronen and Zetterberg, 1996), after which the nant tree species in forests, with an average proportion of about 571 506 influence of moist air masses from the Atlantic Ocean may have 40%. Our study site is situated in a relatively open mire, and 572 507 considerably increased in continental Eurasia. The increase in pre- therefore the pollen values may represent a large region. Picea 573 508 cipitation along the Russian Plain is reflected in the rising of the forests were one of the key elements of the West Siberian taiga 574 509 Caspian Sea about 8800 cal. BP (Chepalyga, 1984). At our study zone during the Atlantic period between 7800 and 3800 cal. BP 575 510 site, several charcoal layers were found within the peat and were (Velichko et al., 1997; Khotinskiy, 1984). Pinus sylvestris was 576 511 dated between 8000 and 9000 cal. BP (Turunen et al., 2001), almost as abundant as Picea according to corrected pollen values 577 512 suggesting climatic fluctuations including drought, at the limit of (Donner, 1972), but in the data from Niznevartovsk and Surgut 578 513 the Boreal and Atlantic periods. (Neustadt, 1974; Khotinskiy, 1984; Neustadt and Selikson, 1985) 579 514 In a vegetation zone map (Khotinskiy, 1984) for a period of over half of the Pinus pollen was identified as P. cembra. How- 580 515 about 9000–10000 cal. BP our study site belongs to the Betula ever, Liss and Berezina (1981) found only minor amounts of P. 581 516 forest zone about 500 km north of the steppe area limit and near cembra pollen during the Atlantic period. Also, Blyakharchuk and 582 517 the southern limit of the Pinus cembra-Picea zone. Despite a sud- Sulerzhitsky (1999) did not find P. cembra pollen to be dominant 583 518 den increase in Betula pollen at our site, the low Betula/NAP ratio during the early Atlantic period; however, their data suggests an 584 519 value 9900 cal. BP indicates that tree pollen was not abundant increase in P. cembra from 6000 cal. BP onwards. According to 585 520 but was still transported from distant sources during the early the vegetation zone map of Khotinskiy (1984), our study area 586 521 Boreal period. Also, the Pinus/NAP ratio indicates a timelag belongs to the dark coniferous middle taiga zone in the P. cembra- 587 522 between the sharp increase of pollen and the actual establishment Picea taiga subdivision between 5300 and 7000 cal. BP. Contrary 588 523 of trees in the region (Figure 6). This pattern suggests that, during to vegetation estimates by Khotinskiy (1984), our pollen data indi- 589 524 the Boreal period (about 9000–10000 cal. BP), trees were just cate the presence of Pinus sylvestris-Picea forests, in which P. 590 525 becoming established in the region and that Betula forests may cembra was only a minor part. 591 526 not have actually existed until the late Boreal period. Our data A continuous Alnus pollen curve with sporadic Abies pollen 592 527 also contrast with the proximity of the Pinus cembra-Picea zone starts about at the level where total AP values are around 90% 593 528 (about 100 km north from our site) reconstructed by Khotinskiy (in core A at 350 cm and in core B at 383 cm). At Bugristoye 594 529 (1984). The near absence of Pinus pollen (Figures 4 and 5) sug- bog (Blyakharchuk and Sulerzhitsky, 1999), a continuous Abies 595 530 gests that both P. sylvestris and P. cembra forests must have been curve starts between 8000 and 9000 cal. BP and is highest about 596 531 a long distance from our study site. 6000–7000 cal. BP. At our study site, a continuous Abies pollen 597 532 The decline in Picea pollen in the AP-NAP zone, most evident curve starts later, about 6500 cal. BP, and values of about 5% 598

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599 may be high enough to indicate its establishment in the region. detailed palaeovegetation studies are needed for better and more 661 600 A sharp decrease of Abies around 6000 cal. BP, followed by a reliable climate reconstructions. 662 601 considerable increase in Pinus cembra with no change in Pinus 602 sylvestris abundance, is reported by Blyakharchuk and Sulerzhit- 603 sky (1999). However, at our study site, there was no evidence of Acknowledgements 663 604 a decline in Abies pollen, and no increase in Pinus cembra pollen. 605 From about 6500 cal. BP onwards, Corylus, Tilia and Ulmus We thank the Academy of Finland for financial support; Dmitry 664 606 pollen were found both at our study site and at Bugristoye bog L. Varlyguin, Olga L. Liss and Georgy Vizgalov for their help 665 607 (Blyakharchuk and Sulerzhitsky, 1999). At 5000–7000 cal. BP and co-operation during the fieldwork; D.L. Varlyguin for co- 666 608 there was a climatic optimum in the Northern Hemisphere, includ- operation during writing of the manuscript and providing a photo- 667 609 ing Siberia. The mean January temperatures were estimated to be graph for Figure 2; and Carrie Turunen for revision of the English. 668 610 about 5°C warmer than today, but mean July temperatures were We also thank A. Andreev, T. Blyakharchuk and Dorothy Peteet 669 611 similar to present in West Siberia. Estimated precipitation values for critical comments on the manuscript. 670 612 were not different from present (Velichko et al., 1997; Monserud 613 et al., 1998).

References 671 614 Forests 4100 cal. BP to present 615 There is evidence of a climatic change in Fennoscandia Aario, L. 1940: Waldgrenzen und subrezente Pollenspektren in Petsamo 672 616 (Digerfeldt, 1988; Korhola, 1995; Hyva¨rinen and Alhonen, 1994; Lappland. Ann. Acad. Scient. Fenn., Ser. A 54(8), 1–120. 673 617 Reinikainen and Hyva¨rinen, 1997) about 4300 cal. BP, coinciding —— 1943: U¨ ber die Wald- und Klimaentwicklung an der Lappla¨ndischen 674 618 with a rapid decrease of Picea pollen after 4100–4300 cal. BP at Eismeerku¨ste in Petsamo. Ann. Bot. Soc. Zool. Bot. Fenn. Vanamo 19(1), 675 619 our study site. In Siberia, the northern tree-line retreated near to 1–158. 676 620 its present limit between 4000 and 3000 cal. BP (MacDonald Andreev. A.A. and Klimanov, V.A. 2000: Quantitative Holocene climatic 677 621 et al., 2000). Decline of Picea pollen values to less than 5% indi- reconstruction from Arctic Russia. Journal of Paleolimnology 24, 81–91. 678 622 cate that spruce probably disappeared from the peatland islands Andreev. A.A., Tarasov, P.E., Romanenko, F.A. and Sulerzhitsky, L.D. 679 623 and may have decreased considerably in large areas. Early Sub- 1997: Younger Dryas pollen records from Svedrup Island (Kara Sea). 680 Quaternary International 41/42, 135–39. 681 624 boreal cooling occurred at 5300–4600 cal. BP and was followed Baulin, V.V. and Danilova, N.S. 1984: Dynamics of late Quaternary per- 682 625 by a warmer period at 4600–3400 cal. BP (Khotinskiy, 1984). mafrost in Siberia. In Velichko, A.A., editor, Late Quaternary environ- 683 626 There is no notion in Russian literature of a rapid decline of Picea ments of the Soviet Union, London: Longman, 179–200. 684 627 pollen around 4000 cal. BP in West Siberia, although in an Berglund, B.E. and Ralska-Jasiewiczowa, M. 1986: Pollen analysis and 685 628 undated diagram from Surgut (Neustadt, 1974) a decline in Picea pollen diagrams. In Berglund, B.E., editor, Handbook of Holocene palaeo- 686 629 pollen correlates with the decline at our study area. Khotinskiy ecology and palaeohydrology, New York: John Wiley, 455–85. 687 630 (1984) only remarks that Picea and Abies forests declined, but Birks, H.J.B. and Birks, H.H. 1980: Quaternary palaeoecology. Lon- 688 631 Pinus cembra and Betula forests became more important during don: Arnold. 689 632 the Subboreal and Subatlantic periods. Blyakharchuk and Sulerzh- Blyakharchuk, T.A. and Sulerzhitsky, L. 1999: Holocene vegetational 690 633 itsky (1999) found evidence of the beginning of palsa formations and climatic changes in the forest zone of Western Siberia according to 691 pollen records from the extrazonal palsa bog Bugristoye. The Holocene 692 634 about 4900 cal. BP at Bugristoye bog, indicating a shift to a cooler 9, 621–28. 693 635 climate, but no sharp decrease of Picea. However, in Bugristoye Bondestam, K., Vasari, A., Vasari, Y., Lemdahl, G. and Eskonen, K. 694 636 bog (Blyakharchuk and Sulerzhitsky, 1999) and at the Niznevar- 1994: Younger Dryas and Preboreal in Salpausselka¨ foreland, Finnish Kar- 695 637 tovsk site (Khotinskiy, 1984; Neustadt and Selikson, 1985) Picea elia. Dissertationes Botanicae 234, 161–206. 696 638 pollen indicates a decrease in spruce between 5000 cal. BP and Botch, M.S. and Masing, V.V. 1983: Mire ecosystems in the USSR. In 697 639 present. The data from our study site suggests only a slight Gore, A.J.P., editor, Mires: swamp, bog, fen and moor, B. Regional 698 640 increase in Pinus cembra, but rather a rapid increase of Pinus studies, Amsterdam: Elsevier, 95–152. 699 641 sylvestris after 4100 cal. BP (Figures 3, 4 and 6). 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BP (van Geel and Donner, J. 1972: Pollen frequencies in the Flandrian sediments of Lake 708 649 Renssen, 1998). Vakoja¨rvi, south Finland. Comment. Biol. 53, 1–19. 709 Donner, J.J., Alhonen, P., Eronen, M., Jungner, H. and Vuorela, I. 710 1978: Biostratigraphy and radiocarbon dating of the Holocene lake sedi- 711 ments and the peats in the adjoining bog Varrassuo west of Lahti in south- 712 650 Conclusions ern Finland. Annales Botanici Fennici 15, 258–80. 713 Eronen, M. and Zetterberg, P. 1996: Climatic changes in northern 714 651 Climatic reconstructions from the Holocene period in West Sib- Europe since late glacial times, with special reference to dendroclimatol- 715 652 eria are based only on a few studies (Khotinskiy, 1984; Velichko ogical studies in northern Finnish Lappland. 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