Did Tree-Betula, Pinus and Picea Survive the Last Glaciation Along The

Home , Betula nana, Betula pubescens, Setesdal

Journal of Biogeography (J. Biogeogr.) (2005) 32, 1461–1471

ORIGINAL Did tree-Betula, Pinus and Picea survive ARTICLE the last glaciation along the west coast of Norway? A review of the evidence, in light of Kullman (2002) Hilary H. Birks1,2,3*, E. Larsen4 and H. J. B. Birks1,2,3

1Department of Biology, University of Bergen, ABSTRACT Bergen, Norway, 2Bjerknes Centre for Climate Aim We discuss the hypotheses proposed by Kullman [Geo-O¨ ko 21 (2000) 141; Research, Bergen, Norway, 3Environmental Change Research Centre, University College Nordic Journal of Botany 21 (2001) 39; Journal of Biogeography 29 (2002) 1117] on London, London, UK and 4Geological Survey of the basis of radiocarbon-dated megafossils of late-glacial age from the central Norway, Trondheim, Norway Swedish mountains that boreal trees survived the glaciation along the south-west coast of Norway and subsequently migrated eastward early in the late-glacial to early deglaciated parts of the central Swedish Scandes mountains. Methods We assess these hypotheses on the basis of glacial geological evidence and four lines of palaeoecological evidence, namely macrofossil records of the tree species, vegetation and climate reconstructions from plant evidence, independent climate reconstructions from other proxies for the late-glacial environment of south-west Norway, and the patterns of post-glacial spread of the tree species. Location South and west Norway, central Swedish Scandes mountains (Ja¨mtland). Results and conclusions South-west Norway and the adjacent continental shelf were under ice at the last-glacial maximum (LGM). The late-glacial vegetation of south-west Norway was treeless and summer temperatures were below the thermal limits for Betula pubescens Ehrh., Pinus sylvestris L. and Picea abies (L.) Karst. Instead of spreading immediately after the onset of Holocene warming, as might have been expected if local populations were surviving, B. pubescens showed a lag of local arrival of 600 to > 1000 years, Pinus lagged by 1500 to > 2000 years, and Picea only reached southern Norway c. 1500 years ago and has not colonized most of south-west Norway west of the watershed. Glacial geological evidence shows the presence of an ice sheet in the Scandes at the LGM and in the Younger Dryas, which was cold-based near or at the area where the late-glacial-dated megafossils were recovered by Kullman. We conclude that the samples dated by Kullman (2002) should be evaluated carefully for possible sources of contamination. All the available evidence shows that the biogeographical hypotheses, based on these radiocarbon dates taken at face value, of late-glacial tree survival at the Norwegian coast and subsequent eastwards spread to the mountains, are unsupportable. *Correspondence: Hilary H. Birks, Department Keywords of Biology, University of Bergen, Alle´gaten 41, N-5007 Bergen, Norway. Betula pubescens, glacial survival of trees, late-glacial, macrofossils, megafossils, E-mail: [email protected] Picea abies, Pinus sylvestris, pollen, south-west Norway.

2001, 2002) from the central Swedish mountains. From this INTRODUCTION evidence Kullman concludes that Betula pubescens Ehrh. ssp. Radiocarbon dates of late-glacial age obtained from ‘megafos- tortuosa (Ledeb.) Nyman, Picea abies (L.) Karst. and Pinus sil’ tree remains have recently been reported by Kullman (2000, sylvestris L. grew at 1360 m a.s.l. on Mt A˚ reskutan, Ja¨mtland,

ª 2005 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi doi:10.1111/j.1365-2699.2005.01287.x 1461 H. H. Birks, E. Larsen and H. J. B. Birks central Sweden, during the late-glacial interstadial (Bølling- localities since at least 12,000 bp’. He maintains that the trees Allerød) and the following cold stadial (Younger Dryas) spread from there eastwards during the late-glacial to the between 14,000 and 10,200 14Cyrbp. Kullman’s radiocarbon central Swedish mountains, where, however, his earliest dated dates are sensational and his conclusions have major implica- remains (Betula) are very early in the late glacial, at tions for our current understanding of late-glacial plant 14,020 ± 80 and 12,870 ± 70 14Cyrbp. geography and vegetation history and of the deglaciation Kullman (2000, 2001, 2002), based on this comparison of history of the Scandinavian ice sheet. pollen and megafossil evidence from other areas (above), uses Following from these results, Kullman (2000, 2001, 2002) arguments based on pollen data to propose that tree-pollen proposes that (1) the trees survived the glacial period on percentages in late-glacial sites along the Norwegian west coast exposed continental shelf areas west and south-west of might derive from small locally surviving tree populations. Norway, (2) the trees migrated into the Scandes mountains However, it could be argued as equally likely that they do not. from the west during the late-glacial period, and thus (3) the Winds were much stronger than today in full- and late-glacial Scandinavian ice sheet was thinner than previously suspected times (e.g. COHMAP members, 1988), thus facilitating long- and that late-glacial nunataks were available and suitable for distance dispersal of pollen. This makes the presence of early tree colonization. These hypotheses are highly contro- macrofossils even more important as proof of local tree versial, going against the generally accepted pattern of glacial presence (Birks & Birks, 2000, 2003; Birks, 2003). Small pollen and forest history of the area in the late-glacial and early percentages of Pinus and Picea and small or sometimes quite Holocene. Therefore, we consider that it is important to large percentages of Betula pollen (some at least of which examine the hypotheses critically in the light of the existing originated from B. nana L.; van Dinter & Birks, 1996) in the palaeoecological fossil and glacial geological evidence. west Norwegian late-glacial should not be taken as proof for local tree occurrences without supporting macrofossil evidence, especially when independent macrofossil evidence for TYPES OF PALAEOECOLOGICAL FOSSIL local vegetation and climate is available from several sites in EVIDENCE western Norway (Fig. 1). Previously Kullman (1995, 1996, p. 97, 1998a, pp. 425–426, 1998b, p. 153, 2000, p. 165, 2001, p. 40) has criticized pollen EXAMINATION OF THE EVIDENCE analysis as providing poor evidence for the local occurrence of trees, asserting that small outlying tree populations are not Palaeoecological evidence detected by pollen analysis because pollen analysts do not generally consider that occasional pollen grains or ‘tails’ in There are four main lines of palaeoecological evidence with pollen diagrams may originate from small locally present tree which we can test Kullman’s proposals: (1) plant macrofossil populations. The only way to prove local presence in these evidence, ignored by Kullman, (2) late-glacial vegetation circumstances is through ﬁnds of macrofossils (including reconstructions in western Norway that show extensive conifer stomata) or megafossils. He suggests that ‘… by alpine-vegetation analogues, (3) independent climate recon- combining mega-, macro-, and microfossil data, a more structions from other proxies and (4) evidence from the routes realistic comprehension can be obtained concerning the critical and timing of tree spread in the early Holocene in western pollen percentage level, that must be exceeded before spatially Norway. precise biogeographical range-limit reconstructions can be inferred from pollen data’ (Kullman, 2000, p. 165). Kullman Plant macrofossil and pollen evidence for late-glacial trees in (2002, p. 1117) states ‘It is increasingly evident, however, that south-west Norway fossil pollen data do not always accurately account for such vital aspects of historical biogeography, such as, location of In spite of numerous late-glacial macrofossil investigations in glacial refugia… geographical spread and elevational shifts’. south-west Norway (Fig. 1), no macrofossils of tree-Betula We largely agree with these statements. Pollen analysis is a have yet been found. Although it is unsatisfactory to argue difﬁcult tool to use in treeless environments and in the from negative evidence, it would appear that tree-Betula was detection of tree-line movements. In these situations records of absent, or very localized and so far undetected in south-west macrofossils and megafossils provide additional complement- Norway (Birks et al., 1993; Birks, 1993; van Dinter & Birks, ary evidence (Birks & Birks, 2000, 2003; Eide, 2003). Later, 1996; Birks & van Dinter, 1997; see Birks, 2003 for a review). however, Kullman (2002, p. 1121) returns to his previous There is similarly no macrofossil or stomatal evidence for the (2000) proposal of calibrating pollen data with macro- or presence of Picea and Pinus to indicate that they survived the megafossil evidence of local presence, and, implying that the glaciation in the south-west Norwegian region or that they calibration has already been made, states: ‘… pollen records were present in the late glacial (e.g. Fægri, 1949; see Giesecke & from south-west Norway (Kristiansen et al., 1988; Paus, 1989) Bennett, 2004). If the highest late-glacial pollen percentages of display pollen sums that, judging from comparisons of pollen Betula (including tree-Betula) (e.g. Paus, 1989) were derived and megafossil evidence from other areas (Kullman, 2000), from long-distance transport (see van Dinter & Birks, 1996), it may suggest presence of birch, pine and spruce at sheltered is probable that the percentages of Picea and Pinus pollen were

1462 Journal of Biogeography 32, 1461–1471, ª 2005 Blackwell Publishing Ltd Glacial survival of boreal trees in south-west Norway?

0° 10° 20° E example, in the late-glacial at Kra˚kenes on the Norwegian west coast (Birks et al., 2000) and in northern Scotland (Birks, 70° 1984). It is curious that Kullman (2002) does not consider any of the late-glacial macrofossil investigations in western Norway and instead relies entirely on pollen analytical data, in contrast to his own advice detailed above. Tromsø A Evidence for the actual vegetation growing during the full- and late-glacial Plant macrofossil and pollen data from west Norwegian glacial 65° deposits (Barstadvik – E. Larsen & H.H. Birks, unpubl. data; Andøya – Alm & Birks, 1991) and late-glacial sites (on Fig. 1) provide abundant evidence that the vegetation was treeless. In the examples cited by Kullman (2002) no tree-Betula macrofossils were found and the local vegetation was dominated by Betula nana or Empetrum L. [pollen diagram by Paus (1989); Trondheim macrofossils by van Dinter & Birks (1996); pollen diagram by Ba Kristiansen et al. (1988); unpublished macrofossil data of H.H. G L Birks]. Other west-Norwegian sites (Fig. 1) have assemblages K N 60° dominated by Salix herbacea L. and taxa associated with snow- beds and fjellfields, etc., all closely analogous to vegetation above the tree-line in the oceanic west Norwegian mountains Bl Bergen Oslo today (Birks, 1993, 2003; Jonsgard & Birks, 1995; Birks & van Younger Dryas Dinter, 1997), or even polar desert (Alm & Birks, 1991). U Stavanger Individual species recorded as macrofossils in the Norwegian N late-glacial include Salix polaris Wahlenb., Ranunculus glacialis E Kristiansand Bj L., Saxifraga rivularis L., S. cespitosa L., Sagina intermedia 400 km Fenzl, Koenigia islandica L., Papaver Sect. Scapiflora (Reich- enb.), Aulacomnium turgidum (Wahlenb.) Schwa¨gr., Polytri- Figure 1 Map of Norway and Sweden showing Mt A˚ reskutan chum sexangulare Brid., etc. Given the vegetation and the (star), full-glacial sites (triangles) and late-glacial sites in south environment suggested by such species it is improbable that Norway with macrofossil records (black dots). The limit of the tree-Betula, Picea and Pinus could have grown in these Younger Dryas re-advance is shown. A ¼ Andøya (Alm & Birks, conditions. 1991), Ba ¼ Barstadvik (E. Larsen & H.H. Birks, unpubl. data), The radiocarbon date of 10,360 ± 170 14Cyrbp (T-5592) L ¼ Lerstadvatn (J. Mangerud & H.H. Birks, unpubl. data), on the layer containing squirrel (Sciurus vulgaris) bones, and G ¼ Godøy (Birks et al., 1993), N ¼ Nordfjord transect (Birks & by inference the occurrence of conifer forest (Kullman, 2002), van Dinter, 1997), K ¼ Kra˚kenes (e.g. Birks et al., 2000), at the top of the Skjonghelleren cave sequence on the west Bl ¼ Blomøy (Birks, 1993), U ¼ Utsira (Birks, 1993), E ¼ Eige- Norwegian coast (Larsen et al., 1987) was measured on a large bakken (van Dinter & Birks, 1996), Bj ¼ Bjerkreim (H.H. Birks, bulk sample of bone material of marine mammals, birds and Aa. Paus & S.J. Brooks, unpubl. data). fish. Thus the bone sample probably covers quite a large age span and the Younger Dryas age obtained should not be taken also long-distance transported. Pinus pollen is notoriously as a definitive age for squirrel (cf. Stewart & Lister, 2001). Also, long-distance transported. The percentage representation of as most of the dated material is of marine origin, it suffers long-distance transported pollen depends on the local pollen from a marine reservoir age of at least 400 and up to 600– production. As macrofossil and other evidence suggests that 1000 years (Bondevik et al., 2001). Taking into account the the west-Norwegian late-glacial vegetation was almost certainly occurrence of lengthy radiocarbon plateaux in the early treeless, low amounts of long-distance transported pollen Holocene (e.g. Bjo¨rck & Wohlfarth, 2001) the squirrel remains would not have been masked by local pollen, and could have could easily have an early Holocene true age. reached high percentages in a total pollen sum (Birks & Birks, 2003). This effect is demonstrable today by the considerable Independent climate reconstructions from late-glacial sites in percentages of tree pollen recorded on Spitsbergen that derive south-west Norway from areas far to the south (van der Knaap, 1987). A similar long-distance component was reported in the Devon Island ice Summer temperature reconstructions using chironomids [at core from arctic Canada (McAndrews, 1984). Pollen influx or Kra˚kenes (Brooks & Birks, 2001) and Bjerkreim (S.J. Brooks, annual accumulation rates give more reliable information, for unpubl. data)], Cladocera [at Kra˚kenes (Duigan & Birks,

Journal of Biogeography 32, 1461–1471, ª 2005 Blackwell Publishing Ltd 1463 H. H. Birks, E. Larsen and H. J. B. Birks

2000)], Coleoptera [at Godøy (Birks et al., 1993) and Kra˚kenes 1985; Liedberg Jo¨nsson, 1988). More than 600 years were (Lemdahl, 2000)], Oribatid mites [at Kra˚kenes (Solhøy & needed for Betula to spread round the Norwegian coast and up Solhøy, 2000)], and glacial geological evidence (Larsen et al., the valleys into the mountains. 1984; Larsen & Stalsberg, 2004) are all consistent (e.g. Birks & Pinus spread rapidly across southern Norway some 800– Ammann, 2000; Birks et al., 2000, 2005). Reconstructed July 1000 years after Betula (Fig. 2c). Radiocarbon dating of its first temperatures in the Allerød and Younger Dryas are well below macrofossil occurrences and pollen influx increases shows that the thermal limit for tree-Betula growth, which needs a mean Pinus reached the Norwegian south coast and also the southern July temperature of c. 11 C at the coast (Odland, 1996) and Hardangervidda (Setesdal) at 10,000–9600 cal yr bp (Eide temperatures were therefore also below the thermal require- et al., 2005) and the Oslo area by 9600–9200 cal yr bp (H.J.B. ments of Picea abies and Pinus sylvestris. Although Kullman Birks & S.M. Peglar, unpubl. data). Pinus expanded near Bergen (2000, 2002) records Picea krummholz growing at mean June– on the west coast at 9160 cal yr bp (J. Larsen, S.M. Pegiar, August temperatures of 6–7 C, krummholz is not a pioneer H.J.B. Birks, unpubl. data). The earliest Pinus megafossils in the growth form, but a persisting, poorly reproducing, relict of south-central Norwegian mountains near Trondheim were former warmer conditions (e.g. Kullman, 1983, 2001; Payette, dated at c. 9250 cal yr bp and on the Hardangervidda at 1983; Holtmeier, 2000). Kullman (2002) reports a 50-year-old c. 9650 cal yr bp (Aas & Faarlund, 1988). At Lake Spa˚ime Picea sapling growing at a mean June–August temperature of c. 45 km SW of A˚ reskutan, the earliest Holocene flora was of an 5 C (comparable with modern Svalbard temperatures) on alpine, pioneer type and B. pubescens macrofossils are first Mt A˚ reskutan, thus illustrating the unique local microclimates recorded at 9500 cal yr bp (Hammarlund et al., 2005). apparently present there today. However, in hollows such as Although Pinus pollen reached c. 70% between 9500 and that where the wood remains were found, temperatures may be c. 8000 cal yr bp, probably indicating its local presence, several degrees lower. In the A˚ reskutan area, hollows in the macrofossils were not found until c. 6000 cal yr bp. The pollen conifer forest are occupied by birch trees, or in the subalpine situation is similar in sites in the Hando¨l valley 40 km SE of Mt birch forest, the hollows are treeless and contain tundra A˚ reskutan, but specific macrofossil records are lacking polygons (J. Lundqvist, pers. comm.). (Segestro¨m & von Stedingk, 2003). Given the lower elevations of Lake Spa˚ime and the Ha¨ndol valley, the lack of a rapid expansion of both Betula and Pinus at these sites argues against Routes and timing of tree spread in the early Holocene nearby late-glacial refugia at Mt A˚ reskutan (1360 m a.s.l.). Kullman (2002) proposes that trees survived in scattered This evidence, based on macrofossils, megafossils and pollen ‘cryptic’ (Stewart & Lister, 2001) refugia with locally favour- influx values, does not support a spread of Betula and Pinus able climatic conditions. If so, these populations would have from coastal refugia or from the Swedish Scandes in the Allerød been expected to expand immediately after the climate became and Younger Dryas but illustrates a post-glacial spread from the generally favourable after the start of the Holocene (11,530 cal south. The documented establishment delays of 600 to yr bp; Gulliksen et al., 1998). However, they did not, as > 2000 years would not be expected if these pioneer trees were discussed below. already locally present in small populations in the mountains The pattern of Holocene tree-Betula spread in southern during the late-glacial or earliest Holocene or on exposed Norway is summarized in Fig. 2b. Tree-Betula was the first tree continental shelf and coastal areas of western and south- to arrive and was locally present (as shown by macrofossils western Norway during the late-glacial. The nearest Younger and/or pollen influx values) in the Oslo area and in Dryas occurrences of Pinus documented by macrofossils, apart southernmost Norway by 10,800 cal yr bp (Eide et al., 2005; from Kullman’s records, are from southern England (Lambert, H.J.B. Birks & S.M. Peglar, unpubl. data) and at the west coast 1964), and south Sweden [Kullaberg (Liedberg Jo¨nsson, 1988); near Bergen at 10,600 cal yr bp (J. Larsen, S.M. Pegiar, H.J.B. Ska˚ne (Gertz (1926); Blekinge (Berglund, 1966)]. Birks., unpubl. data). The high-resolution radiocarbon chro- The case of Picea abies is different. Picea can be a pioneer nology and macrofossil stratigraphy at Kra˚kenes revealed that tree on open soils (Giesecke & Bennett, 2004) but its seedlings tree-Betula took c. 650 years to expand locally (10,900 cal are shade tolerant and well adapted to regeneration within yr bp) after the start of the Holocene (Birks et al., 2000). forest, where it grows best on acid soils with adequate nutrients Betula arrived near Trondheim at c. 10,600–9800 cal yr bp and competes strongly with other forest trees (Nikolov & (H.J.B. Birks & S.M. Peglar, unpubl. data). Macrofossils of Helmisaari, 1992). However, Mt A˚ reskutan has virtually no Betula indicate its presence in the Dovre mountains by soil, just bare bedrock (Lundqvist, 1969). Picea has relatively 10,000 cal yr bp (Eide, 2003) and on the southern Hard- low pollen production. Picea pollen records show that it spread angervidda plateau by 10,800–10,000 cal yr bp (Eide et al., from the east late in the Holocene towards Ja¨mtland and 2005). The oldest megafossils (large wood remains) of Betula western Norway and it never naturally reached the coast except in the S. Norwegian mountains are dated to c. 9250 in Dovre in the lowlands in the Trondheim area (e.g. Huntley & Birks, (Barth et al., 1980) and c. 7950 cal yr bp on the Hardangervi- 1983; Hafsten, 1992; Kullman, 2001; Giesecke & Bennett, dda (Aas & Faarlund, 1988). Tree-Betula appears to have 2004). This pattern suggests that temperatures in the migrated from its nearest proven (by macrofossils) Younger mountains had already fallen below the thermal limit of Picea Dryas refugia in Denmark and south Sweden (e.g. Jensen, before it could cross the watersheds and its southward and

1464 Journal of Biogeography 32, 1461–1471, ª 2005 Blackwell Publishing Ltd Glacial survival of boreal trees in south-west Norway?

(a) (b) 10° E Betula

Åreskutan Trondheim 10.6 Spåime 9.5 Dovre 10.9 10.0 9.25 60° N H v 7.95 10.8 Bergen H Oslo 10.6 10.6 YD

Setesdal 10.8 N Kristiansand >10.0

400 km

9.0 Figure 2 Maps showing the earliest 14C-dated macrofossil/pollen-influx increase 9.25 records (black dots) and megafossil records 9.65 9.6 (stump symbol) in the Holocene for 2b 2.8 9.16 10 Betula L., 2c Pinus L., and 2d Picea A.Dietr. 9.6 0.7 Ages are given in calendar years bp ) (1950) · 10 3. Place names mentioned in the 9.9 0.9 text are shown in 2a (Hv ¼ Hardangervidda, H ¼ Haukeligrend). Mt A˚ reskutan is shown ~10.0 ~1.5 (0.75 mac) by a star. The Younger Dryas (YD) ice re-advance limit is shown. westward spread in south Norway was blocked, so it has never Without specific macrofossil evidence, the record of ‘conifer- been native in south-west Norway (e.g. Fægri, 1949; Huntley & ous wood’ should not be used to confirm local presence of Birks, 1983; Hafsten, 1992). In contrast, Kullman (2000, 2001) Picea from small numbers of pollen grains. interprets early Holocene dates of Picea megafossils to indicate The nearest last-glacial Picea macrofossil record to the that small populations of Picea grew at high elevations in Scandinavian mountains is from Byelorussia (Punning et al., moist, snow-rich habitats in the Swedish Scandes. He proposed 1983), which supports the spreading pattern based on pollen that Picea spread downwards and eastwards. As climate records (Giesecke & Bennett, 2004). There are no other became cooler and moister around 3000 14Cyrbp, the documented records of late-glacial Picea in the Swedish scattered Picea populations increased in the eastern lowlands, Scandes from mountains that might be more suitable thus accounting for its apparent late spread from the east as ecologically than Mt A˚ reskutan for its growth. Picea pollen deduced from pollen records (see Giesecke & Bennett, 2004). peaks associated with Picea macrofossils (needles) occurred Pollen analyses of Holocene peat sections 40–45 km SE of Mt transiently in eastern and southern Finland in the early A˚ reskutan (Segestro¨m & von Stedingk, 2003) record single Holocene before 9000 14Cyrbp (e.g. Vasari, 1962; Tolonen, Picea pollen grains back to c. 9000 cal yr bp, but not earlier. 1967; Giesecke & Bennett, 2004), showing that this area could Their records of ‘coniferous wood’ macrofossils from one site be a source for early Holocene spread and establishment of could be ascribed either to Picea (one pollen grain), Pinus scattered small Picea populations to the west, even though (whose pollen was abundant since c. 10,000 cal yr bp), or Picea subsequently became extinct in Finland and recolonized Juniperus, which has a continuous pollen record at 2–3%. later in the Holocene. Spruce can spread extremely quickly, as

Journal of Biogeography 32, 1461–1471, ª 2005 Blackwell Publishing Ltd 1465 H. H. Birks, E. Larsen and H. J. B. Birks

Figure 3 The maximum extent of the Scandinavian ice sheet, the extent of cold- based ice at the last-glacial maximum, the limit of the Younger Dryas re-advance, and the areas of cold-based ice at the start of deglaciation. Mt A˚ reskutan is shown by a star. the seeds are shed in winter and travel long distances across a yet met and coalesced (Fig. 2d). If Picea had survived during smooth snow-covered landscape (Ritchie & MacDonald, 1996; the late-glacial on the south-west coast of Norway, it is Giesecke & Bennett, 2004). However, its late-Holocene surprising that it went extinct at the start of the Holocene, and expansion through already forested northern Europe was slow. was, in fact, absent from all of north-west Europe except at An example of actual spreading routes can be traced in detail high elevations in the Swedish Scandes. by combined Picea macrofossil and pollen inﬂux records in Genetic studies also suggest a westward Holocene spread of Setesdal, southern Norway (Fig. 2d). It entered Setesdal from Picea abies from the east and south. Vendramin et al. (2000) the south near Kristiansand by 1500 cal yr bp (Eide et al., identiﬁed two main genetic types, a central European-alpine 2005) and spread some 60 km north up the valley by 980 cal and an east European-Scandinavian, that relate to the yr bp. In contrast, Picea reached northern Setesdal from the locations of the glacial refugia proposed from the pollen eastern lowlands, arriving in the Haukeligrend area by 2800 cal record by Huntley & Birks (1983) and Giesecke & Bennett yr bp (H.J.B. Birks & S.M. Peglar, unpubl. data), but it never (2004). Further analyses by Bucci & Vendramin (2000) reached the southern Hardangervidda plateau (Eide et al., delimited a genetically homogeneous area in central Sweden 2005). Spruce is sparse in the upper pine forest today and its and the Trondheim area, which was closely related to an current altitudinal species limit is similar to that of Pinus at adjacent area in south-east Norway and south Sweden. They around 900 m. The lowest pass across the mountains is at propose that genetic variability increases with geographical 1150 m. Picea penetrated northern Setesdal as far south as the isolation (distance). Thus one might expect – as indeed was Hovden area by 700 cal yr bp. The central part of Setesdal predicted as a generalization by Kullman (2002) – that lacks native Picea where the two invading populations have not populations of spruce derived from proposed refugia in

1466 Journal of Biogeography 32, 1461–1471, ª 2005 Blackwell Publishing Ltd Glacial survival of boreal trees in south-west Norway? western Norway and central Sweden would be genetically margin retreated north during the early deglaciation, allowing distinct from the populations that spread from refugia in open water to penetrate along the Norwegian coast, the reverse south and east Europe. However, this hypothesis is not happened during the Younger Dryas, with persistent sea-ice supported by the genetic data available so far. south to the Lofoten-Vøring Plateau area (Koç et al., 1993; Birks & Koç, 2002; Birks et al., 2005). On land, the ice withdrew from the coast during the early deglaciation (c. 15–13 ka) but Glacial geological evidence: the extent and character re-advanced during the Younger Dryas to the limits mapped by of the Scandinavian ice sheet Andersen et al. (1995) and in coastal cirques (Fig. 3). A narrow How do the late-glacial megafossils in central Sweden relate to coastal strip of land was ice-free during the Younger Dryas the extent of the Scandinavian ice sheet? Kullman (2002) where climatic conditions were severe (Birks et al., 2005). Mean quotes evidence to support a proposal that deglaciation was July temperatures were c. 7 C lower than today at Kra˚kenes more complex than previously supposed, with early emerging (i.e. mean July c. 5–6 C) (Larsen et al., 1984; Birks et al., 2000; nunataks. Trees could occupy these, perhaps in krummholz Larsen & Stalsberg, 2004) and at nearby Godøy (Birks et al., form, during the Allerød and Younger Dryas, having migrated 1993). These temperatures typify the high- or mid-alpine from glacial refugia in western Norway. Norwegian vegetation zones and central Spitsbergen today. The most recent reconstruction of the maximum extent and configuration of the Scandinavian ice sheet (Svendsen DISCUSSION – ALTERNATIVE HYPOTHESES et al., 2004) parallels earlier reconstructions (e.g. Kullman, 2002, Fig. 1) (Fig. 3). The ice sheet was thickest (> 2500 m) The hypotheses of tree survival and migration can now be over southern Scandinavia, including the A˚ reskutan area. considered in the light of all the available evidence. At the The central part was cold-based, namely frozen to the LGM, the Scandinavian ice sheet covered western Norway substrate. Little erosion occurred, and by mapping uneroded and the continental shelf. Where could the trees survive? landscapes and ribbed moraines, Kleman et al. (1997) and Plant-macrofossil evidence, inferred past vegetation, and Kleman & Ha¨ttestrand (1999) have delimited the cold-based climate reconstructions, all falsify the hypothesis of tree area, both for the last-glacial maximum (LGM) and for the survival in south-west Norway during the late-glacial. The start of deglaciation (see Fig. 3). Mt A˚ reskutan lies close to occurrence of ice-free nunataks is still controversial (Nesje the boundary between cold- and warm-based ice during the et al., 1987; Larsen et al., 1995), but if present, these were glaciation and deglaciation and may have experienced certainly mountain peaks with little or no soil and a harsh both states, as there are glacial striations and scouring on climate. The existence of locally favourable environments for the abundant bare bedrock up to the summit (Lundqvist, tree growth in the mountains during the early late-glacial is 1969). not supported by geological evidence or by the geomor- Seasonally sea-ice free conditions in the Norwegian-Green- phology of Mt A˚ reskutan, with its predominance of bare land Sea occurred during the full-glacial period (e.g. Sarnthein bedrock and sparse soil development (Lundqvist, 1969): an et al., 1995) corresponding to warmer periods of the unlikely habitat for Picea. Therefore, from all these lines of Dansgaard-Oeschger cycles (Dokken & Hald, 1996). Dokken evidence, Kullman’s (2000, 2001, 2002) hypotheses of glacial & Hald (1996) and Siegert & Marsiat (2001) proposed that survival of boreal trees at the south-west Norwegian coast moisture collected by mid-Atlantic storms passing over these and their subsequent eastward migration to mountain open seas and guided up the Norwegian coast built up the nunataks during the Allerød and Younger Dryas cannot be Scandinavian ice sheet at modelled rates of accumulation of supported. ) c. 500 mm yr 1 water-equivalent at the west coast. Although The main problem in rejecting the tree-survival hypotheses conditions were relatively warm, the modelled annual temper- is the radiocarbon-dated wood remains implying late-glacial atures along the west coast are between )5 and )12 C (Siegert tree growth on Mt A˚ reskutan. We can ask if the dates are & Marsiat, 2001). The ice margin oscillated during the correct ages? Examination or treatment of the material for Weichselian, allowing biota to spread periodically (e.g. during possible sources of contamination was not reported, so we the A˚ lesund interstadial 40 ka; e.g. Forsstro¨m & Punkari, 1997) have to take the dates at face value. If the dates are not but it is universally agreed that the ice sheet extended beyond correct ages, they must be either too old or too young. the Norwegian coast to the continental shelf edge at the LGM Although it seems to be an obvious idea that the trees are of (c. 22,000 14Cyrbp) (Fig. 3), both from geological evidence Holocene age and the wood has been contaminated with old and modelling evidence (e.g. Siegert & Marsiat, 2001; Svendsen carbon, it is difficult to propose a mechanism for this. The et al., 2004), leaving nowhere for trees or other plants to grow main ice-movement from the east may have transported (Brochmann et al., 2003). limestone to the non-calcareous Mt A˚ reskutan from strata One of the earliest deglaciated areas close to the ice-sheet situated 4–6 km east and south-east and 800–900 m lower margin (Svendsen et al., 2004) was Andøya (north-west than the summit (Sveriges Geologiske Underso¨kning, 1984). Norway), but macrofossil evidence from soon after the LGM Calcareous contamination may then have entered the (c. 19,000 14Cyrbp) shows that the vegetation there was polar groundwater from unweathered till (Lundqvist, 1969, pers. desert (Alm & Birks, 1991). Although the summer sea-ice comm.).

Journal of Biogeography 32, 1461–1471, ª 2005 Blackwell Publishing Ltd 1467 H. H. Birks, E. Larsen and H. J. B. Birks

Alternatively, we ask if the tree remains are ancient, perhaps ACKNOWLEDGEMENTS older than the limit of radiocarbon dating? Remains of all three species are recorded near the end of the last interglacial, We are grateful to many people for helpful discussion and both near sea-level in western Norway and south Sweden, and input of ideas to this paper, in particular Keith Bennett, Anne in the Hardangervidda mountains at 900 m (summarized by Bjune, Svante Bjo¨rck, Keith Briffa, Wenche Eide, Michael Donner, 1995). Remains of the trees that grew at high altitudes Friedrich, Jan Lundqvist, Heikki Seppa¨, John Inge Svendsen in the central Swedish mountains could have been preserved and Ruth Terhuërne-Berson. We thank Jorunn Larsen and under cold-based ice, where erosion was minimal, during the Sylvia Peglar for the use of unpublished fossil data and Cathy full-glacial and the Younger Dryas (Fig. 3). However, Picea did Jenks and Bjørg Svendga˚rd for help with the figures. This is not grow in Ja¨mtland during MIS 5a or 5c when the area was publication Nr A91 from the Bjerknes Centre for Climate free of ice, and it seems improbable that tree remains from Research, Bergen. MIS 5e (interglacial) have been preserved through three glacial episodes and two interstadials (J. Lundqvist, pers. comm.). REFERENCES Kullman (2002) proposed that a small cold-based ice body existed for some part of the later Holocene (Neoglaciation) on Aas, B. & Faarlund, T. (1988) Postglasiale skoggrenser i sen- Mt A˚ reskutan, contributing to the preservation of the ancient trale sørnorske fjelltrakter (Postglacial forest limits in central plant remains. However, such snow-beds are rare on south Norwegian mountains). Norsk Geografiske Tiddskrift, Mt A˚ reskutan today and would probably have disappeared 42, 25–61. during the Holocene thermal maximum (Lundqvist, 1969). Alm, T. & Birks, H.H. (1991) Late Weichselian flora and Whatever their true ages, the fact is that the wood remains vegetation of Andøya, Northern Norway – macrofossil (seed were preserved by cold temperatures and waterlogging in a and fruit) evidence from Nedre Æra˚svatn. Nordic Journal of snow-bed area and a small pond at A˚ reskutan through the Botany, 11, 465–476. Holocene. Andersen, B., Lundqvist, J. & Saarnisto, M. (1995) The Relatively small amounts of modern contamination by Younger Dryas margin of the Scandinavian ice sheet – an organic material in very old samples could result in late-glacial introduction. Quaternary International, 28, 145–146. and early Holocene radiocarbon dates. We can only speculate Barth, E.K., Lilma-De-Faria, A. & Berglund, B.E. (1980) Two about the amount of undetectable contamination that may 14C dates of wood samples from Rondane, Norway. Bota- have taken place on wood buried for the length of the Holocene niske Notiser, 133, 643–644. or longer in snow-beds or under shallow water and overgrown Berglund, B.E. (1966) Late-Quaternary vegetation in eastern by moss. Betula and Picea wood especially could have been Blekinge, south-eastern Sweden. 1. Late-glacial time. Opera penetrated by rootlets and microbes. If such contamination is Botanica, 12, 180. suspected, individually teased out wood fibres should be dated Birks, C.J.A. & Koç, N. (2002) A high-resolution diatom (M. Friedrich, pers. comm.). However, if the wood remains record of late-Quaternary sea-surface temperatures and were all of interglacial age, it seems unlikely that they would oceanographic conditions from the eastern Norwegian Sea. have received the same 20–25% modern contamination neces- Boreas, 31, 323–344. sary to result in late-glacial and early Holocene radiocarbon Birks, H.H. (1984) Late-Quaternary pollen and plant macro- ages. A wider range of ages from 10 to 30 ka bp might have been fossil stratigraphy at Lochan an Druim, north-west Scotland. expected (S. Bjo¨rck, pers. comm.). Lake sediments and environmental history (ed. by E.Y. Independently, if the pine remains are well enough Haworth and J.W.G. Lund), pp. 377–405. University of preserved, their ring sequences could be analysed and any Leicester Press, Leicester. match with the Scandinavian Holocene tree-ring record Birks, H.H. (1993) The importance of plant macrofossils (K. Briffa & M. Friedrich, pers. comm.; e.g. Briffa & Osborn, in late-glacial climatic reconstructions: an example 2002) would determine if they are of Holocene age. A non- from western Norway. Quaternary Science Reviews, 12, match could mean that the trees were not Holocene, or that 719–726. they did not match the Holocene tree-ring pattern. Birks, H.H. 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Proceedings of the National samples should be carefully assessed for possible sources of Academy of Sciences of the United States of America, 97, contamination and submitted (if suitable) to independent age 1390–1394. assessment by tree-ring analysis before basing unsupported Birks, H.H. & Birks, H.J.B. (2000) Future uses of pollen ana- hypotheses on them about glacial and late-glacial survival of lysis must include plant macrofossils. Journal of Biogeo- the tree species at the south-west coast of Norway. graphy, 27, 31–35.

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Ingo´lfson, O., Jakobsson, M., Kjær, K., Larsen, E., Lokrantz, presence of population subdivision in Norway spruce (Picea H., Lunkka, J., Lysa˚, A., Mangerud, J., Matiouchkov, A., abies K.). Genome, 43, 68–78. Murray, A., Mo¨ller, P., Niessen, F., Nikolskaya, O., Polyak, P., Saarnisto, M., Siegert, C., Siegert, M., Spielhagen, R. & Stein, R. (2004) Late Quaternary ice sheet history of nor- BIOSKETCHES thern Eurasia. Quaternary Science Reviews, 23, 1229–1271. Sveriges Geologiske Underso¨kning (1984) Karta o¨ver berg- Hilary H. Birks is a Quaternary palaeoecologist at the grunnen i Ja¨mtlands la¨n, scale 1 : 200,000. Sveriges Geolog- University of Bergen. She specializes in plant macrofossil iske Underso¨kning (Ser. Ca.), 53. analysis, and has used this technique to investigate the changes ¨ Tolonen, K. (1967) Uber die Entwicklung eines nordkarelis- in past vegetation and climate in several parts of the world, but chen Moores im Lichte der C14-datierung. Das Moor particularly during the late-glacial and early Holocene in Puohtiinsuo in Ilomantsi (Ost-Finnland). Annales Botanici Norway. Societas Zoologicae Botanicae Fennicæ ‘Vanamo’, 18, 41–57. Van der Knaap, W.O. (1987) Long-distance transported pollen Eiliv Larsen is a Quaternary geologist at the Geological and spores in Spitsbergen and Jan Mayen. Pollen et Spores, Survey of Norway, Trondheim, Norway. His main research is 24, 449–453. on the reconstruction of glaciations during the last-glacial Van Dinter, M. & Birks, H.H. (1996) Distinguishing fossil cycle in Scandinavia and Russia and the progress and processes Betula nana and B. pubescens using their wingless fruits: of deglaciation. implications for the late-glacial vegetational history of John Birks is a plant ecologist and Quaternary palaeoecol- western Norway. Vegetation History and Archaeobotany, 5, ogist at the University of Bergen, Norway, and University 229–240. College London, UK. His research interests are centred on late- Vasari, Y. (1962) A study of the vegetational history of the Quaternary environmental and biotic change and on quanti- Kuusamo district (North East Finland) during the late- tative palaeoecology, ecology and plant geography, particularly Quaternary period. Annales Botanici Societatis Fennicæ in arctic and alpine areas. ‘Vanamo’, 33, 1–140. Vendramin, G.G., Anzidei, M., Madaghiele, A., Sperisen, C. & Bucci, G. (2000) Chloroplast microsite analysis reveals the Editor: Mark Bush

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