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 evi- dence, 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.