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Changes in Betula in the Holocene record from Iceland—a palaeoclimatic record or evidence for early Holocene hybridisation?
Review of Palaeobotany and Palynology 117 (2001) 139±152
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Changes in Betula in the Holocene record from IcelandÐa palaeoclimatic record or evidence for early Holocene hybridisation? Chris Caseldine* Department of Geography, School of Geography and Archaeology, University of Exeter, Amory Building, Rennes Drive, Exeter EX4 4RJ, UK
Abstract Tree birch in its mountain form is the only woodland-forming tree found in the Holocene record of Iceland. Given the close relationship between tree-lines and summer temperature in Fennoscandia, it should therefore provide a valuable proxy temperature indicator for the Holocene in Iceland. Following a review of issues relating to the taxonomy of mountain birch, and the identi®cation of tree birch in the palynological record, data are presented from the TroÈllaskagi area of northern Iceland relating to the development of birch woodland in the early Holocene. It is argued that following the development of communities in which dwarf birch, Betula nana L., was important, true woodland communities only became established after a period of relative instability in the vegetation cover. Data from morphometric analyses of Betula pollen from this period are interpreted as representing probable hybridisation between the different forms of birch, rather than re¯ecting a clear climatic signal. The question is also raised of whether the progenitor of tree birch in Iceland was an immigrant form of mountain birch, (Betula pubescens Ehrh. ssp. tortuosa (Lebed.) Nyman as de®ned in Scandinavia), or whether Icelandic mountain birch developed by hybridisation in Iceland between Betula nana L. and Betula pubescens Ehrh. during the Holocene. The use of birch as a palaeoclimatic indicator, especially prior to the establishment of birch woodland at suitable locations and altitudes, should not therefore be accepted unless accompanied by other independent climatic evidence. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Betula; Iceland; Holocene; hybridisation; palaeoclimate; pollen
1. Introduction Birch is the only forest-forming tree in Iceland, hence understanding changes in the composition and extent of birch woodland is crucial to establishing the potential of palaeoecological data for palaeoclimatic information, both in terms of tree-line movement and community change. Studies in Scandinavia have shown a link between climatic parameters, particularly summer temperature, and the oscillations of the * Fax: 144-139-226-3342. E-mail address: [email protected] (C. Caseldine).
sub-arctic (alpine) birch tree-line (Odland, 1996; Mook and Vorren, 1996). Correlations of between 0.92 and 0.87 have been found between tree-line altitude and a variety of summer temperature proxies (mean July temperature, mean maximum July temperature, mean tritherm [June, July, August] temperature and mean maximum tetratherm [including September] temperature. Thus by analogy it would appear likely that by determining patterns of change in tree birches in Iceland, data on Holocene palaeoclimatic ¯uctuations could be produced to complement the developing glacial record (StoÈtter et al., 1999). Previous work in the country has adopted
0034-6667/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0034-666 7(01)00082-3
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such an approach at a general level (Einarsson, 1961, 1963; Bartley, 1973; HallsdoÂttir, 1987; Vasari and Vasari, 1990), seeing oscillations in the Betula pollen curve as broadly re¯ecting climate changes, albeit also in relation to expanding mires and the in¯uence of increasing contributions from wetland taxa, notably Cyperaceae. Uncertainties, however, still exist in a number of areas concerning the species identi®cation of Betula pollen, and interpretation of its relative and absolute changes through time, which need to be addressed before palaeoclimatic information can be derived with con®dence. 2. Probable Betula species in Iceland Identi®cation of what is commonly termed mountain birch (Betula tortuosa) to species level is still an area of considerable debate amongst taxonomists (Vaarama and Valanne, 1973; Kallio et al., 1983). The evolution of birch species in the subarctic appears to have been a feature of Holocene vegetation change and is still occurring. The key area for evolution is where the subsections Albae and Nanae meet in conditions allowing successful introgressive hybridisation, unlike the situation further south within the area dominated by Albae where limiting factors prevent such a process taking place. A number of features characteristic of tortuosa forms have been identi®ed by Vaarama and Valanne (1973) which indicate the in¯uence of B. nana in producing a species (or subspecies) particularly able to adapt to the climatic severity of the subarcticÐ`the morphological features common to B. tortuosa and B. nana and absent from the southern provenances of B. pubescens are suf®cient to warrant the assumption that gene ¯ow has taken place through hybridisation and introgression from B. nana to the northern B. pubescens called B. tortuosa' (p. 82). Thus according to these authors, mountain birch, either as a separate species or, in other researchers' views as a subspecies of B. pubescens, has developed because of the hybridisation of B. nana and B. pubescens. In Iceland the de®nition of species varies between ¯oras. All agree in the presence of a form of dwarf birch, Betula nana L., which apparently shows little variation, but there are then a series of forms of tree or shrub birch. GroÈntved (1942) includes all non-dwarf
forms within B. pubescens Ehrh., variations being due to introgression with B. nana, and it was his ®ndings that have been quoted by Vaarama and Valanne (1973) as crucial evidence for effective hybridisation. StefaÂnsson (1948) divides the non-dwarf forms into B. pubescens Ehrh., B. coriaceae Gunnarss., and a hybrid of B. nana L. and B. pubescens Ehrh. LoÈve (1945) also includes B. callosa NotoÈ., but rede®nes the most common form, B. pubescens, as B. tortuosa Lebed. In 1956 LoÈve and LoÈve also introduce B. concinna Gunnarss., suggesting that apart from B. nana L. the remaining tree/shrub forms are combinations of the ®ve different species they de®ne. These authors also doubted the hybridisation evolution of B. tortuosa, arguing that there was no evidence to show that hybrids were not sterile. More recent studies both in the ®eld (Elkington, 1968) and in controlled experiments (Vaarama and Valanne, 1973) seem to have disproved this, although successful seed production only occurred in favourable years, and was not the normal pattern. Cytogenetic evidence has also been used to describe the process of hybrid introgression in Iceland (Anamthawat-JoÂnsson and ToÂmasson, 1990; Anamthawat-JoÂnsson, 1994). Perhaps crucially for the Icelandic situation Vaarama and Valanne (1973) suggest that `it is reasonable to assume that there have been times earlier in the post-glacial period when a more favourable climate has allowed frequent and effective gene exchange between species' (p. 82). In their pollen and macrofossil diagrams Vasari and Vasari (1990) use the subsection term B. alba, but refer to B. pubescens in the text. The most recent guide to ¯owering plants by Kristinsson (1987) simply de®nes Betula pubescens Ehrh. and B. nana L., although comments on hybridisation between the two, especially where the latter is common. Most pollen analysis in Iceland refers to Betula pubescens and B. nana (Einarsson, 1961; HallsdoÂttir, 1987), as does Glawion (1985) in his detailed ecological study of Icelandic vegetation, or does not go beyond Betula (Bartley, 1973; PaÊhlsson, 1981). Thus in terms of which species of tree/shrub birch exist, or existed in Iceland during the Holocene, there is still a lot of uncertainty, just as there appears to be in much of Scandinavia. One of the few ecological studies of birch in Iceland de®nes the major tree birch species as Betula pubescens Ehrh. ssp. tortuosa (Lebed.) Nyman (SveinbjoÈrnsson, 1993), also the
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assumed form for tree birch in Greenland (Fredskild, 1991). A similar approach is adopted by AnamthawatJoÂnsson (1994) who states, `Icelandic woodland birch is similar to the variety B. pubescens Ehrh. spp. tortuosa (Lebed.) Nyman, often known as mountain birch in the alpine zone of Fennoscandia' (p. 10). Despite this, it is clear that although Betula nana L. can be clearly identi®ed as a distinct species, the remaining tree/shrub birches are highly polymorphic and potentially include a number of hybrid forms, with perhaps many more in the past, especially in locations suitable for hybridisation (Elkington, 1968). Although other birch species, notably B. pendula, are found in Iceland today it is likely that B. pubescens and/or B. tortuosa were the dominant species in the evolution of the Holocene ¯ora. 3. Species identi®cation of Betula pollen The separation of pollen of different Betula species has been the subject of several studies starting with morphometric separation of B. nana, pendula and pubescens by Jentys-Szaferova over 70 years ago (1928), and the literature has been summarised recently by MaÈkelaÈ (1996, 1999). Different workers favour different approaches to separation of species, particularly the separation of B. nana. Praglowski (1962, 1966), following TerasmaÈe (1951), proposed a morphological separation, and a number of laboratories use this approach, with or without taking measurements. Birks (1968), following Walker (1955) proposed separation of B. nana using a morphometric approach, utilising a ratio between the diameter of the full grain and the pore depth. This was extended by more formal statistical approaches adopted by Usinger (1975), and by Gordon and Prentice (1977) who developed analyses based on distinguishabilities between species. MaÈkelaÈ (1996) favours a morphometric approach, arguing that it is more objective than identi®cations based on a subjective assessment of morphology (with or without taking into account relative size) and also used a form of distinguishability analysis on modern material to attempt to separate species. Arising out of the range of approaches so far adopted are two basic questions: (1) is it possible to separate out individual species from the range of birch species and subspecies that occur in
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northern Scandinavia (the focus of most of this work): and (2) is it at least possible to reliably separate B. nana from shrub/tree forms? MaÈkelaÈ (1996, 1999) was particularly concerned to try to distinguish between the major tree birch species and subspecies, as well as separating out B. nana, and used Scandinavian herbarium and collected specimens from a range of locations as a basis for detailed morphometric analysis, concentrating on maximum grain diameter. Previous morphometric work, starting with Jentys-Szaferova (1928), had suggested some gradation in size between B. nana, pendula, pubescens and tortuosa, but not always in the same order (Eneroth, 1951), and the variations between species were often less than those within species (Berglund and Digerfeldt, 1970). Comparability of observations between different workers also proved dif®cult because different criteria and preparation procedures were used in deriving sets of measurements (Andersen, 1980; Usinger, 1981; Fredskild, 1991). The question also remains, given the uncertain taxonomy of mountain birch (something re¯ected in the taxonomic af®nities given to samples used by MaÈkelaÈ (1996)), whether morphometric separation should be possible within an overall group, especially as both Eneroth (1951) and Usinger (1975) were able to demonstrate increases in pollen size for the same species with altitude and latitude. Where fossil pollen assemblages include two or more species, the use of frequency distributions (Prentice, 1981; Gordon and Prentice, 1977), either in a simple graphical form, or in a more rigorous multivariate statistical analysis, have, however, suggested that separate populations could be identi®ed whatever their taxonomic origin. On the basis of her work MaÈkelaÈ (1996) discovered just how dif®cult it was to separate the main tree birch forms, although she did ®nd a general trend in size from B. pendula ! B. pubescens. She had two separate groups of B. pubescens ssp. tortuosa from her Finnish site, and assumed that one set derived from one-stemmed, monocormic trees, whereas the other was from polycormic, multi-stemmed forms. Because her B. tortuosa pollen had large grain diameters she further assumed that the polycormic form was the group with the larger of the diameters, and was therefore encouraged at the possibility of separating B. tortuosa and hybrids involving this species from the much smaller pollen of B. nana and, possibly, B.
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pendula. There were also two groups of B. tortuosa sizes, hence replicable differentiation remains highly questionable. In conclusion she comments that there do appear to be systematic size differences between the populations she measured and individual populations do conform to normal Gaussian distributions at an individual level, albeit with overlaps in the sizes at 1 s. In terms of absolute sizes though there are still considerable within-species variations when samples are taken from different locations. All her measurements were taken from fresh material and MaÈkelaÈ does point out that `It is also possible that the pollen sizes in the very variable B. pubescens group have changed through time, since hybridisation and introgression seem to have played a major role in the evolution of mountain birch' (1996, p. 255). This latter point is probably the case in Iceland, especially in more marginal locations (Elkington, 1968). The most encouraging ®nding of MaÈkelaÈ's work, however, is the clear separation of the pollen of B. nana simply on grain diameter (17.31 ^ 0.88 [1 s], n 110, in silicon oil) with very little overlap with the other species, and none with one of the sets of B. pubescens ssp. tortuosa (25.98 ^ 2.30, n 615, the other being 22.0 ^ 1.57, n 324), perhaps the most likely form of tree birches in Iceland, especially if the group represents the polycormic form. Thus, to return to the two questions posed earlier it would appear possible to distinguish between dwarf birch and other forms of birch on the basis of grain diameter, especially if supported by pore depth measurements following Walker (1955) and Birks (1968). Birks (1968) quotes a range of average grain diameter: pore depth ratios from 10 to 11.5 for B. nana, and 7.3±7.91 for B. pubescens. Attribution of possible tree/shrub forms for larger diameter grains to species or subspecies is though not possible, especially where there is a likelihood of hybridisation. Shifts in mean size of grains in populations clearly larger than B.nana may re¯ect the increasing in¯uence of B. tortuosa in a polycormic form, but this would also be very speculative on the available evidence. Past palynological work in Iceland has either used separation of B. nana by morphology (HallsdoÂttir, 1987, although then produced a single curve for Betula), not differentiated between species (Bartley, 1973; Einarsson, 1961, 1963; PaÊhlsson, 1981; Vasari and Vasari, 1990; Caseldine and Hatton, 1994), or
used a size/frequency distribution approach (HallsdoÂttir, 1990; Fri riksdoÂttir, 1973 quoted in HallsdoÂttir, 1995). Rundgren (1995, 1998) (pers. commun.) for his work on Skagi identi®ed three categories on morphometric and morphological criteriaÐB. nana, B. pubescens ssp. tortuosa and Betula undiff. The size criteria used in Iceland by Rundgren and HallsdoÂttir are outlined in Table 1. Because both authors used glycerol as the mounting medium the sizes are larger than those quoted by MaÈkelaÈ, who used silicon oil. Correction can be made (Fñgri and Iversen, 1989; Moore, Webb and Collinson, 1991) but quoted ®gures vary from 1.1 to 1.3 £ (glycerol larger than silicone mountant, Fñgri and Iversen, 1989, p. 285). Thus for Iceland, on the basis of very limited observations it appears that mean diameters for B. nana are around 17 mm when mounted in silicon oil, certainly no smaller, a ®gure that agrees well with other studies. The sizes of the non-dwarf forms are, however, more dif®cult to assess and compare. For palynological work in Iceland, although only based so far on a small number of observations, it would seem reasonable to argue that it is possible to separate B. nana pollen from non-dwarf forms for particular assemblages (not necessarily for every grain) by morphometric means using grain diameter ^ pore depth. Once grains are found which lie consistently outside the range for B. nana, it is only possible to identify them as tree/shrub forms on this basis, although frequency distributions would help in de®ning whether discrete populations were present. It may be that large grain sizes refer to B. tortuosa, or B. pubescens ssp. tortuosa, but the morphometric evidence alone would not be suf®cient to con®rm such an interpretation. 4. Macrofossil evidence The presence of macrofossils identi®able to species level provides the most convincing evidence for the local presence of speci®c forms of dwarf or tree birches. Separation of the fruits of B. nana from B. pubescens or B. pendula can be made relatively easily when still winged, and van Dinter and Birks (1996) have shown separation of various Betula fruit bodies which have lost their wings on sub-fossilisation. Although separation of dwarf birch from tree birches proved possible there was considerable overlap
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Table 1 Measurements of Betula pollen (^1 s) from a selection of sources. Species attributions are as in original texts. All are based on modern material except those of HallsdoÂttir (1990) which are representative samples of Holocene age, and the samples from North Iceland from this paper, which were taken from surface samples. The North Iceland samples were collected from VaglaskoÂgur, FnjoÂskadalur, those from South Iceland from Skaftafell Source
Taxon
Size details (mm)
Comments
This paper
B. nana (N.Iceland) B. pubescens ssp. tortuosa (N.Iceland) B. pubescens ssp. tortuosa (S. Iceland)
19.23 ^ 1.43 24.06 ^ 1.90
Silicon oil mount
Rundgren 1998, pers. comm.
B. pubescens ssp. tortuosa Betula undiff B. nana B. nana
. 27.2 or 24±27 1 morph 24±27 24±27 1 morph , 24 1 pore depth ,4.1
Glycerol mount
HallsdoÂttir, 1990
B. nana B. pubescens
23.5 30
Glycerol mount, estimated from graph
MaÈkelaÈ, 1996
B. B. B. B.
nana pubescens ssp. tortuosa pubescens ssp. tortuosa pubescens hybrid
17.31 ^ 0.88 22.06 ^ 2.25 25.98 ^ 2.30 24.26 ^ 1.59
Silicon oil mount
Birks, 1968
B. B. B. B. B. B. B. B.
nana nana nana pubescens pubescens tortuosa tortuosa tortuosa
18.71 ^ 1.30 18.60 ^ 1.09 18.72 ^ 1.21 22.63 ^ 1.43 23.31 ^ 1.59 26.42 ^ 1.60 23.43 ^ 1.38 26.07 ^ 1.72
Silicon oil mount
Eneroth, 1951
B. nana B. pubescens B. tortuosa
19.4 25.0 26.1
Glycerol mount
between the tree forms and their hybrids with B. nana. The presence of megafossils in the form of buried trunks is often further evidence for local presence. Birks (1993) and van Dinter and Birks (1996) have successfully used macrofossil analysis to demonstrate that tree birches failed to reach south-west Norway during the Allerùd, despite high percentage and in¯ux values for the pollen of tree birches (up to 40±50% and 900 grains cm 22 yr 21, Paus 1989). It should, however, be noted that long-distance transport is possible for some macrofossils, especially if only occasional or isolated remains are found (Birks, 1973). Glaser (1981), and see comments in Birks (1991) has produced a model for macrofossil dispersal in tundra environments and winter dispersal, possibly of material from a distant tree-line can be important,
26.94 ^ 2.10
especially where deposition is facilitated by a lack of local macrofossils. The converse is of course also true, absence of macrofossils does not necessarily imply local absence of any particular tree species (Barnekow, 1999). In Iceland there has been relatively little use of macrofossil analysis apart from the description of large birch remains in peat pro®les (Bartley, 1973; Haraldsson, 1981; see Fig. 5 in HallsdoÂttir, 1995). Vasari and Vasari (1990) identi®ed scales and fruits de®ned as Betula alba, and leaves, scales and fruits of B. nana at HafratjoÈrn in northern Iceland, but at LomatjoÈrn in the south they only separated out unde®ned macrofossils of B. pubescens and B. nana. Also in northern Iceland for the Late Weichselian and Early Holocene, Rundgren (1998) identi®ed
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Table 2 Published in¯ux rates for Betula from modern studies. Timescale
Source
Vegetation character and altitude
In¯ux rate (Range or average in pollen cm 22 yr 21)
Alm (1996)
Sub-alpine birch forest with scattered pine, 400±160 m from tree-line
234±2770
1991±1993
133±527
1991±1993
16±283
1991±1993
500 1600 2000
1984, 1986±1991 1984, 1986±1991 1984, 1986±1991
Sub-alpine birch forest, 520 m, 240 m from tree-line Low alpine heath, 600 m, 140 m from tree-line Hicks (1996)
Treeless Mountain birch woodland Birch and pine woodland
macrofossils of B. nana. There is clearly a lot of potential for further macrofossil analysis of birch remains in Iceland, as they are likely to provide the key evidence for the presence of tree birches at sites, although they will be unlikely to allow determination of what species of tree birch were present. Suf®cient material from good preserving environments may be found from peat sites, especially those in open sections; cores have so far derived mainly from sites with relatively limited macrofossil records, and thus it may not always prove easy to derive this form of data, in combination with other palaeoecological indicators, to the level of rigour that would be preferred. 5. Pollen in¯ux rates Recent studies of surface pollen deposition, particularly collecting by means of Tauber traps, have been undertaken with the aim of ®nding threshold values for certain types of plant community, and much of this has concentrated on the tree-line of Fennoscandia. Collection periods range from only 2 or 3 years of trapping in Norway (Alm, 1996; Vorren and Stavseth, 1996) to 7 years in Finland (Hicks, 1996). Not surprisingly results have varied considerably (Table 2), with differences also between collecting media, traps against glass jars for instance (Alm, 1996), but some recurrent patterns have emerged and Hicks (1996) offers hope for this approach to vegetation reconstruction. By contrast in their introduction to the volume on Holocene Treeline Oscillations, Dendrochronology and Palaeoclimate, Birks et al. (1996b) are more cautious, echoing van Dinter and
Birks (1996): `It seems that the percentages and in¯ux of Betula pollen cannot be interpreted in any strict way in terms at which Betula trees may be locally represented' (p. 237). What surface pollen in¯ux studies have shown is that where tree birches are present at or close to the tree limit pollen in¯ux can be as low as ,200 grains cm 22 yr 21(Vorren and Stavseth, 1996) re¯ecting varying actual annual rates over a 3-year period of 133±527 grains cm 22 yr 21 (Alm, 1996). Hicks (1996) ®nds an average value over 7 years of 1600 grains cm 22 yr 21 within birch woodland in Finland, contrasting with 500 grains cm 22 yr 21 for treeless areas. Because of this variation most workers have been guarded about de®ning thresholds and Alm (1996) emphasises the good vertical and horizontal mixing of Betula compared to Pinus, suggesting a threshold of 500 grains cm 22 yr 21 as likely to indicate local presence. In contrast Vorren and Stavseth (1996) prefer the lower value of 240±360 grains cm 22 yr 21, 20% of which derives from winter in¯ux. All these values are well below the maximum values for the Allerùd treeless environment of southern Norway described earlier. Alm (1996) does, however, also show a reduction of up to 75% in collection between the woodland and collecting mires 20±30 m away from the trees, allowing some sort of calibration between peat sites and neighbouring woodland. In Iceland only HallsdoÂttir (1995, 1996) has presented pollen in¯ux rates from palaeoecological sites for the period when birch woodland was most likely to have existed. Results from Vatnskotsvatn (at 64 m asl) show values for Betula of over 2000 grains cm 22 yr 21 in the lake sediment, but after
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Fig. 1. Betula pollen data from HaÂmundarsta ahaÂls 8: Total Betula is expressed as percent Total Land Pollen (TLP); Betula mean diameters are for all measurable grains per level or a sample of 30.
ca. 6000 BP at both this site and Hestvatn in southern Iceland values are much lower at around 150 grains cm 22 yr 21. There are so far no published data on tree birch in¯uxes in Iceland from within current communities considered at all comparable to Holocene examples, and any data collected would provide an uncertain analogue due to the dif®culties of comparing the overall community structure of present woodland with the woodland that probably existed earlier in the Holocene. 6. Betula in the Early Holocene from Northern IcelandÐdata from EyjafjoÈr ur Fig. 1 shows the record for the total Betula pollen count from HaÂmundarsta ahaÂls, a small (90 £ 60 m) enclosed basin at 100 m asl on the western margin of EyjafjoÈr ur, Northern Iceland (Figs. 2 and 3). The record begins after the deposition of the Saksunarvatn
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ash which here has its upper limit at 480 cm and the lower limit is dated to 9195 ^ 65 14C yr BP (AA27591) from an adjacent core (StoÈtter and Wastl, in press) (compared to the date range of 8930±9060 14C yr BP (Birks et al., 1996a), and 10180 ^ 60 ice core years (GroÈnvold et al., 1995)). Although the pro®le is not yet fully dated the tephra H5, dated in the area to 6065 ^ 55 14C yr BP (Wastl et al., in preparation), occurs between 25 and 32 cm above the peat/gyttja transition in ®ve adjacent cores, whereas in the analysed core which has a relatively sharp transition to a sedge peat at 292 cm it was not found. It is likely therefore that the record covers the period between ca. 8500 and 6200±6500 14C yr BP during which birch woodland became established in most suitable locations in Iceland (HallsdoÂttir, 1995). The pollen record is taken only from the gyttja which has extremely good pollen preservation and is not affected by a high autochthonous pollen component, i.e. Cyperaceae or aquatic taxa. Lying at only 100 m, despite its relatively proximity to the fjord, the site would be expected to be suitable for such woodland establishment by comparison with patterns established elsewhere in the region (Einarsson, 1961; Bartley, 1973; HallsdoÂttir, 1990; Caseldine and Hatton, 1994ÐFig.2). Fig. 1 shows values for total Betula as percent Total Land Pollen (TLP), rising to up to 80% in HH vii, and concentrations of Betula pollen as grains cm 23. It is not possible to estimate in¯ux values in the absence of more dates but values in excess of 1000±2000 grains cm 22 yr 21 are likely for the upper levels with highest concentrations. In the ®nal column the mean diameter of all, or a sample of, measureable Betula grains is included. The Betula record can be divided into fairly clear zones as outlined in Fig. 1. The lowermost zone, HH iii, has few grains and those that occur have large diameters and are assumed to represent long-distance transport, either from outside Iceland or from very limited areas of Iceland supporting early tree birch growth. HH iv is dominated by what appears to be B. nana from the mean diameters and the low percent and concentrations. In HH v, although there is an increase in concentrations and in %TLP values the diameters still indicate small grains characteristic of B. nana. This is in marked contrast to the uppermost zone, HH vii, which has a much larger mean diameter throughout, over double the %TLP values, and
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Fig. 2. Photograph of the HaÂmundarsta ahaÂls site looking southwest. The core was taken from close to the centre of the main basin.
considerably increased concentrations of birch pollen, interpreted as re¯ecting local closed birch woodland around the margins of the site almost exclusively comprising tree birches. Notwithstanding uncertainties over the chronology in¯ux values appear to be in excess of those found from small mires within birch woodland (Table 2). The intermediate zone, HH vi, whilst showing a remarkably consistent curve for %TLP, and relatively uniform concentrations, shows transitional values for mean grain diameters. The period covered by HH vi thus represents the change from a marginal community in which dwarf forms were dominant to a full woodland community. The zone may last several hundred years (constant sedimentation would give 600 14C yr), and is clearly a dynamic phase in the local vegetation by the end of which tree birches were strongly established. In order to examine this period more closely, and in particular to try to understand why tree birches took so long to become well established, and what the relative in¯uences of tree and dwarf forms were during this period, more detailed morphometric analyses were carried out.
Fig. 4 shows histograms for samples of Betula mean diameters throughout the record. Zones HH iv and v have mostly unimodal forms with peaks between 17.5 and 19.6 mm, the variability probably due to the small sample sizes available for analysis, re¯ecting the dominance of B. nana. This is supported by the mean diameters of modern B. nana from the area as also shown in Fig. 4 and Table 1 (19.23 ^ 1.43 mm, n 23). The top zone, HH vii, also shows a predominantly unimodal pattern, especially in midzone, with a peak diameter around 22±23 mm, overlapping at ^1 s with modern data on tree birch in the area (24.06 ^ 1.9 mm, n 60), albeit generally slightly smaller in maximum diameter, but well below values for tree birches from Skaftafell in southern Iceland (26.94 ^ 2.1 mm, n 100). HH vi has a more complex pattern characterised by a peak in several levels at around 21 mm, not the same group as in either the zones above or below. There are some variations in all zones, as in the clear occurrence of B. nana grains at times throughout the record, and an interesting shift to an HH vi like pattern between 314±5 and 298±9 in HH vii. What is missing,
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Fig. 3. Location map for the TroÈllaskagi peninsula, northern Iceland, showing HaÂmundarsta ahaÂls and other published pollen sites mentioned in the text.
however, is any clear bimodality in the record for grain diameters as might be expected if both dwarf birch and tree birches were present and there was a gradual replacement of one by the other. The pattern is in fact very similar to that shown in a more limited data set at KrosshoÂlsmyÂri in Flateyjardalur to the east for a similar stratigraphic position (HallsdoÂttir, 1990). As additional evidence a more limited series of samples were examined for their diameter:pore ratio
(Fig. 5). The contrast between the B. nana-dominated zones HH iv and v with their high ratios and only occasional tree birch grains, and the dominantly low ratio HH vii deriving from tree birches is again clear. HH vi seems to show close af®nity to tree birch ratios, albeit with a signi®cant number of ratios intermediate between the extremes, and one level, 416±7 cm, with a good bimodal distribution. Examination of all the data from HH vi supports the
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Fig. 4. Histograms of samples of mean diameters of Betula grains from the four zones at HaÂmundarsta ahaÂls. Data from modern samples of Betula nana were prepared from samples collected from SkõÂ adalur, for Betula pubescens ssp. tortuosa, samples were collected east of this area at VaglaskoÂgur, FnjoÂskadalur, and from Skaftafell, southern Iceland.
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Fig. 5. Betula diameter:pore ratios (following Walker, 1955) for selected levels from HaÂmundarsta ahaÂls. Modern samples are as in Fig. 4.
interpretation of a relatively complex period of vegetation change in the early Holocene leading up to the establishment of closed birch woodland. The transitional nature of the morphometric data and the steplike changes in both %TLP and pollen concentrations is interpreted as representing a phase when there was considerable hybridisation taking place between wellestablished B. nana and the incoming form of tree birch. Eventually this hybridisation ceased at this low altitude, well below the presumed tree-line, and woodland comparable to that in Fennoscandia today probably emerged. Changes towards the top of HH vii, when there is a shift towards smaller mean diameters (Figs. 4 and 5), could be indicative of a return to local hybridisation even at this low altitude. Why such a development should have taken place
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with the migration of tree birches is uncertain, as is the control on such a process. Climatic conditions in the early Holocene, as seen in the offshore record (KocË et al., 1996; Jiang et al., 1999) and in the early optimum indicated at high latitudes in the North Atlantic region, appear to have been suitable for the establishment of birch woodland, although there is little direct detailed climatic evidence available for Iceland for this period. Complications in the record for Betula as indicated here make the use of this taxon as a widespread palaeoclimatic indicator in Iceland uncertain. It could, however, be that understanding the Betula record, and the meaning of the changes discussed above, may lead to deriving more precise climatic signals, especially if hybridisation was, at least in part, climatically controlled. The hybridisation hypothesis outlined above lacks suitable analogue data from Iceland. Hybrid forms can be observed in the area today, both at relatively high sites and on drained but abandoned ®elds (M. Wastl and P. Wookey, pers. comm.). We do not have morphometric data on pollen from such specimens, indeed it has even proved dif®cult to ®nd pollen released from B. nana in the area today, and the results shown in Figs. 4 and 5 are based on relatively small samples despite using areas with fruiting individuals. Comparison of the Holocene data with the present material (Figs. 4 and 5) reveals higher mean diameters for present day tree birches both in the region (24.06 ^ 1.9 mm) and especially in southern Iceland (26.94 ^ 2.10 mm), as against 22±23 mm throughout HH vii. For B. nana the diameters are virtually identical. It does therefore seem likely that the taxonomy of tree birches has continued to change during the Holocene in Iceland, even if only to a small degree, as implied for Fennoscandia by MaÈkelaÈ (1996), and it may be pertinent to ask the question: what was the form of tree birch that Icelandic trees derived from? Irrespective of whether tree birches survived the Younger Dryas (or even the Last Glacial Maximum) (Rundgren and IngoÂlfsson, 1999), or whether they immigrated at the end of the last glacial stage from Scandinavia (Buckland and Dugmore, 1991), it is not known whether the original trees were a form of B. tortuosa already developed by hybridisation in Fennoscandia, or whether B. pubescens arrived to hybridise with B. nana in Iceland and thus form a speci®cally Icelandic subspecies. If it was
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the latter then this could explain why the spread of tree birches was relatively slow and not the rapid migration seen elsewhere in northern Europe at the opening of the Holocene, as trees already well adapted to sub-arctic conditions did not undergo signi®cant hybridisation with dwarf birch. Genetic studies of Icelandic birch have so far indicated a likely single gene pool despite the observed high genetic variability (Anamthawat-JoÂnsson, 1994), with links to populations from Scandinavia (Anamthawat-JoÂnsson quoted in Rundgren and IngoÂlfsson, 1999), but what is needed is a much more rigorous analysis of the Holocene palynological, and where possible, macrofossil data before we can more fully understand the nature of woodland development and its relationship to climatic variability during the Holocene.
Acknowledgements I am grateful to Margret HallsdoÂttir and Mats Rundgren for helping with unpublished Icelandic data and to Kevin Edwards and Margret HallsdoÂttir for comments on the text. Much of this was written whilst at the University of Innsbruck and I thank Hans StoÈtter and Maria Wastl for their hospitality and help. Original work on the pollen from HaÂmundarsta ahaÂls was undertaken by Jackie Hatton.The diagrams were drawn by Helen Jones in the School of Geography and Archaeology at Exeter. The core was collected by Hans StoÈtter, Maria Wastl, Georg Albert, Nancy Bertler, Markus Maukisch and Johannes Simstich.
References Alm, T., 1996. Pollen in¯ux in traps along a height transect on  djit, Troms, Norway. PalaÈoklimaforschung 20, 157± Mount A 171. Anamthawat-JoÂnsson, K., 1994. Genetic variation in Icelandic birch. Norwegian Journal of Agricultural Sciences, 9±14 (Supplement 18). Anamthawat-JoÂnsson, K., ToÂmasson, Th., 1990. Cytogenics of hybrid introgression in Icelandic birch. Hereditas 112, 65±70. Andersen, S., Th, ., 1980. Early and Late Weichselian chronology and birch assemblages in Denmark. Boreas 9, 53±69. Barnekow, L., 1999. Holocene tree-line dynamics and inferred Ê bisko area, northern Sweden, based climatic changes in the A on macrofossil and pollen records. The Holocene 9, 253±266. Bartley, D.D., 1973. The stratigraphy and pollen analysis of peat
deposits at Ytri-Baegisa near Akureyri, Iceland. Geologiska FoÈreningens i Stockholm FoÈrhandlingar 95, 410±414. Berglund, B.E., Digerfeldt, G., 1970. A palaeoecological study of the Late-Glacial lake at Torreberga, Scania, South Sweden. Oikos 21, 98±128. Birks, H.H., 1973. Modern macrofossil assemblages in lake sediments in Minnesota. In: Birks, H.J.B., West, R.G. (Eds.), Quaternary Plant Ecology. Blackwells Scienti®c Publications, Oxford, pp. 173±189. Birks, H.H., 1991. Holocene vegetational history and climatic change in west SpitsbergenÐplant macrofossils from Skardtjùrna, an Arctic lake. The Holocene 1, 209±218. Birks, H.H., 1993. The importance of plant macrofossils in LateGlacial climatic reconstructions: an example from Western Norway. Quaternary Science Reviews 12, 719±726. Birks, H.H., Gulliksen, S., Ha¯idason, H., Mangerud, J., Possnert, G., 1996a. New radiocarbon dates for the Vedde Ash and the Salsunarvatn Ash from western Norway. Quaternary Research 45, 119±127. Birks, H.H., Vorren, K.-D., Birks, H.J.B., 1996b. Holocene treelines, dendro-chronology and palaeoclimate. PalaÈoklimaforschung 20, 1±18. Birks, H.J.B., 1968. The identi®cation of Betula nana pollen. New Phytologist 67, 309±314. Buckland, P.C., Dugmore, A.D., 1991. If this is a refugium, why are my feet so bloody cold? The origins of the Icelandic biota in the light of recent research. In: Maizels, J.K., Caseldine, C. (Eds.), Environmental Change in Iceland: Past and Present. Kluwer, Dordrecht, pp. 107±126. Caseldine, C., Hatton, J., 1994. Interpretation of Holocene climatic change for the EyjafjoÈr ur area of northern Iceland from pollenanalytical data: comments and preliminary results. In: StoÈtter, J., Wilhelm, F. (Eds.), Environmental Change in Iceland. MuÈnchener Geographische Abhandlungen, Reihe, pp. 41±62 (B 12). Einarsson, ., 1961. Pollenanalytische Untersuchungen zur spaÈtund postglazialen Klimageschichte Islands. SonderveroÈffentlichungen des Geologisches Instituts der UniversitaÈt KoÈln 6, 1±52. Einarsson, , 1963. Pollen-analytical studies on the vegetation and climate history of Iceland in Late and Post-glacial times. In: LoÈve, A., LoÈve, D. (Eds.), North Atlantic Biota and Their History. Pergamon, Oxford, pp. 355±365. Elkington, T.T., 1968. Introgressive hybridization between Betula nana L. and B. pubescens Ehrh. in North-West Iceland. New Phytologist 67, 109±118. Eneroth, O., 1951. UndersoÈkning rorande mojligheterna att i fossilt material urskilja de olika Betula-arternas pollen. Geologiska FoÈreningens i Stockholm FoÈrhandlingar 73, 343±408. Fñgri, K., Iversen, J., 1989. Textbook of Pollen Analysis, 4th ed. John Wiley and Sons, Chichester. Fredskild, B., 1991. The genus Betula in GreenlandÐHolocene history, present distribution and synecology. Nordic Journal of Botany 11, 393±412. Fri riksdoÂttir, S.P., 1973. FrjoÂgreining a jar vegi uÂr Tjarnarveri og SoÂleyjarhoÈf a, B.Sc. thesis. Faculty of Engineering and Science, University of Iceland, ReykjaviÂk, 24 pp. Glaser, P.H., 1981. Transport and deposition of leaves and seeds on
C. Caseldine / Review of Palaeobotany and Palynology 117 (2001) 139±152 tundra: a late-glacial analog. Arctic and Alpine Research 13, 173±182. Glawion, R., 1985. Die natuÈrliche Vegetation Islands als Ausdruck des oÈkologisches Raumpotentials. Bochumer Geographische Arbeiten 45, 208 . Gordon, A.D., Prentice, I.C., 1977. Numerical methods in Quaternary palaeoecology IV. Separating mixtures of morphologically similar pollen taxa. Review of Palaeobotany and Palynology 23, 349±362. GroÈntved, J., 1942. The Pteridophyta and Spermatophyta of Iceland. The Botany of Iceland, Vol. 4. (Part 1). Copenhagen. Â skarsson, N., Johnsen, S., Clausen, H.B., Hammer, GroÈnvold, K., O C.U., Bond, G., Bard, E., 1995. Ash layers from Iceland in the Greenland GRIP ice core correlated with oceanic and land sediments. Earth and Planetary Science Letters 135, 149±155. HallsdoÂttir, M., 1987. Pollen Analytical Studies of Human In¯uence on Vegetation in Relation to the LandnaÂm Tephra Layer in Southern Iceland. Lundqua Thesis 18. Lund University, Lund, p. 46. HallsdoÂttir, M., 1990. Studies in the vegetational history of north Iceland. A radiocarbon-dated pollen diagram from Flateyjardalur. JoÈkull 40, 67±81. HallsdoÂttir, M., 1995. On the pre-settlement history of Icelandic vegetation. BuÂvõÂsindi 9, 17±29. HallsdoÂttir, M., 1996. Synthesis of the Holocene history of vegetation in northern Iceland. PalaÈoklimaforschung 20, 203±214. Haraldsson, H., 1981. The Marka¯joÂt sandur area, southern Iceland: sedimentological, petrographical and stratigraphical studies. Striae 15, 1±64. Hicks, S., 1996. The feasibility of using pollen deposition data as climatic indices. PalaÈoklimaforschung 20, 173±187. Jiang, H., Seidenkrantz, M.-S., Knudsen, K.-L., Eiriksson, J., Ha¯idason, H. 1999. Reconstruction of summer sea-surface temperature since 16,000 Cal. Years BP on the North Icelandic shelf using diatom-based transfer functions. Geological Society of America Annual Meeting and Exposition, Abstracts, A-74. Jentys-Szaferova, J., 1928. La structure des membranes du pollen de Corylus, de Myrica et des espeÁces europeÂenes de Betula et leur deÂtermination a l'eÂtat fossile. Bulletin international de l'Academie Polonaise de Sciences et des Lettres, Ser. B 68, 75±125. Kallio, P., Niemi, S., Sulkinoja, M., 1983. The Fennoscandian birch and its evolution in the marginal forest zone. Nordicana 47, 101±110. KocË, N., Jansen, E., Ha¯idason, H., 1996. Paleoceanographic reconstructions of surface ocean conditions in the Greenland, Icelandic and Norwegian Seas through the last 14 ka based on diatoms. Quaternary Science Reviews 12, 115±140. Kristinsson, H., 1987. A Guide to the Flowering Plants and Ferns of È rn og O È rlygur, ReykjavõÂk, p. 312. Iceland. O Â ., 1945. IÂslenzkar Jurtir. Ejnar Munksgaard, KaupmannaLoÈve, A hoÈfn, p. 291. MaÈkelaÈ, E.M., 1996. Size distinctions between Betula pollen typesÐa review. Grana 35, 248±256. MaÈkelaÈ, E.M., 1999. The Holocene history of birch in northeastern FennoscandiaÐan interpretation based on fossil birch pollen measurements. University of Helsinki, Department of Geology, Division of Geology and Palaeontology, Helsinki.
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Mook, R., Vorren, K.D., 1996. The temperature climate at the altitudinal vegetation limits in Skibotn, northern Norway. PalaÈoklimaforschung 20, 61±74. Moore, P.D., Webb, J.A., Collinson, M.E., 1991. Pollen Analysis, 2nd ed. Blackwell, Oxford. Odland, A., 1996. Differences in the vertical distribution patterns of Betula pubescens in Norway and its ecological signi®cance. PalaÈoklimaforschung 20, 43±59. PaÊhlsson, I., 1981. A pollen analytical study on a peat deposit at LaÂgafell, southern Iceland. Striae 15, 60±64. Paus, Aa., 1989. Late Weichselian vegetation, climate and ¯oral migration at Eigebakken, South Rogaland, southwestern Norway. Review of Palaeobotany and Palynology 61, 177±203. Praglowski, J.R., 1962. Notes on the pollen morphology of Swedish trees and shrubs. Grana Palynologica 3, 45±65. Praglowski, J.R., 1966. On pollen size variations and the occurrence of Betula nana in different layers of a bog. Grana Palynologica 6, 528±543. Prentice, I.C., 1981. Quantitative birch (Betula L.) pollen separation by analysis of size frequency data. New Phytologist 89, 145± 157. Rundgren, M., 1995. Biostratigraphic evidence of the AllerùdYounger Dryas-Preboreal oscillation in northern Iceland. Quaternary Research 44, 405±416. Rundgren, M., 1998. Early-Holocene vegetation of northern Iceland: pollen and plant macrofossil evidence from the Skagi peninsula. The Holocene 8, 553±564. Â ., 1999. Plant survival in Iceland Rundgren, M., IngoÂlfsson, O during periods of glaciation? Journal of Biogeography 26, 387±396. StefaÂnsson, S., 1948. FloÂra Âislands. Hi Âõslenzka naÂttuÂrufrñ ifeÂlag, 3rd ed. Akureyri, p. 407. StoÈtter, J., Wastl, M., 2001. Palaeoclimatic investigations in Northern IcelandÐterrestrial reference for the variations in the system of ocean, atmosphere and ice distribution in the North Atlantic during the Holocene. PalaÈoklimaforschung. in press. StoÈtter, J., Wastl, M., Caseldine, C.J., HaÈberle, T., 1999. Holocene Palaeoclimatic reconstruction in Northern Iceland: approaches and results. Quaternary Science Reviews 18, 457±474. SveinbjoÈrnsson, B., 1993. Climate and growth of mountain birch near the treeline in Northern Sweden and Iceland. PalaÈoklimaforschung 9, 57±68. TerasmaÈe, J., 1951. On the pollen morphology of Betula nana. Svensk Botanisk Tidskrift 45, 358±361. Usinger, H., 1975. Pollenanalytische und stratigraphische Untersuchungen an zwei SpaÈtglazial-Vorkommen in SchleswigHolstein. Mitteilungen der Arbeitsgemeinschaft Geobotanik in Schleswig-Holstein und Hamburg 25, 1±173. Usinger, H., 1981. Zur spaÈt- und fruÈhen-postglazialen Vegetationsgeschichte der Schleswig-Holsteinen Geest nach einem Pollenund Pollendicht-diagramm aus dem Esinger Moor. Pollen et Spores 23, 389±432. Vaarama, A., Valanne, T., 1973. On the taxonomy, biology and origin of Betula tortuosa Lebed. Reports from Kevo Subarctic Research Station 10, 70±84. van Dinter, M., Birks, H.H., 1996. Distinguishing fossil Betula nana and Betula pubescens using their wingless fruits: implications
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for the late-glacial history of western Norway. Vegetation History and Archaeobotany 5, 229±240. Vasari, Y., Vasari, A., 1990. L'histoire holoceÁne des lacs Islandais. In: Devers, S. (Ed.), Pour Jean Malaurie. 102 teÂmoignages en hommage aÁ quarante ans d'eÂtudes arctiques. EÂditions Plon, Paris, pp. 277±293.
Vorren, K.-D., Stavseth, A., 1996. Late Holocene sediments of an alpine lake in North Norway. PalaÈoklimaforschung 20, 257± 269. Walker, D., 1955. Studies in the post-glacial history of British vegetation XIV. Skelsmergh Tarn and Kentmere, Westmorland. New Phytologist 54, 222±254.
www.elsevier.com/locate/revpalbo
Changes in Betula in the Holocene record from IcelandÐa palaeoclimatic record or evidence for early Holocene hybridisation? Chris Caseldine* Department of Geography, School of Geography and Archaeology, University of Exeter, Amory Building, Rennes Drive, Exeter EX4 4RJ, UK
Abstract Tree birch in its mountain form is the only woodland-forming tree found in the Holocene record of Iceland. Given the close relationship between tree-lines and summer temperature in Fennoscandia, it should therefore provide a valuable proxy temperature indicator for the Holocene in Iceland. Following a review of issues relating to the taxonomy of mountain birch, and the identi®cation of tree birch in the palynological record, data are presented from the TroÈllaskagi area of northern Iceland relating to the development of birch woodland in the early Holocene. It is argued that following the development of communities in which dwarf birch, Betula nana L., was important, true woodland communities only became established after a period of relative instability in the vegetation cover. Data from morphometric analyses of Betula pollen from this period are interpreted as representing probable hybridisation between the different forms of birch, rather than re¯ecting a clear climatic signal. The question is also raised of whether the progenitor of tree birch in Iceland was an immigrant form of mountain birch, (Betula pubescens Ehrh. ssp. tortuosa (Lebed.) Nyman as de®ned in Scandinavia), or whether Icelandic mountain birch developed by hybridisation in Iceland between Betula nana L. and Betula pubescens Ehrh. during the Holocene. The use of birch as a palaeoclimatic indicator, especially prior to the establishment of birch woodland at suitable locations and altitudes, should not therefore be accepted unless accompanied by other independent climatic evidence. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Betula; Iceland; Holocene; hybridisation; palaeoclimate; pollen
1. Introduction Birch is the only forest-forming tree in Iceland, hence understanding changes in the composition and extent of birch woodland is crucial to establishing the potential of palaeoecological data for palaeoclimatic information, both in terms of tree-line movement and community change. Studies in Scandinavia have shown a link between climatic parameters, particularly summer temperature, and the oscillations of the * Fax: 144-139-226-3342. E-mail address: [email protected] (C. Caseldine).
sub-arctic (alpine) birch tree-line (Odland, 1996; Mook and Vorren, 1996). Correlations of between 0.92 and 0.87 have been found between tree-line altitude and a variety of summer temperature proxies (mean July temperature, mean maximum July temperature, mean tritherm [June, July, August] temperature and mean maximum tetratherm [including September] temperature. Thus by analogy it would appear likely that by determining patterns of change in tree birches in Iceland, data on Holocene palaeoclimatic ¯uctuations could be produced to complement the developing glacial record (StoÈtter et al., 1999). Previous work in the country has adopted
0034-6667/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0034-666 7(01)00082-3
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such an approach at a general level (Einarsson, 1961, 1963; Bartley, 1973; HallsdoÂttir, 1987; Vasari and Vasari, 1990), seeing oscillations in the Betula pollen curve as broadly re¯ecting climate changes, albeit also in relation to expanding mires and the in¯uence of increasing contributions from wetland taxa, notably Cyperaceae. Uncertainties, however, still exist in a number of areas concerning the species identi®cation of Betula pollen, and interpretation of its relative and absolute changes through time, which need to be addressed before palaeoclimatic information can be derived with con®dence. 2. Probable Betula species in Iceland Identi®cation of what is commonly termed mountain birch (Betula tortuosa) to species level is still an area of considerable debate amongst taxonomists (Vaarama and Valanne, 1973; Kallio et al., 1983). The evolution of birch species in the subarctic appears to have been a feature of Holocene vegetation change and is still occurring. The key area for evolution is where the subsections Albae and Nanae meet in conditions allowing successful introgressive hybridisation, unlike the situation further south within the area dominated by Albae where limiting factors prevent such a process taking place. A number of features characteristic of tortuosa forms have been identi®ed by Vaarama and Valanne (1973) which indicate the in¯uence of B. nana in producing a species (or subspecies) particularly able to adapt to the climatic severity of the subarcticÐ`the morphological features common to B. tortuosa and B. nana and absent from the southern provenances of B. pubescens are suf®cient to warrant the assumption that gene ¯ow has taken place through hybridisation and introgression from B. nana to the northern B. pubescens called B. tortuosa' (p. 82). Thus according to these authors, mountain birch, either as a separate species or, in other researchers' views as a subspecies of B. pubescens, has developed because of the hybridisation of B. nana and B. pubescens. In Iceland the de®nition of species varies between ¯oras. All agree in the presence of a form of dwarf birch, Betula nana L., which apparently shows little variation, but there are then a series of forms of tree or shrub birch. GroÈntved (1942) includes all non-dwarf
forms within B. pubescens Ehrh., variations being due to introgression with B. nana, and it was his ®ndings that have been quoted by Vaarama and Valanne (1973) as crucial evidence for effective hybridisation. StefaÂnsson (1948) divides the non-dwarf forms into B. pubescens Ehrh., B. coriaceae Gunnarss., and a hybrid of B. nana L. and B. pubescens Ehrh. LoÈve (1945) also includes B. callosa NotoÈ., but rede®nes the most common form, B. pubescens, as B. tortuosa Lebed. In 1956 LoÈve and LoÈve also introduce B. concinna Gunnarss., suggesting that apart from B. nana L. the remaining tree/shrub forms are combinations of the ®ve different species they de®ne. These authors also doubted the hybridisation evolution of B. tortuosa, arguing that there was no evidence to show that hybrids were not sterile. More recent studies both in the ®eld (Elkington, 1968) and in controlled experiments (Vaarama and Valanne, 1973) seem to have disproved this, although successful seed production only occurred in favourable years, and was not the normal pattern. Cytogenetic evidence has also been used to describe the process of hybrid introgression in Iceland (Anamthawat-JoÂnsson and ToÂmasson, 1990; Anamthawat-JoÂnsson, 1994). Perhaps crucially for the Icelandic situation Vaarama and Valanne (1973) suggest that `it is reasonable to assume that there have been times earlier in the post-glacial period when a more favourable climate has allowed frequent and effective gene exchange between species' (p. 82). In their pollen and macrofossil diagrams Vasari and Vasari (1990) use the subsection term B. alba, but refer to B. pubescens in the text. The most recent guide to ¯owering plants by Kristinsson (1987) simply de®nes Betula pubescens Ehrh. and B. nana L., although comments on hybridisation between the two, especially where the latter is common. Most pollen analysis in Iceland refers to Betula pubescens and B. nana (Einarsson, 1961; HallsdoÂttir, 1987), as does Glawion (1985) in his detailed ecological study of Icelandic vegetation, or does not go beyond Betula (Bartley, 1973; PaÊhlsson, 1981). Thus in terms of which species of tree/shrub birch exist, or existed in Iceland during the Holocene, there is still a lot of uncertainty, just as there appears to be in much of Scandinavia. One of the few ecological studies of birch in Iceland de®nes the major tree birch species as Betula pubescens Ehrh. ssp. tortuosa (Lebed.) Nyman (SveinbjoÈrnsson, 1993), also the
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assumed form for tree birch in Greenland (Fredskild, 1991). A similar approach is adopted by AnamthawatJoÂnsson (1994) who states, `Icelandic woodland birch is similar to the variety B. pubescens Ehrh. spp. tortuosa (Lebed.) Nyman, often known as mountain birch in the alpine zone of Fennoscandia' (p. 10). Despite this, it is clear that although Betula nana L. can be clearly identi®ed as a distinct species, the remaining tree/shrub birches are highly polymorphic and potentially include a number of hybrid forms, with perhaps many more in the past, especially in locations suitable for hybridisation (Elkington, 1968). Although other birch species, notably B. pendula, are found in Iceland today it is likely that B. pubescens and/or B. tortuosa were the dominant species in the evolution of the Holocene ¯ora. 3. Species identi®cation of Betula pollen The separation of pollen of different Betula species has been the subject of several studies starting with morphometric separation of B. nana, pendula and pubescens by Jentys-Szaferova over 70 years ago (1928), and the literature has been summarised recently by MaÈkelaÈ (1996, 1999). Different workers favour different approaches to separation of species, particularly the separation of B. nana. Praglowski (1962, 1966), following TerasmaÈe (1951), proposed a morphological separation, and a number of laboratories use this approach, with or without taking measurements. Birks (1968), following Walker (1955) proposed separation of B. nana using a morphometric approach, utilising a ratio between the diameter of the full grain and the pore depth. This was extended by more formal statistical approaches adopted by Usinger (1975), and by Gordon and Prentice (1977) who developed analyses based on distinguishabilities between species. MaÈkelaÈ (1996) favours a morphometric approach, arguing that it is more objective than identi®cations based on a subjective assessment of morphology (with or without taking into account relative size) and also used a form of distinguishability analysis on modern material to attempt to separate species. Arising out of the range of approaches so far adopted are two basic questions: (1) is it possible to separate out individual species from the range of birch species and subspecies that occur in
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northern Scandinavia (the focus of most of this work): and (2) is it at least possible to reliably separate B. nana from shrub/tree forms? MaÈkelaÈ (1996, 1999) was particularly concerned to try to distinguish between the major tree birch species and subspecies, as well as separating out B. nana, and used Scandinavian herbarium and collected specimens from a range of locations as a basis for detailed morphometric analysis, concentrating on maximum grain diameter. Previous morphometric work, starting with Jentys-Szaferova (1928), had suggested some gradation in size between B. nana, pendula, pubescens and tortuosa, but not always in the same order (Eneroth, 1951), and the variations between species were often less than those within species (Berglund and Digerfeldt, 1970). Comparability of observations between different workers also proved dif®cult because different criteria and preparation procedures were used in deriving sets of measurements (Andersen, 1980; Usinger, 1981; Fredskild, 1991). The question also remains, given the uncertain taxonomy of mountain birch (something re¯ected in the taxonomic af®nities given to samples used by MaÈkelaÈ (1996)), whether morphometric separation should be possible within an overall group, especially as both Eneroth (1951) and Usinger (1975) were able to demonstrate increases in pollen size for the same species with altitude and latitude. Where fossil pollen assemblages include two or more species, the use of frequency distributions (Prentice, 1981; Gordon and Prentice, 1977), either in a simple graphical form, or in a more rigorous multivariate statistical analysis, have, however, suggested that separate populations could be identi®ed whatever their taxonomic origin. On the basis of her work MaÈkelaÈ (1996) discovered just how dif®cult it was to separate the main tree birch forms, although she did ®nd a general trend in size from B. pendula ! B. pubescens. She had two separate groups of B. pubescens ssp. tortuosa from her Finnish site, and assumed that one set derived from one-stemmed, monocormic trees, whereas the other was from polycormic, multi-stemmed forms. Because her B. tortuosa pollen had large grain diameters she further assumed that the polycormic form was the group with the larger of the diameters, and was therefore encouraged at the possibility of separating B. tortuosa and hybrids involving this species from the much smaller pollen of B. nana and, possibly, B.
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pendula. There were also two groups of B. tortuosa sizes, hence replicable differentiation remains highly questionable. In conclusion she comments that there do appear to be systematic size differences between the populations she measured and individual populations do conform to normal Gaussian distributions at an individual level, albeit with overlaps in the sizes at 1 s. In terms of absolute sizes though there are still considerable within-species variations when samples are taken from different locations. All her measurements were taken from fresh material and MaÈkelaÈ does point out that `It is also possible that the pollen sizes in the very variable B. pubescens group have changed through time, since hybridisation and introgression seem to have played a major role in the evolution of mountain birch' (1996, p. 255). This latter point is probably the case in Iceland, especially in more marginal locations (Elkington, 1968). The most encouraging ®nding of MaÈkelaÈ's work, however, is the clear separation of the pollen of B. nana simply on grain diameter (17.31 ^ 0.88 [1 s], n 110, in silicon oil) with very little overlap with the other species, and none with one of the sets of B. pubescens ssp. tortuosa (25.98 ^ 2.30, n 615, the other being 22.0 ^ 1.57, n 324), perhaps the most likely form of tree birches in Iceland, especially if the group represents the polycormic form. Thus, to return to the two questions posed earlier it would appear possible to distinguish between dwarf birch and other forms of birch on the basis of grain diameter, especially if supported by pore depth measurements following Walker (1955) and Birks (1968). Birks (1968) quotes a range of average grain diameter: pore depth ratios from 10 to 11.5 for B. nana, and 7.3±7.91 for B. pubescens. Attribution of possible tree/shrub forms for larger diameter grains to species or subspecies is though not possible, especially where there is a likelihood of hybridisation. Shifts in mean size of grains in populations clearly larger than B.nana may re¯ect the increasing in¯uence of B. tortuosa in a polycormic form, but this would also be very speculative on the available evidence. Past palynological work in Iceland has either used separation of B. nana by morphology (HallsdoÂttir, 1987, although then produced a single curve for Betula), not differentiated between species (Bartley, 1973; Einarsson, 1961, 1963; PaÊhlsson, 1981; Vasari and Vasari, 1990; Caseldine and Hatton, 1994), or
used a size/frequency distribution approach (HallsdoÂttir, 1990; Fri riksdoÂttir, 1973 quoted in HallsdoÂttir, 1995). Rundgren (1995, 1998) (pers. commun.) for his work on Skagi identi®ed three categories on morphometric and morphological criteriaÐB. nana, B. pubescens ssp. tortuosa and Betula undiff. The size criteria used in Iceland by Rundgren and HallsdoÂttir are outlined in Table 1. Because both authors used glycerol as the mounting medium the sizes are larger than those quoted by MaÈkelaÈ, who used silicon oil. Correction can be made (Fñgri and Iversen, 1989; Moore, Webb and Collinson, 1991) but quoted ®gures vary from 1.1 to 1.3 £ (glycerol larger than silicone mountant, Fñgri and Iversen, 1989, p. 285). Thus for Iceland, on the basis of very limited observations it appears that mean diameters for B. nana are around 17 mm when mounted in silicon oil, certainly no smaller, a ®gure that agrees well with other studies. The sizes of the non-dwarf forms are, however, more dif®cult to assess and compare. For palynological work in Iceland, although only based so far on a small number of observations, it would seem reasonable to argue that it is possible to separate B. nana pollen from non-dwarf forms for particular assemblages (not necessarily for every grain) by morphometric means using grain diameter ^ pore depth. Once grains are found which lie consistently outside the range for B. nana, it is only possible to identify them as tree/shrub forms on this basis, although frequency distributions would help in de®ning whether discrete populations were present. It may be that large grain sizes refer to B. tortuosa, or B. pubescens ssp. tortuosa, but the morphometric evidence alone would not be suf®cient to con®rm such an interpretation. 4. Macrofossil evidence The presence of macrofossils identi®able to species level provides the most convincing evidence for the local presence of speci®c forms of dwarf or tree birches. Separation of the fruits of B. nana from B. pubescens or B. pendula can be made relatively easily when still winged, and van Dinter and Birks (1996) have shown separation of various Betula fruit bodies which have lost their wings on sub-fossilisation. Although separation of dwarf birch from tree birches proved possible there was considerable overlap
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Table 1 Measurements of Betula pollen (^1 s) from a selection of sources. Species attributions are as in original texts. All are based on modern material except those of HallsdoÂttir (1990) which are representative samples of Holocene age, and the samples from North Iceland from this paper, which were taken from surface samples. The North Iceland samples were collected from VaglaskoÂgur, FnjoÂskadalur, those from South Iceland from Skaftafell Source
Taxon
Size details (mm)
Comments
This paper
B. nana (N.Iceland) B. pubescens ssp. tortuosa (N.Iceland) B. pubescens ssp. tortuosa (S. Iceland)
19.23 ^ 1.43 24.06 ^ 1.90
Silicon oil mount
Rundgren 1998, pers. comm.
B. pubescens ssp. tortuosa Betula undiff B. nana B. nana
. 27.2 or 24±27 1 morph 24±27 24±27 1 morph , 24 1 pore depth ,4.1
Glycerol mount
HallsdoÂttir, 1990
B. nana B. pubescens
23.5 30
Glycerol mount, estimated from graph
MaÈkelaÈ, 1996
B. B. B. B.
nana pubescens ssp. tortuosa pubescens ssp. tortuosa pubescens hybrid
17.31 ^ 0.88 22.06 ^ 2.25 25.98 ^ 2.30 24.26 ^ 1.59
Silicon oil mount
Birks, 1968
B. B. B. B. B. B. B. B.
nana nana nana pubescens pubescens tortuosa tortuosa tortuosa
18.71 ^ 1.30 18.60 ^ 1.09 18.72 ^ 1.21 22.63 ^ 1.43 23.31 ^ 1.59 26.42 ^ 1.60 23.43 ^ 1.38 26.07 ^ 1.72
Silicon oil mount
Eneroth, 1951
B. nana B. pubescens B. tortuosa
19.4 25.0 26.1
Glycerol mount
between the tree forms and their hybrids with B. nana. The presence of megafossils in the form of buried trunks is often further evidence for local presence. Birks (1993) and van Dinter and Birks (1996) have successfully used macrofossil analysis to demonstrate that tree birches failed to reach south-west Norway during the Allerùd, despite high percentage and in¯ux values for the pollen of tree birches (up to 40±50% and 900 grains cm 22 yr 21, Paus 1989). It should, however, be noted that long-distance transport is possible for some macrofossils, especially if only occasional or isolated remains are found (Birks, 1973). Glaser (1981), and see comments in Birks (1991) has produced a model for macrofossil dispersal in tundra environments and winter dispersal, possibly of material from a distant tree-line can be important,
26.94 ^ 2.10
especially where deposition is facilitated by a lack of local macrofossils. The converse is of course also true, absence of macrofossils does not necessarily imply local absence of any particular tree species (Barnekow, 1999). In Iceland there has been relatively little use of macrofossil analysis apart from the description of large birch remains in peat pro®les (Bartley, 1973; Haraldsson, 1981; see Fig. 5 in HallsdoÂttir, 1995). Vasari and Vasari (1990) identi®ed scales and fruits de®ned as Betula alba, and leaves, scales and fruits of B. nana at HafratjoÈrn in northern Iceland, but at LomatjoÈrn in the south they only separated out unde®ned macrofossils of B. pubescens and B. nana. Also in northern Iceland for the Late Weichselian and Early Holocene, Rundgren (1998) identi®ed
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Table 2 Published in¯ux rates for Betula from modern studies. Timescale
Source
Vegetation character and altitude
In¯ux rate (Range or average in pollen cm 22 yr 21)
Alm (1996)
Sub-alpine birch forest with scattered pine, 400±160 m from tree-line
234±2770
1991±1993
133±527
1991±1993
16±283
1991±1993
500 1600 2000
1984, 1986±1991 1984, 1986±1991 1984, 1986±1991
Sub-alpine birch forest, 520 m, 240 m from tree-line Low alpine heath, 600 m, 140 m from tree-line Hicks (1996)
Treeless Mountain birch woodland Birch and pine woodland
macrofossils of B. nana. There is clearly a lot of potential for further macrofossil analysis of birch remains in Iceland, as they are likely to provide the key evidence for the presence of tree birches at sites, although they will be unlikely to allow determination of what species of tree birch were present. Suf®cient material from good preserving environments may be found from peat sites, especially those in open sections; cores have so far derived mainly from sites with relatively limited macrofossil records, and thus it may not always prove easy to derive this form of data, in combination with other palaeoecological indicators, to the level of rigour that would be preferred. 5. Pollen in¯ux rates Recent studies of surface pollen deposition, particularly collecting by means of Tauber traps, have been undertaken with the aim of ®nding threshold values for certain types of plant community, and much of this has concentrated on the tree-line of Fennoscandia. Collection periods range from only 2 or 3 years of trapping in Norway (Alm, 1996; Vorren and Stavseth, 1996) to 7 years in Finland (Hicks, 1996). Not surprisingly results have varied considerably (Table 2), with differences also between collecting media, traps against glass jars for instance (Alm, 1996), but some recurrent patterns have emerged and Hicks (1996) offers hope for this approach to vegetation reconstruction. By contrast in their introduction to the volume on Holocene Treeline Oscillations, Dendrochronology and Palaeoclimate, Birks et al. (1996b) are more cautious, echoing van Dinter and
Birks (1996): `It seems that the percentages and in¯ux of Betula pollen cannot be interpreted in any strict way in terms at which Betula trees may be locally represented' (p. 237). What surface pollen in¯ux studies have shown is that where tree birches are present at or close to the tree limit pollen in¯ux can be as low as ,200 grains cm 22 yr 21(Vorren and Stavseth, 1996) re¯ecting varying actual annual rates over a 3-year period of 133±527 grains cm 22 yr 21 (Alm, 1996). Hicks (1996) ®nds an average value over 7 years of 1600 grains cm 22 yr 21 within birch woodland in Finland, contrasting with 500 grains cm 22 yr 21 for treeless areas. Because of this variation most workers have been guarded about de®ning thresholds and Alm (1996) emphasises the good vertical and horizontal mixing of Betula compared to Pinus, suggesting a threshold of 500 grains cm 22 yr 21 as likely to indicate local presence. In contrast Vorren and Stavseth (1996) prefer the lower value of 240±360 grains cm 22 yr 21, 20% of which derives from winter in¯ux. All these values are well below the maximum values for the Allerùd treeless environment of southern Norway described earlier. Alm (1996) does, however, also show a reduction of up to 75% in collection between the woodland and collecting mires 20±30 m away from the trees, allowing some sort of calibration between peat sites and neighbouring woodland. In Iceland only HallsdoÂttir (1995, 1996) has presented pollen in¯ux rates from palaeoecological sites for the period when birch woodland was most likely to have existed. Results from Vatnskotsvatn (at 64 m asl) show values for Betula of over 2000 grains cm 22 yr 21 in the lake sediment, but after
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Fig. 1. Betula pollen data from HaÂmundarsta ahaÂls 8: Total Betula is expressed as percent Total Land Pollen (TLP); Betula mean diameters are for all measurable grains per level or a sample of 30.
ca. 6000 BP at both this site and Hestvatn in southern Iceland values are much lower at around 150 grains cm 22 yr 21. There are so far no published data on tree birch in¯uxes in Iceland from within current communities considered at all comparable to Holocene examples, and any data collected would provide an uncertain analogue due to the dif®culties of comparing the overall community structure of present woodland with the woodland that probably existed earlier in the Holocene. 6. Betula in the Early Holocene from Northern IcelandÐdata from EyjafjoÈr ur Fig. 1 shows the record for the total Betula pollen count from HaÂmundarsta ahaÂls, a small (90 £ 60 m) enclosed basin at 100 m asl on the western margin of EyjafjoÈr ur, Northern Iceland (Figs. 2 and 3). The record begins after the deposition of the Saksunarvatn
145
ash which here has its upper limit at 480 cm and the lower limit is dated to 9195 ^ 65 14C yr BP (AA27591) from an adjacent core (StoÈtter and Wastl, in press) (compared to the date range of 8930±9060 14C yr BP (Birks et al., 1996a), and 10180 ^ 60 ice core years (GroÈnvold et al., 1995)). Although the pro®le is not yet fully dated the tephra H5, dated in the area to 6065 ^ 55 14C yr BP (Wastl et al., in preparation), occurs between 25 and 32 cm above the peat/gyttja transition in ®ve adjacent cores, whereas in the analysed core which has a relatively sharp transition to a sedge peat at 292 cm it was not found. It is likely therefore that the record covers the period between ca. 8500 and 6200±6500 14C yr BP during which birch woodland became established in most suitable locations in Iceland (HallsdoÂttir, 1995). The pollen record is taken only from the gyttja which has extremely good pollen preservation and is not affected by a high autochthonous pollen component, i.e. Cyperaceae or aquatic taxa. Lying at only 100 m, despite its relatively proximity to the fjord, the site would be expected to be suitable for such woodland establishment by comparison with patterns established elsewhere in the region (Einarsson, 1961; Bartley, 1973; HallsdoÂttir, 1990; Caseldine and Hatton, 1994ÐFig.2). Fig. 1 shows values for total Betula as percent Total Land Pollen (TLP), rising to up to 80% in HH vii, and concentrations of Betula pollen as grains cm 23. It is not possible to estimate in¯ux values in the absence of more dates but values in excess of 1000±2000 grains cm 22 yr 21 are likely for the upper levels with highest concentrations. In the ®nal column the mean diameter of all, or a sample of, measureable Betula grains is included. The Betula record can be divided into fairly clear zones as outlined in Fig. 1. The lowermost zone, HH iii, has few grains and those that occur have large diameters and are assumed to represent long-distance transport, either from outside Iceland or from very limited areas of Iceland supporting early tree birch growth. HH iv is dominated by what appears to be B. nana from the mean diameters and the low percent and concentrations. In HH v, although there is an increase in concentrations and in %TLP values the diameters still indicate small grains characteristic of B. nana. This is in marked contrast to the uppermost zone, HH vii, which has a much larger mean diameter throughout, over double the %TLP values, and
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Fig. 2. Photograph of the HaÂmundarsta ahaÂls site looking southwest. The core was taken from close to the centre of the main basin.
considerably increased concentrations of birch pollen, interpreted as re¯ecting local closed birch woodland around the margins of the site almost exclusively comprising tree birches. Notwithstanding uncertainties over the chronology in¯ux values appear to be in excess of those found from small mires within birch woodland (Table 2). The intermediate zone, HH vi, whilst showing a remarkably consistent curve for %TLP, and relatively uniform concentrations, shows transitional values for mean grain diameters. The period covered by HH vi thus represents the change from a marginal community in which dwarf forms were dominant to a full woodland community. The zone may last several hundred years (constant sedimentation would give 600 14C yr), and is clearly a dynamic phase in the local vegetation by the end of which tree birches were strongly established. In order to examine this period more closely, and in particular to try to understand why tree birches took so long to become well established, and what the relative in¯uences of tree and dwarf forms were during this period, more detailed morphometric analyses were carried out.
Fig. 4 shows histograms for samples of Betula mean diameters throughout the record. Zones HH iv and v have mostly unimodal forms with peaks between 17.5 and 19.6 mm, the variability probably due to the small sample sizes available for analysis, re¯ecting the dominance of B. nana. This is supported by the mean diameters of modern B. nana from the area as also shown in Fig. 4 and Table 1 (19.23 ^ 1.43 mm, n 23). The top zone, HH vii, also shows a predominantly unimodal pattern, especially in midzone, with a peak diameter around 22±23 mm, overlapping at ^1 s with modern data on tree birch in the area (24.06 ^ 1.9 mm, n 60), albeit generally slightly smaller in maximum diameter, but well below values for tree birches from Skaftafell in southern Iceland (26.94 ^ 2.1 mm, n 100). HH vi has a more complex pattern characterised by a peak in several levels at around 21 mm, not the same group as in either the zones above or below. There are some variations in all zones, as in the clear occurrence of B. nana grains at times throughout the record, and an interesting shift to an HH vi like pattern between 314±5 and 298±9 in HH vii. What is missing,
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Fig. 3. Location map for the TroÈllaskagi peninsula, northern Iceland, showing HaÂmundarsta ahaÂls and other published pollen sites mentioned in the text.
however, is any clear bimodality in the record for grain diameters as might be expected if both dwarf birch and tree birches were present and there was a gradual replacement of one by the other. The pattern is in fact very similar to that shown in a more limited data set at KrosshoÂlsmyÂri in Flateyjardalur to the east for a similar stratigraphic position (HallsdoÂttir, 1990). As additional evidence a more limited series of samples were examined for their diameter:pore ratio
(Fig. 5). The contrast between the B. nana-dominated zones HH iv and v with their high ratios and only occasional tree birch grains, and the dominantly low ratio HH vii deriving from tree birches is again clear. HH vi seems to show close af®nity to tree birch ratios, albeit with a signi®cant number of ratios intermediate between the extremes, and one level, 416±7 cm, with a good bimodal distribution. Examination of all the data from HH vi supports the
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Fig. 4. Histograms of samples of mean diameters of Betula grains from the four zones at HaÂmundarsta ahaÂls. Data from modern samples of Betula nana were prepared from samples collected from SkõÂ adalur, for Betula pubescens ssp. tortuosa, samples were collected east of this area at VaglaskoÂgur, FnjoÂskadalur, and from Skaftafell, southern Iceland.
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Fig. 5. Betula diameter:pore ratios (following Walker, 1955) for selected levels from HaÂmundarsta ahaÂls. Modern samples are as in Fig. 4.
interpretation of a relatively complex period of vegetation change in the early Holocene leading up to the establishment of closed birch woodland. The transitional nature of the morphometric data and the steplike changes in both %TLP and pollen concentrations is interpreted as representing a phase when there was considerable hybridisation taking place between wellestablished B. nana and the incoming form of tree birch. Eventually this hybridisation ceased at this low altitude, well below the presumed tree-line, and woodland comparable to that in Fennoscandia today probably emerged. Changes towards the top of HH vii, when there is a shift towards smaller mean diameters (Figs. 4 and 5), could be indicative of a return to local hybridisation even at this low altitude. Why such a development should have taken place
149
with the migration of tree birches is uncertain, as is the control on such a process. Climatic conditions in the early Holocene, as seen in the offshore record (KocË et al., 1996; Jiang et al., 1999) and in the early optimum indicated at high latitudes in the North Atlantic region, appear to have been suitable for the establishment of birch woodland, although there is little direct detailed climatic evidence available for Iceland for this period. Complications in the record for Betula as indicated here make the use of this taxon as a widespread palaeoclimatic indicator in Iceland uncertain. It could, however, be that understanding the Betula record, and the meaning of the changes discussed above, may lead to deriving more precise climatic signals, especially if hybridisation was, at least in part, climatically controlled. The hybridisation hypothesis outlined above lacks suitable analogue data from Iceland. Hybrid forms can be observed in the area today, both at relatively high sites and on drained but abandoned ®elds (M. Wastl and P. Wookey, pers. comm.). We do not have morphometric data on pollen from such specimens, indeed it has even proved dif®cult to ®nd pollen released from B. nana in the area today, and the results shown in Figs. 4 and 5 are based on relatively small samples despite using areas with fruiting individuals. Comparison of the Holocene data with the present material (Figs. 4 and 5) reveals higher mean diameters for present day tree birches both in the region (24.06 ^ 1.9 mm) and especially in southern Iceland (26.94 ^ 2.10 mm), as against 22±23 mm throughout HH vii. For B. nana the diameters are virtually identical. It does therefore seem likely that the taxonomy of tree birches has continued to change during the Holocene in Iceland, even if only to a small degree, as implied for Fennoscandia by MaÈkelaÈ (1996), and it may be pertinent to ask the question: what was the form of tree birch that Icelandic trees derived from? Irrespective of whether tree birches survived the Younger Dryas (or even the Last Glacial Maximum) (Rundgren and IngoÂlfsson, 1999), or whether they immigrated at the end of the last glacial stage from Scandinavia (Buckland and Dugmore, 1991), it is not known whether the original trees were a form of B. tortuosa already developed by hybridisation in Fennoscandia, or whether B. pubescens arrived to hybridise with B. nana in Iceland and thus form a speci®cally Icelandic subspecies. If it was
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the latter then this could explain why the spread of tree birches was relatively slow and not the rapid migration seen elsewhere in northern Europe at the opening of the Holocene, as trees already well adapted to sub-arctic conditions did not undergo signi®cant hybridisation with dwarf birch. Genetic studies of Icelandic birch have so far indicated a likely single gene pool despite the observed high genetic variability (Anamthawat-JoÂnsson, 1994), with links to populations from Scandinavia (Anamthawat-JoÂnsson quoted in Rundgren and IngoÂlfsson, 1999), but what is needed is a much more rigorous analysis of the Holocene palynological, and where possible, macrofossil data before we can more fully understand the nature of woodland development and its relationship to climatic variability during the Holocene.
Acknowledgements I am grateful to Margret HallsdoÂttir and Mats Rundgren for helping with unpublished Icelandic data and to Kevin Edwards and Margret HallsdoÂttir for comments on the text. Much of this was written whilst at the University of Innsbruck and I thank Hans StoÈtter and Maria Wastl for their hospitality and help. Original work on the pollen from HaÂmundarsta ahaÂls was undertaken by Jackie Hatton.The diagrams were drawn by Helen Jones in the School of Geography and Archaeology at Exeter. The core was collected by Hans StoÈtter, Maria Wastl, Georg Albert, Nancy Bertler, Markus Maukisch and Johannes Simstich.
References Alm, T., 1996. Pollen in¯ux in traps along a height transect on  djit, Troms, Norway. PalaÈoklimaforschung 20, 157± Mount A 171. Anamthawat-JoÂnsson, K., 1994. Genetic variation in Icelandic birch. Norwegian Journal of Agricultural Sciences, 9±14 (Supplement 18). Anamthawat-JoÂnsson, K., ToÂmasson, Th., 1990. Cytogenics of hybrid introgression in Icelandic birch. Hereditas 112, 65±70. Andersen, S., Th, ., 1980. Early and Late Weichselian chronology and birch assemblages in Denmark. Boreas 9, 53±69. Barnekow, L., 1999. Holocene tree-line dynamics and inferred Ê bisko area, northern Sweden, based climatic changes in the A on macrofossil and pollen records. The Holocene 9, 253±266. Bartley, D.D., 1973. The stratigraphy and pollen analysis of peat
deposits at Ytri-Baegisa near Akureyri, Iceland. Geologiska FoÈreningens i Stockholm FoÈrhandlingar 95, 410±414. Berglund, B.E., Digerfeldt, G., 1970. A palaeoecological study of the Late-Glacial lake at Torreberga, Scania, South Sweden. Oikos 21, 98±128. Birks, H.H., 1973. Modern macrofossil assemblages in lake sediments in Minnesota. In: Birks, H.J.B., West, R.G. (Eds.), Quaternary Plant Ecology. Blackwells Scienti®c Publications, Oxford, pp. 173±189. Birks, H.H., 1991. Holocene vegetational history and climatic change in west SpitsbergenÐplant macrofossils from Skardtjùrna, an Arctic lake. The Holocene 1, 209±218. Birks, H.H., 1993. The importance of plant macrofossils in LateGlacial climatic reconstructions: an example from Western Norway. Quaternary Science Reviews 12, 719±726. Birks, H.H., Gulliksen, S., Ha¯idason, H., Mangerud, J., Possnert, G., 1996a. New radiocarbon dates for the Vedde Ash and the Salsunarvatn Ash from western Norway. Quaternary Research 45, 119±127. Birks, H.H., Vorren, K.-D., Birks, H.J.B., 1996b. Holocene treelines, dendro-chronology and palaeoclimate. PalaÈoklimaforschung 20, 1±18. Birks, H.J.B., 1968. The identi®cation of Betula nana pollen. New Phytologist 67, 309±314. Buckland, P.C., Dugmore, A.D., 1991. If this is a refugium, why are my feet so bloody cold? The origins of the Icelandic biota in the light of recent research. In: Maizels, J.K., Caseldine, C. (Eds.), Environmental Change in Iceland: Past and Present. Kluwer, Dordrecht, pp. 107±126. Caseldine, C., Hatton, J., 1994. Interpretation of Holocene climatic change for the EyjafjoÈr ur area of northern Iceland from pollenanalytical data: comments and preliminary results. In: StoÈtter, J., Wilhelm, F. (Eds.), Environmental Change in Iceland. MuÈnchener Geographische Abhandlungen, Reihe, pp. 41±62 (B 12). Einarsson, ., 1961. Pollenanalytische Untersuchungen zur spaÈtund postglazialen Klimageschichte Islands. SonderveroÈffentlichungen des Geologisches Instituts der UniversitaÈt KoÈln 6, 1±52. Einarsson, , 1963. Pollen-analytical studies on the vegetation and climate history of Iceland in Late and Post-glacial times. In: LoÈve, A., LoÈve, D. (Eds.), North Atlantic Biota and Their History. Pergamon, Oxford, pp. 355±365. Elkington, T.T., 1968. Introgressive hybridization between Betula nana L. and B. pubescens Ehrh. in North-West Iceland. New Phytologist 67, 109±118. Eneroth, O., 1951. UndersoÈkning rorande mojligheterna att i fossilt material urskilja de olika Betula-arternas pollen. Geologiska FoÈreningens i Stockholm FoÈrhandlingar 73, 343±408. Fñgri, K., Iversen, J., 1989. Textbook of Pollen Analysis, 4th ed. John Wiley and Sons, Chichester. Fredskild, B., 1991. The genus Betula in GreenlandÐHolocene history, present distribution and synecology. Nordic Journal of Botany 11, 393±412. Fri riksdoÂttir, S.P., 1973. FrjoÂgreining a jar vegi uÂr Tjarnarveri og SoÂleyjarhoÈf a, B.Sc. thesis. Faculty of Engineering and Science, University of Iceland, ReykjaviÂk, 24 pp. Glaser, P.H., 1981. Transport and deposition of leaves and seeds on
C. Caseldine / Review of Palaeobotany and Palynology 117 (2001) 139±152 tundra: a late-glacial analog. Arctic and Alpine Research 13, 173±182. Glawion, R., 1985. Die natuÈrliche Vegetation Islands als Ausdruck des oÈkologisches Raumpotentials. Bochumer Geographische Arbeiten 45, 208 . Gordon, A.D., Prentice, I.C., 1977. Numerical methods in Quaternary palaeoecology IV. Separating mixtures of morphologically similar pollen taxa. Review of Palaeobotany and Palynology 23, 349±362. GroÈntved, J., 1942. The Pteridophyta and Spermatophyta of Iceland. The Botany of Iceland, Vol. 4. (Part 1). Copenhagen. Â skarsson, N., Johnsen, S., Clausen, H.B., Hammer, GroÈnvold, K., O C.U., Bond, G., Bard, E., 1995. Ash layers from Iceland in the Greenland GRIP ice core correlated with oceanic and land sediments. Earth and Planetary Science Letters 135, 149±155. HallsdoÂttir, M., 1987. Pollen Analytical Studies of Human In¯uence on Vegetation in Relation to the LandnaÂm Tephra Layer in Southern Iceland. Lundqua Thesis 18. Lund University, Lund, p. 46. HallsdoÂttir, M., 1990. Studies in the vegetational history of north Iceland. A radiocarbon-dated pollen diagram from Flateyjardalur. JoÈkull 40, 67±81. HallsdoÂttir, M., 1995. On the pre-settlement history of Icelandic vegetation. BuÂvõÂsindi 9, 17±29. HallsdoÂttir, M., 1996. Synthesis of the Holocene history of vegetation in northern Iceland. PalaÈoklimaforschung 20, 203±214. Haraldsson, H., 1981. The Marka¯joÂt sandur area, southern Iceland: sedimentological, petrographical and stratigraphical studies. Striae 15, 1±64. Hicks, S., 1996. The feasibility of using pollen deposition data as climatic indices. PalaÈoklimaforschung 20, 173±187. Jiang, H., Seidenkrantz, M.-S., Knudsen, K.-L., Eiriksson, J., Ha¯idason, H. 1999. Reconstruction of summer sea-surface temperature since 16,000 Cal. Years BP on the North Icelandic shelf using diatom-based transfer functions. Geological Society of America Annual Meeting and Exposition, Abstracts, A-74. Jentys-Szaferova, J., 1928. La structure des membranes du pollen de Corylus, de Myrica et des espeÁces europeÂenes de Betula et leur deÂtermination a l'eÂtat fossile. Bulletin international de l'Academie Polonaise de Sciences et des Lettres, Ser. B 68, 75±125. Kallio, P., Niemi, S., Sulkinoja, M., 1983. The Fennoscandian birch and its evolution in the marginal forest zone. Nordicana 47, 101±110. KocË, N., Jansen, E., Ha¯idason, H., 1996. Paleoceanographic reconstructions of surface ocean conditions in the Greenland, Icelandic and Norwegian Seas through the last 14 ka based on diatoms. Quaternary Science Reviews 12, 115±140. Kristinsson, H., 1987. A Guide to the Flowering Plants and Ferns of È rn og O È rlygur, ReykjavõÂk, p. 312. Iceland. O Â ., 1945. IÂslenzkar Jurtir. Ejnar Munksgaard, KaupmannaLoÈve, A hoÈfn, p. 291. MaÈkelaÈ, E.M., 1996. Size distinctions between Betula pollen typesÐa review. Grana 35, 248±256. MaÈkelaÈ, E.M., 1999. The Holocene history of birch in northeastern FennoscandiaÐan interpretation based on fossil birch pollen measurements. University of Helsinki, Department of Geology, Division of Geology and Palaeontology, Helsinki.
151
Mook, R., Vorren, K.D., 1996. The temperature climate at the altitudinal vegetation limits in Skibotn, northern Norway. PalaÈoklimaforschung 20, 61±74. Moore, P.D., Webb, J.A., Collinson, M.E., 1991. Pollen Analysis, 2nd ed. Blackwell, Oxford. Odland, A., 1996. Differences in the vertical distribution patterns of Betula pubescens in Norway and its ecological signi®cance. PalaÈoklimaforschung 20, 43±59. PaÊhlsson, I., 1981. A pollen analytical study on a peat deposit at LaÂgafell, southern Iceland. Striae 15, 60±64. Paus, Aa., 1989. Late Weichselian vegetation, climate and ¯oral migration at Eigebakken, South Rogaland, southwestern Norway. Review of Palaeobotany and Palynology 61, 177±203. Praglowski, J.R., 1962. Notes on the pollen morphology of Swedish trees and shrubs. Grana Palynologica 3, 45±65. Praglowski, J.R., 1966. On pollen size variations and the occurrence of Betula nana in different layers of a bog. Grana Palynologica 6, 528±543. Prentice, I.C., 1981. Quantitative birch (Betula L.) pollen separation by analysis of size frequency data. New Phytologist 89, 145± 157. Rundgren, M., 1995. Biostratigraphic evidence of the AllerùdYounger Dryas-Preboreal oscillation in northern Iceland. Quaternary Research 44, 405±416. Rundgren, M., 1998. Early-Holocene vegetation of northern Iceland: pollen and plant macrofossil evidence from the Skagi peninsula. The Holocene 8, 553±564. Â ., 1999. Plant survival in Iceland Rundgren, M., IngoÂlfsson, O during periods of glaciation? Journal of Biogeography 26, 387±396. StefaÂnsson, S., 1948. FloÂra Âislands. Hi Âõslenzka naÂttuÂrufrñ ifeÂlag, 3rd ed. Akureyri, p. 407. StoÈtter, J., Wastl, M., 2001. Palaeoclimatic investigations in Northern IcelandÐterrestrial reference for the variations in the system of ocean, atmosphere and ice distribution in the North Atlantic during the Holocene. PalaÈoklimaforschung. in press. StoÈtter, J., Wastl, M., Caseldine, C.J., HaÈberle, T., 1999. Holocene Palaeoclimatic reconstruction in Northern Iceland: approaches and results. Quaternary Science Reviews 18, 457±474. SveinbjoÈrnsson, B., 1993. Climate and growth of mountain birch near the treeline in Northern Sweden and Iceland. PalaÈoklimaforschung 9, 57±68. TerasmaÈe, J., 1951. On the pollen morphology of Betula nana. Svensk Botanisk Tidskrift 45, 358±361. Usinger, H., 1975. Pollenanalytische und stratigraphische Untersuchungen an zwei SpaÈtglazial-Vorkommen in SchleswigHolstein. Mitteilungen der Arbeitsgemeinschaft Geobotanik in Schleswig-Holstein und Hamburg 25, 1±173. Usinger, H., 1981. Zur spaÈt- und fruÈhen-postglazialen Vegetationsgeschichte der Schleswig-Holsteinen Geest nach einem Pollenund Pollendicht-diagramm aus dem Esinger Moor. Pollen et Spores 23, 389±432. Vaarama, A., Valanne, T., 1973. On the taxonomy, biology and origin of Betula tortuosa Lebed. Reports from Kevo Subarctic Research Station 10, 70±84. van Dinter, M., Birks, H.H., 1996. Distinguishing fossil Betula nana and Betula pubescens using their wingless fruits: implications
152
C. Caseldine / Review of Palaeobotany and Palynology 117 (2001) 139±152
for the late-glacial history of western Norway. Vegetation History and Archaeobotany 5, 229±240. Vasari, Y., Vasari, A., 1990. L'histoire holoceÁne des lacs Islandais. In: Devers, S. (Ed.), Pour Jean Malaurie. 102 teÂmoignages en hommage aÁ quarante ans d'eÂtudes arctiques. EÂditions Plon, Paris, pp. 277±293.
Vorren, K.-D., Stavseth, A., 1996. Late Holocene sediments of an alpine lake in North Norway. PalaÈoklimaforschung 20, 257± 269. Walker, D., 1955. Studies in the post-glacial history of British vegetation XIV. Skelsmergh Tarn and Kentmere, Westmorland. New Phytologist 54, 222±254.