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14. The Holocene vegetation history of Iceland, state-of-the-art and future research
14. T h e H o l o c e n e v e g e t a t i o n h i s t o r y of Iceland, state-of-the-art a n d f u t u r e research Margr6t Hallsd6ttir ~and Chris J. Caseldine 2 ~Icelandic Institute of Natural History, Reykjavik, Iceland 2Department of Geography, University of Exeter, Exeter, UK.
In the Late Preboreal and Early Boreal Chronozones dwarf-shrub heath and shrub heath, followed by juniper and mountain birch copses, replaced snow beds and fellfield vegetation characteristic of the Lateglacial/Early Preboreal newly deglaciated landscape of Iceland. During the Late Boreal and Early Atlantic Chronozones birch woodland established itself in the more favourable places, especially fjord lowlands and inland valleys. The development of birch woodland suffered a setback due to a transient climatic oscillation some 7500 ~4C years ago, but recovered again relatively quickly and more than 6000 ~4C years ago birch woodland covered the lowland areas both in northern and southern Iceland. At that time it reached its highest altitude, at least in northern Iceland. During the Late Atlantic and Subboreal Chronozones the birch woodland showed a retrogressive succession towards a more open landscape with expanding mires and heaths. There is some conflict between the evidence from pollen percentages, which indicate that the woodland regenerated several times during the latter half of the Holocene, and pollen influx values which reflect no such regeneration of the woodland. New habitats were created for birch after a period of cool climate and instability during the Early Subatlantic Chronozone as fresh screes and sandur plains became vegetated, at least partly, by woodland. This development was halted at the beginning of the Norse settlement, which resulted in further opening of the woodland. The birch woodland closest to the farms in the lowland of Iceland was cut and utilized for timber and fuel. Grazing of domestic animals opened the landscape still further and the previous woodland never re-established itself. This happened within only half a century from the arrival of the first settlers. During the ensuing 1100 years of human influence the sub-alpine birch woodland has been so intensively utilized that only in fenced, protected areas and at the most inaccessible and remote places has birch survived. The shrub and dwarf-shrub heaths, widespread mires, fell fields, and hay fields so characteristic for the Icelandic landscape at present thus developed as a relatively recent phenomenon in the form in which they appear today. A number of areas are identified for future research: elucidating the origins of the Icelandic flora, deriving palaeoclimatic data from the vegetation record, correlating terrestrial with marine and ice core records and expanding our understanding of the human impact on vegetation.
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14.1 INTRODUCTION
14.1.1 Background and research history Pollen analysis was introduced for the first time in Iceland during the 1940s, when, in his well known doctoral thesis, famous for introducing the method of tephrochronology, Sigurbur I~6rarinsson (1944) published two pollen diagrams from abandoned farms in I~j6rs~rdalur, southern Iceland (see Table 14.1 for a detailed list of Icelandic pollen sites). More than ten years later he published the first diagram that covered almost the whole Holocene (I~6rarinsson, 1955). Around that time Einarsson (1956, 1957) published his first pollen diagrams of peat sections from southwestern Iceland. His doctoral thesis on the climate history of Iceland during the Lateglacial and Holocene was published in 1961 (Einarsson, 1961), based to a large extent on pollen analysis of peat sections as well as one lake sediment core from 9 sites in Iceland. Others were also working in Iceland, for instance the Finn Okko (1956) published two pollen diagrams from Iceland in his doctoral thesis (cf. Hoffell and Laugardalur) and in the same year the German Straka (1956) published a paper including a pollen diagram from H66insvik at Tj6rnes, northern Iceland. Most of these analyses were carried out on peat sections with poor time resolution. In the early days of radiocarbon dating absolute dates were few, but the tephra layers proved of great value, both as tools for dating and for correlation. Based on these data Einarsson (1968) published his first zonation scheme of the vegetation history of Iceland, comprising five periods: namely the birch free period, the lower birch period, the lower mire period, the upper birch period, and the upper mire period, the latter with the partly superimposed settlement and historical time. In the 1970s the first pollen diagrams from Icelandic lakes were published, from Hafratj6m in northern Iceland and L6matj6rn in southern Iceland (Vasari, 1972, 1973) and in these studies Vasari showed that the pollen of ]uniperus had been an important part of the pollen dispersal at the time of shrub-heath formation in the Boreal Chronozone (1972, p. 243). Later this work was expanded with the first macrofossil diagrams on Icelandic lake sediments and some new radiocarbon dates, which led to a revision of the timing of the pollen zones (1990), although examination of the chronology reveals problems assumed to be due to erroneous radiocarbon dates. Over the next few years a number of papers on diverse and in some cases specific aspects of the vegetation history were published, covering sites at altitudes from sea level up to the highlands and most often dealing with a part of the Holocene only (Bartley, 1968; Fri6riksd6ttir, 1973; Skaftad6ttir, 1974; Sigb6rsd6ttir, 1976; Schwar, 1978; Hallsd6ttir, 1991; Hansom and Briggs, 1991; Caseldine and Hatton, 1991; Hallsd6ttir, 1995, 1996; Wastl et al., 2001), almost all of these studies relying on peat sections. The most recent palynological work of Rundgren (1995, 1998, 1999), with detailed studies on Skagi in northern Iceland which was deglaciated relatively early, looking in particular at the pioneer stages in the vegetational development in Iceland, is however solely based on lake sediment cores using both pollen and macrofossil analysis. There have also been several studies where the main topic has been human influence on vegetation, thus concentrating on the last 1100 years. These started with I~6rarinsson (1944), continued with Einarsson (1963) and Pfihlsson (1981), and were significantly enhanced by Hallsd6ttir (1982,1984,1987,1992,1993) and more recently by Zutter (1997).
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14.1.2 The situation at the end of the last glacial As with other isolated islands there has been a lively discussion on the origin of the Icelandic flora. This was originally stimulated by the appearance of Steinddr Steinddrsson's (1937,1954,1962) hypothesis on nunatak refugia, areas where he believed about half of the flora should have survived during the Ice Age. This he supported by geographical and geological research on the extent of the main ice sheet. Contrary to this hypothesis is the tabula rasa hypothesis, which is supported in a renewed form by e.g. Buckland et al. (1991), where it is argued that little or nothing of the flora could have survived the Last Glacial with colonisation occurring p r e d o m i n a n t l y by transportation as ice-rafted debris during a short episode at the Weichselian/Holocene transition around 10,000 ~4C yr BE Midway between these extreme views is the idea that arctic/alpine floral elements may have survived on nunataks, but that other plants, especially trees, probably immigrated during the Lateglacial and Early Holocene by aerial transport, by ocean currents and with the help of migrating birds (Johansen and Hytteborn, 2001). In recent years it has become more apparent that the Weichselian glacial period was not a continuously cold period, but rather a series of cold spells with intermittent and short lived warm periods (cf. Allen et al., 1999). In this way it may be argued that there were ice-free areas, not only on nunataks but also on lower lying ground adjacent to the sea or submerged at present. The plants had therefore both more time and more space for transport by luck than previously perceived. The latest palaeobotanical research north of Skagi (Rundgren and Ingdlfsson, 1999) supports the view that a part of the flora that flourished there during the Aller6d interstadial survived the Younger Dryas stadial. As the climate during that stadial was very harsh, this adds further weight to the argument that plants could have survived conditions prior to the Aller6d thus originating from well before the interstadial. 14.1.3 The current situation- prevailing climatic conditions and altitudinal and latitudinal tree lines. Birch woodland is the natural climax vegetation in Iceland and lowland areas up to about 300 m asl. lie within the sub-alpine vegetation zone. Above this limit and in the outermost coastal districts in the northwest, north and northeast, arctic-alpine vegetation dominates. This is also the case in the westernmost part of the Sn0efellsnes and Reykjanes peninsulas. On the sandur fields in southern Iceland, where the strength of the wind and shifting nature of the substratum prevents the growth of birch trees, plant cover is discontinuous and L e y m u s arenarius dominates. At present, Iceland is almost devoid of woodlands, mainly due to intensive land use over the last 1100 years. Natural woodlands and plantations cover only 1.2 % (1250 km 2) of the country (Sigurc3sson, 1977) and the maximum altitude for growing birch trees is to be found in the district of Skagafj6rOur, northern Iceland, at 620 m asl. (Guc3bergsson, 1992). At present, 46% of the total land area is vegetated, in the sense that the vegetation cover is higher than 50% (of which mires cover about 8%, grass heath and dwarf-shrub heath 25%, moss heath 10%, woodland 1%, and cultivated land 1%). Land with less than 50% vegetation cover may be interpreted as fellfield and covers 41% of the country at present, and is thus the largest single landscape type. It mainly occurs at altitudes greater than 500 m, but is also to be seen in the lowlands.
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Table 14.1 c o n t i n u e d Reference numbers for pollen sites: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
I~6rarinsson, 1944 Okko, 1956 Straka, 1956 Einarsson, 1957 Einarsson, 1961 Vasari, 1972 Bartley, 1973 Fri6riksd6ttir, 1973 Skaftad6ttir, 1974 Sigb6rsd6ttir, 1976
11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Schwar, 1978 Hallsd6ttir, 1987 Hallsd6ttir, 1991 Hansom and Briggs, 1991 Hallsd6ttir, 1995 Hallsd6ttir, 1996 Bj6rck et al., 1992 Caseldine and Hatton, 1994 Rundgren, 1995 Rundgren, 1998
21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
Hallsd6ttir et al., 1998 Wastl et al., 2001 I~6rarinsson, 1944 Einarsson, 1963 Pahlsson, 1981 Hallsd6ttir, 1982 Hallsd6ttir, 1984 Hallsd6ttir, 1992 Hallsd6ttir, 1993 Zutter, 1997
The proportions above are based on the 1:500,000 Vegetation Map of Iceland (N~itt6rufr0e6istofnun islands, 1998). Although we rely principally on pollen analysis and palaeoecology to reconstruct Icelandic vegetation history during the Holocene, for historical time proxies such as place names, historical documents (chronicles, land surveys etc.), and accounts given and books written by foreign travellers have also proved valuable sources of information.
14.2 THE VEGETATION HISTORY OF ICELAND 14.2.1 Grass heath and shrub heath, the Preboreal vegetation In northern Iceland evidence of grass tundra is preserved in lake sediments from early Preboreal times on the Skagi peninsula. Unstable habitats prevailed and there are indications of a relatively diverse herbaceous flora including pollen types of Capsella bu rsa-pastoris, Saxifraga sp., Chenopodiaceae and Caryophyllaceae as well as Poaceae (Rundgren, 1995, 1999). Vegetation succession appears to have been slow in the wake of continuing deglaciation. Herb tundra with the characteristic pollen taxa Oxyria/ Rumex and Koenigia islandica, both representing plants of disturbed and discontinuous plant cover, followed the grass tundra. Betula nana was expanding as the Preboreal cooling (9800-9700 ~4C yr BP, cf. Bj6rck et al., 1997) occurred and further slowed down the succession to dwarf-shrub tundra. It seems likely that the Preboreal cooling was the main reason for the late establishment of dwarf-shrub tundra in Iceland. By mid-Preboreal times a large expansion of dwarf shrubs is evident where the main components were Satix (herbacea ?), E mpetru m n igru m and other Ericales species, and by the late Preboreal in the lowland of northern Iceland the vegetation cover probably became more or less closed (Rundgren, 1999), although the synchroneity of vegetation change during this period is not as yet based on a very extensive dating record. On Flateyjarskagi (Krossh61sm~ri) east of Eyjafj6rOur both Juniperus communis and Betula sp. pollen are found in sediments with a radiocarbon date indicating late Preboreal age, suggesting either a survival of those species in the mountains/nunataks of Flateyjarskagi or an earlier re-immigration than elsewhere known in Iceland (Hallsd6ttir, 1991). The
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only other alternative explanation would be a problem with the radiocarbon date reinforcing the need for further examination of such key sites to establish the robustness of the record. At the transition between the Preboreal and Boreal (ca 9000 ~4Cyr BP) a 'great' volcanic eruption occurred and a huge tephra layer was spread all over northern Iceland (Tv-4 in Bj6rck et al. (1992) and hereafter referred to as the Preboreal/Boreal tephra marker horizon, the Saksunarvatn ash layer). The air fall component of this tephra layer is in most places about 10 cm thick but the upper re-sedimented, secondary, part can be as thick as ten times that (P6tursson and Larsen, 1992). This event had a drastic influence on vegetational succession and it is evident that the tephra fall was the beginning of several decades, even centuries of unstable environment with sandstorms and mudflows, favouring some plant species adapted to such a hostile habitat, e.g. some within the Poaceae, Salix lanata and Triglochin maritima. Later Cyperaceae, Equisetum (arvense ?) and Potentilla (anserina ?) flourished but the succession of delicate and easily physically damaged species at such conditions was halted. The delayed appearance of Juniperus communis at places other than Flateyjardalur might be the result of this tephra fall. Pollen-analytical work on a peat section with high-resolution stratigraphy (6 cm 100 yr -~) at Hella in Eyjafj6rc3ur implies that it was only after some period of instability due to the tephra fall that Juniperus communis succeeded in establishing itself (Hailsd6ttir, unpub.)(Fig.14.1). 14.2.2 The juniper stage during the Boreal 8800 - 8500/8000 ~4C yr BP
The expansion of Juniperus communis together with Betula nana and Empetrum nigrum on Skagi apparently shaded out Salix (herbacea?) (Rundgren, 1998, 1999). At the inland sites of the fjord bottoms and valleys of northern Iceland east of Skagi, both Salix and Ericales are shaded out as Juniperus, Betula nana and B. pubescens expanded (Hallsd6ttir, 1995) but west of Skagi (Hafratj6rn) Salix vegetation was at first reduced somewhat but then it was restored together with expanding Juniperus and the first appearance of Betula nana (Vasari and Vasari, 1990). This difference may imply that in the west the more shrubby species of Salix (S. callicarpaea, S. lanata, or S. phylicifolia) were already well established. At the same time the herb flora was relatively diverse and rich, particularly the spore plants, indicating the openness of the shrub vegetation acting first and foremost as shelter in the otherwise windy environment. In southern Iceland the vegetational development was similar to that of the west part of northern Iceland (Vasari and Vasari, 1990; Hallsd6ttir, 1987, 1995). A prolonged stage with a prevailing park tundra, characterized at first by Juniperus communis, Salix sp. and later on by Betula nana, came to an end as birch woodland developed in the early Atlantic Chronozone. 14.2.3 Birch w o o d l a n d in the Late Boreal and Atlantic 8500-6000 ~4C yr BP
Birch woodland developed in Skagafj6r6ur and Eyjafj6rOur in the Late Boreal as implied by Betula pollen influx values (Hallsd6ttir, 1996) and macroscopic remains in peat near Ytri-B0egisfi (Bartley, 1973) probably representative of the general picture at
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that time in the inland valleys of central north Iceland. At Faxafl6i birch was expanding in the Late Boreal, as indicated by a rising Betula pollen curve in the peat stratigraphy well above the Preboreal/Boreal tephra marker horizon (cf. Kv/Gr in Sigurgeirsson and Le6sson, 1993). Birch woodland was rather sparse in western north Iceland (H6navatnss~sla district) until Atlantic times, not expanding at Efstadalsvatn on southern Isafj6r6ur until 6750 ~4C yr BP (Caseldine et al., 2003), and as previously described, birch was spreading in southern Iceland at the Boreal/Atlantic boundary with dense woodland first appearing well into the Atlantic Chronozone, at ca 7300 ~4C yr BP (Vasari and Vasari, 1990). In Flateyjardalur the shrubs and scattered birch trees were replaced by birch woodland close to the boundary of the Early and Mid-Atlantic (6900 ~4C BP). A recent study on the upper limit of birch scrub in a valley on Tr611askagi, northern Iceland, indicates that this was reached between 6700 and 6000 x4C yr BP at an altitude of 450-500 m asl. (Wastl et al., 2001). From pollen concentration and pollen influx values it is evident that the birch woodland in Iceland was at its most extensive before 6000 14C yr BP (Hallsd6ttir, 1996). Since then the peatland has been expanding out from the lowland basins where the birch trees have had few possibilities to recover. In many places peat began to accumulate from this time onwards. 14.2.4 Expanding heaths and mires in the Late Atlantic, Subboreal and Early Subatlantic 6000-1200 ~4C yr BP During the latter half of the Holocene there was a retrogressive succession towards more open birch woodland with widespread mires and heathland. On the drier hills, on the well-drained mountain slopes and along valleys and fjords, birch could thrive. On the other hand, mires were expanding across the lowlands and there woodland was overcome and buried in peat. The landscape became more open and the birch woodland more patchy in favour of shade-intolerant herbs and ferns, as indicated by increasing relative values of the pollen and spores of these two groups. The first evidence of Sorbus cf. aucuparia pollen (rowan or mountain ash) is seen during this phase of vegetational development and appears earlier in northern than in southern Iceland. In the pollen diagram from Hegranes in Skagafj6rcSur the pollen curve is almost continuous after 5500 ~4C yr BP, while in the south it is first seen just below the tephra layer Hekla-4 ca 4000 14C yr BP (Hallsd6ttir, 1995) (Fig.14.2). The expansion of mires was not continuous though, as evidenced by wood remains in peat sequences. This is most clearly seen in sections near the edges of the mires, with alternating layers of wood remains and relatively homogeneous Care x peat. Recent research, supported by radiocarbon dates, indicates that oscillations in the mire vegetation, which are reflected in macroscopic remains in the peat, have not necessarily been contemporaneous in all parts of the country (Zutter, 1997). However, in some cases there may indeed have been local fluctuations in the groundwater table, which can blur the general picture. From southern Iceland we can see the lowest birch pollen values, indicating the most drastic opening up of the woodland, in pre-historic times occurring at the same level as the thickest Katla tephra in the Holocene (Kn), which has been radiocarbon dated to 3300 ~4C yr BP (R6bertsd6ttir, 1992). The Betula minimum is more pronounced in the peat stratigraphy than in lakes, sometimes it is seen below the tephra layer and
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sometimes just above it, indicating that factors other than the volcanic eruption itself must have operated to diminish the woodland cover, probably a deterioration in climate. Two, and sometimes three, late Holocene woodland regenerations can be seen in southern Iceland younger than the tephra Ke (ca 2850 ~4C yr BP, R6bertsd6ttir, 1992). The latest is dated to ca 160014C yr BP (Haraldsson, 1981) and appears in most of the socalled landnfim profiles as a transient Betula pollen peak below the Landnam tephra (V6, now dated to AD 871+_2 by means of ice core chronology, Gr6nvold et al., 1995).
14.2.5 Post-settlement history of vegetation Changes in vegetation composition were rapid in the wake of the settlement, and using tephrochronology it can be shown that birch woodland vanished in the vicinity of the farms during only one generation. This happened in the period between the tephra horizons V6-871 and K-920 in southern Iceland, where the influence of settlement has been most intensely studied (Hallsd6ttir, 1987). The woodland could not regenerate itself due to almost unlimited sheep grazing; grass heath, dwarf-shrub heath and mires expanded at the expense of the woodland, as Poaceae, Ericales, Galium, Thalictrum alpinum and Carex pollen types become an important part of the pollen spectra. In the pollen diagrams there are also indications of grain cultivation up to the 16th century AD, probably both barley and oats (I~6rarinsson, 1944; Hallsd6ttir, 1987, 1993), as well as flax (Linum usitatissimum) during the first centuries of settlement (Einarsson, 1962). New pollen types appear and give proxy evidence about cultivation such as the occurrence of weed plants like Polygonum aviculare, Urtica sp. and Spergula arvensis, and some native weed species (apophytes) flourish. Prior to the 15th to 16th century AD iron was extracted from bog-iron in Iceland and this industry required much birch-charcoal. Making this charcoal was devastating for the birch woods (I~6rarinsson, 1974), as is shown by numerous remains of ancient charcoal pits in areas of severe soil erosion, such as Haukadalsheic3i in southern Iceland and B~irc3ardalur in the north. Cooler climate increased the requirements for fuel, which also caused pressure on the woods, although turf and peat cutting for fuel had been undertaken simultaneously for a long time. Grazing all round the year, as it was practised up to the 19th or even 20th century further prevented the regeneration of the woodland. All these adverse influences put together resulted in the woods becoming more and more sparse, and finally the landscape approached the character seen at present, almost treeless and with vegetation covering less than half of that at the time of settlement (Arnalds, 1987). It appears that this change was slower in the highlands than in the more densely populated lowlands, in valleys or by the coast. This has, however, not been thoroughly investigated as yet. In the vicinity of Tjarnarver by the I~6rs~i river, at about 575 m asl., some birch scrub and more abundant dwarf birch survived up to at least 1300 AD, when it was overtaken by willow, which later becomes dominant in the pollen spectrum in the 14th century AD (Fri6riksd6ttir, 1973). Intensive land use began quite early in the lowlands, but in spite of a vegetation more sensitive to interference the interior highlands changed later, albeit responding more quickly after the initiation of intense land use, resulting in many places in desertification (cf. Dugmore and Buckland, 1991). This was accomplished by the unified action of cooler climate, intense utilization of the scrub
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vegetation, and overgrazing, as the sheep had unlimited access to the rangeland in most districts. The rangeland vegetation, dwarf-shrub heath, willow heath, grasslands and mires, was really a better pastureland than the dense birch scrub, which had an adverse effect on wool production by tearing the fleece of the sheep, but wool products were the main merchandise of the farmers in early times, or up to the 14th and 15th centuries AD when they were overtaken by stockfish (Porl~iksson, 1991).
14.3. FUTURE DIRECTIONS IN RESEARCH
From the above review it is clear that considerable progress has been made in developing an understanding of the Holocene vegetation of Iceland but there are still several areas for which further research needs to be undertaken. Underlying current understanding is also a lack of chronological control due to the relative paucity of well dated sequences, although this is perhaps less true of later Holocene profiles which can use well established marker tephra horizons such as the Hekla tephras and the Landn~im series. In concluding the review an attempt is made to identify some central research questions for the future suggesting areas which not only require more work but which would potentially repay the research effort. 14.3.1 The origins of the Holocene flora
The work by Rundgren and Ing61fsson (1999) has highlighted the problem of determining the origins and nature of colonisation of the Icelandic Holocene flora. As yet the sites on Skagi remain the only published sites from which Lateglacial pollen and macrofossil records have been published, but as the work of Nor6dahl and P6tursson (this volume) has shown there must be several locations around the coastal fringe of Iceland at which Lateglacial sites have survived providing potential data on the geographical distribution of taxa at this important period for ecosystem development. The realisation that almost all of the Icelandic flora present before the Younger Dryas/ Preboreal Oscillations quickly recolonised, implying survival through that period of climatic deterioration, adds strength to the view that refugia may have played an important part in vegetation development but until more well-dated sites are examined any discussion will suffer from a lack of primary data to inform the debate. Improvement of our knowledge of the location of the Icelandic ice sheet(s) through the Late Weichselian as a whole will provide a good basis for testing hypotheses of vegetation development. What the data from the Holocene show is that the flora is fundamentally northwest European in character and any debate has to concentrate not on from where the species originated but how they arrived in Iceland. A similar origin can be postulated for the fauna such as Coleoptera (Buckland, 1988) and Chironomidae (Caseldine et al., 2003), and to a lesser extent for microfauna such as testate amoebae (Caseldine et al., in press). The question of hybridisation and the special character of Icelandic species has only attracted limited attention although genetic characterisation of species such as birch has recently provided useful new data (Anamthawat-J6nsson, 1994), and Caseldine (2001) has argued on palynological grounds for Icelandic t ree birches representing possible hybridisation between tree species originating in northern Fennoscandia and
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dwarf birch that was already present in Iceland through the Lateglacial. The taxonomy of tree birch in Iceland and changes in tree birches through the Holocene remains a matter of contention and more detailed macrofossil studies would help to inform the debate. 14.3.2 Palaeoclimatic reconstruction from v e g e t a t i o n records in the H o l o c e n e
Because of the nature of the Icelandic flora there are only limited opportunities for using the record to derive detailed and precise palaeoclimatic data, although change and directions of change i.e. deterioration or amelioration of temperatures can be inferred from some of the pollen and macrofossil data. Because the altitudinal limits of tree birches are closely linked to summer temperature the tree birch record has been used to estimate periods of treeline expansion and regression (Wastl et al., 2001). The lack of analogue data from current treelines in Iceland, due to the heavy influence of grazing, means that any palaeoclimatic inferences rely on extrapolation from Scandinavia where several recent studies have isolated varying temperature parameters (summarised in Caseldine, 2001), but overall uncertainty over the birch data inevitably leads to significant errors in any temperature reconstructions. Nevertheless long-term potential vegetationclimate links have been modelled using birch for the whole of Iceland by Olafsd6ttir e t al. (2001) examining the relative importance of climate and human activity for recent historical landscape change. The importance of [uniperus in the early Holocene and its interrelationship with birch may have a climatic significance, possibly related to snow cover/precipitation rather than temperature but this remains a speculative observation until more detailed comparison with independent climatic information is forthcoming, as from ice core and/or marine data, or perhaps from lake level data, a source of possible palaeoclimatic information as yet virtually untapped in Iceland. Providing a rigorous terrestrial palaeoclimatic record is a major challenge to future palaeoecological research and will require a broad multi-proxy record to be effective, both in establishing the reliability of individual proxies and in understanding the importance of non-climatic factors in determining vegetation change. The first work comparing pollen, lithostratigraphic and chironomid data from a site in NW Iceland has provided an encouraging basis for the future (Caseldine et al., 2003), but also underlines the importance of establishing Icelandic training sets and transfer functions as a foundation for temperature reconstruction, rather than relying on extrapolation from apparently similar environments such as northern Fennoscandia. 14.3.3 Wider correlations - marine and ice core records
The close proximity of high resolution palaeoclimatic records from marine sediments, especially to the north and west of Iceland (Eiriksson et al., 2000a; Jiang et al., 2002) and from the Greenland ice sheet (Johnsen et al., 2001) offers significant opportunities for establishing links between terrestrial, marine and atmospheric systems in and around Iceland. Despite the potential limitations of the floral record it still has an important role in combination with other proxies to understand how systems operated in the past, and how they may develop in the future. Accurate correlation of the records requires a rigorous chronological framework which in Iceland is facilitated by the detailed tephrochronology available for both onshore and offshore records (Eiriksson et al., 2000a).
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Margrdt Hallsd6ttir and C.J. Caseldine
Reconstruction of offshore and nearshore water temperatures and the location of currents through time (Eiriksson et al., 2000a; Andrews et al., 2001a, Jiang et al., 2002) now provides a valuable backdrop against which vegetation change can be evaluated leading into questions of the importance of lags and thresholds in the terrestrial ecosystem. Here again the objective needs to be to focused on more specific issues using the marine and ice core records to generate questions for examination in Iceland e.g. was there a terrestrial response to the 8.2 cal. yr BP event (Alley et al., 1997)? Pollen evidence from a lake core in northern Iceland hints at such an evidence, but needs to be followed up by further research at other sites (Hallsd6ttir, 1996 p. 211).
14.3.4 Human record- fleshing out the picture Despite the relatively low resolution of traditional peat and lake records for determining the detailed impact of human settlement on the biota a broad picture has emerged as reviewed above, assisted especially by reconstruction of patterns of soil erosion anchored firmly by the tephrochronological framework available for southern Iceland (Dugmore et al., 2000). In recent years these records have been supplemented by results derived from archaeological sites at which palaeobotanical analyses have been carried out (Zutter, 1997, 2000). Integration of well-dated excavations with local palaeoecological records offers an important opportunity for a much more informative understanding of how communities interacted with their local environments, developing ideas first examined by Hallsd6ttir (1987).
14.4 C O N C L U S I O N Almost 60 years of palaeoecological study in Iceland may not have produced a history of vegetation change comparable to that derived for the more intensively studied areas of North West Europe. Nevertheless it has laid the foundations for future work which will be able to focus on more specific questions, the answers to which will form a key part of our understanding regarding the evolution and future of North Atlantic terrestrial, marine and atmospheric systems. The challenge for researchers in this field is to ensure that the terrestrial record is not overlooked as attention is paid increasingly to the p e r h a p s more attractive high resolution marine and ice core records. Understanding terrestrial responses to climatic change and hence how such systems will evolve in the future cannot be achieved without detail from the terrestrial record itself.
In the Late Preboreal and Early Boreal Chronozones dwarf-shrub heath and shrub heath, followed by juniper and mountain birch copses, replaced snow beds and fellfield vegetation characteristic of the Lateglacial/Early Preboreal newly deglaciated landscape of Iceland. During the Late Boreal and Early Atlantic Chronozones birch woodland established itself in the more favourable places, especially fjord lowlands and inland valleys. The development of birch woodland suffered a setback due to a transient climatic oscillation some 7500 ~4C years ago, but recovered again relatively quickly and more than 6000 ~4C years ago birch woodland covered the lowland areas both in northern and southern Iceland. At that time it reached its highest altitude, at least in northern Iceland. During the Late Atlantic and Subboreal Chronozones the birch woodland showed a retrogressive succession towards a more open landscape with expanding mires and heaths. There is some conflict between the evidence from pollen percentages, which indicate that the woodland regenerated several times during the latter half of the Holocene, and pollen influx values which reflect no such regeneration of the woodland. New habitats were created for birch after a period of cool climate and instability during the Early Subatlantic Chronozone as fresh screes and sandur plains became vegetated, at least partly, by woodland. This development was halted at the beginning of the Norse settlement, which resulted in further opening of the woodland. The birch woodland closest to the farms in the lowland of Iceland was cut and utilized for timber and fuel. Grazing of domestic animals opened the landscape still further and the previous woodland never re-established itself. This happened within only half a century from the arrival of the first settlers. During the ensuing 1100 years of human influence the sub-alpine birch woodland has been so intensively utilized that only in fenced, protected areas and at the most inaccessible and remote places has birch survived. The shrub and dwarf-shrub heaths, widespread mires, fell fields, and hay fields so characteristic for the Icelandic landscape at present thus developed as a relatively recent phenomenon in the form in which they appear today. A number of areas are identified for future research: elucidating the origins of the Icelandic flora, deriving palaeoclimatic data from the vegetation record, correlating terrestrial with marine and ice core records and expanding our understanding of the human impact on vegetation.
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14.1 INTRODUCTION
14.1.1 Background and research history Pollen analysis was introduced for the first time in Iceland during the 1940s, when, in his well known doctoral thesis, famous for introducing the method of tephrochronology, Sigurbur I~6rarinsson (1944) published two pollen diagrams from abandoned farms in I~j6rs~rdalur, southern Iceland (see Table 14.1 for a detailed list of Icelandic pollen sites). More than ten years later he published the first diagram that covered almost the whole Holocene (I~6rarinsson, 1955). Around that time Einarsson (1956, 1957) published his first pollen diagrams of peat sections from southwestern Iceland. His doctoral thesis on the climate history of Iceland during the Lateglacial and Holocene was published in 1961 (Einarsson, 1961), based to a large extent on pollen analysis of peat sections as well as one lake sediment core from 9 sites in Iceland. Others were also working in Iceland, for instance the Finn Okko (1956) published two pollen diagrams from Iceland in his doctoral thesis (cf. Hoffell and Laugardalur) and in the same year the German Straka (1956) published a paper including a pollen diagram from H66insvik at Tj6rnes, northern Iceland. Most of these analyses were carried out on peat sections with poor time resolution. In the early days of radiocarbon dating absolute dates were few, but the tephra layers proved of great value, both as tools for dating and for correlation. Based on these data Einarsson (1968) published his first zonation scheme of the vegetation history of Iceland, comprising five periods: namely the birch free period, the lower birch period, the lower mire period, the upper birch period, and the upper mire period, the latter with the partly superimposed settlement and historical time. In the 1970s the first pollen diagrams from Icelandic lakes were published, from Hafratj6m in northern Iceland and L6matj6rn in southern Iceland (Vasari, 1972, 1973) and in these studies Vasari showed that the pollen of ]uniperus had been an important part of the pollen dispersal at the time of shrub-heath formation in the Boreal Chronozone (1972, p. 243). Later this work was expanded with the first macrofossil diagrams on Icelandic lake sediments and some new radiocarbon dates, which led to a revision of the timing of the pollen zones (1990), although examination of the chronology reveals problems assumed to be due to erroneous radiocarbon dates. Over the next few years a number of papers on diverse and in some cases specific aspects of the vegetation history were published, covering sites at altitudes from sea level up to the highlands and most often dealing with a part of the Holocene only (Bartley, 1968; Fri6riksd6ttir, 1973; Skaftad6ttir, 1974; Sigb6rsd6ttir, 1976; Schwar, 1978; Hallsd6ttir, 1991; Hansom and Briggs, 1991; Caseldine and Hatton, 1991; Hallsd6ttir, 1995, 1996; Wastl et al., 2001), almost all of these studies relying on peat sections. The most recent palynological work of Rundgren (1995, 1998, 1999), with detailed studies on Skagi in northern Iceland which was deglaciated relatively early, looking in particular at the pioneer stages in the vegetational development in Iceland, is however solely based on lake sediment cores using both pollen and macrofossil analysis. There have also been several studies where the main topic has been human influence on vegetation, thus concentrating on the last 1100 years. These started with I~6rarinsson (1944), continued with Einarsson (1963) and Pfihlsson (1981), and were significantly enhanced by Hallsd6ttir (1982,1984,1987,1992,1993) and more recently by Zutter (1997).
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14.1.2 The situation at the end of the last glacial As with other isolated islands there has been a lively discussion on the origin of the Icelandic flora. This was originally stimulated by the appearance of Steinddr Steinddrsson's (1937,1954,1962) hypothesis on nunatak refugia, areas where he believed about half of the flora should have survived during the Ice Age. This he supported by geographical and geological research on the extent of the main ice sheet. Contrary to this hypothesis is the tabula rasa hypothesis, which is supported in a renewed form by e.g. Buckland et al. (1991), where it is argued that little or nothing of the flora could have survived the Last Glacial with colonisation occurring p r e d o m i n a n t l y by transportation as ice-rafted debris during a short episode at the Weichselian/Holocene transition around 10,000 ~4C yr BE Midway between these extreme views is the idea that arctic/alpine floral elements may have survived on nunataks, but that other plants, especially trees, probably immigrated during the Lateglacial and Early Holocene by aerial transport, by ocean currents and with the help of migrating birds (Johansen and Hytteborn, 2001). In recent years it has become more apparent that the Weichselian glacial period was not a continuously cold period, but rather a series of cold spells with intermittent and short lived warm periods (cf. Allen et al., 1999). In this way it may be argued that there were ice-free areas, not only on nunataks but also on lower lying ground adjacent to the sea or submerged at present. The plants had therefore both more time and more space for transport by luck than previously perceived. The latest palaeobotanical research north of Skagi (Rundgren and Ingdlfsson, 1999) supports the view that a part of the flora that flourished there during the Aller6d interstadial survived the Younger Dryas stadial. As the climate during that stadial was very harsh, this adds further weight to the argument that plants could have survived conditions prior to the Aller6d thus originating from well before the interstadial. 14.1.3 The current situation- prevailing climatic conditions and altitudinal and latitudinal tree lines. Birch woodland is the natural climax vegetation in Iceland and lowland areas up to about 300 m asl. lie within the sub-alpine vegetation zone. Above this limit and in the outermost coastal districts in the northwest, north and northeast, arctic-alpine vegetation dominates. This is also the case in the westernmost part of the Sn0efellsnes and Reykjanes peninsulas. On the sandur fields in southern Iceland, where the strength of the wind and shifting nature of the substratum prevents the growth of birch trees, plant cover is discontinuous and L e y m u s arenarius dominates. At present, Iceland is almost devoid of woodlands, mainly due to intensive land use over the last 1100 years. Natural woodlands and plantations cover only 1.2 % (1250 km 2) of the country (Sigurc3sson, 1977) and the maximum altitude for growing birch trees is to be found in the district of Skagafj6rOur, northern Iceland, at 620 m asl. (Guc3bergsson, 1992). At present, 46% of the total land area is vegetated, in the sense that the vegetation cover is higher than 50% (of which mires cover about 8%, grass heath and dwarf-shrub heath 25%, moss heath 10%, woodland 1%, and cultivated land 1%). Land with less than 50% vegetation cover may be interpreted as fellfield and covers 41% of the country at present, and is thus the largest single landscape type. It mainly occurs at altitudes greater than 500 m, but is also to be seen in the lowlands.
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Table 14.1 c o n t i n u e d Reference numbers for pollen sites: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
I~6rarinsson, 1944 Okko, 1956 Straka, 1956 Einarsson, 1957 Einarsson, 1961 Vasari, 1972 Bartley, 1973 Fri6riksd6ttir, 1973 Skaftad6ttir, 1974 Sigb6rsd6ttir, 1976
11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Schwar, 1978 Hallsd6ttir, 1987 Hallsd6ttir, 1991 Hansom and Briggs, 1991 Hallsd6ttir, 1995 Hallsd6ttir, 1996 Bj6rck et al., 1992 Caseldine and Hatton, 1994 Rundgren, 1995 Rundgren, 1998
21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
Hallsd6ttir et al., 1998 Wastl et al., 2001 I~6rarinsson, 1944 Einarsson, 1963 Pahlsson, 1981 Hallsd6ttir, 1982 Hallsd6ttir, 1984 Hallsd6ttir, 1992 Hallsd6ttir, 1993 Zutter, 1997
The proportions above are based on the 1:500,000 Vegetation Map of Iceland (N~itt6rufr0e6istofnun islands, 1998). Although we rely principally on pollen analysis and palaeoecology to reconstruct Icelandic vegetation history during the Holocene, for historical time proxies such as place names, historical documents (chronicles, land surveys etc.), and accounts given and books written by foreign travellers have also proved valuable sources of information.
14.2 THE VEGETATION HISTORY OF ICELAND 14.2.1 Grass heath and shrub heath, the Preboreal vegetation In northern Iceland evidence of grass tundra is preserved in lake sediments from early Preboreal times on the Skagi peninsula. Unstable habitats prevailed and there are indications of a relatively diverse herbaceous flora including pollen types of Capsella bu rsa-pastoris, Saxifraga sp., Chenopodiaceae and Caryophyllaceae as well as Poaceae (Rundgren, 1995, 1999). Vegetation succession appears to have been slow in the wake of continuing deglaciation. Herb tundra with the characteristic pollen taxa Oxyria/ Rumex and Koenigia islandica, both representing plants of disturbed and discontinuous plant cover, followed the grass tundra. Betula nana was expanding as the Preboreal cooling (9800-9700 ~4C yr BP, cf. Bj6rck et al., 1997) occurred and further slowed down the succession to dwarf-shrub tundra. It seems likely that the Preboreal cooling was the main reason for the late establishment of dwarf-shrub tundra in Iceland. By mid-Preboreal times a large expansion of dwarf shrubs is evident where the main components were Satix (herbacea ?), E mpetru m n igru m and other Ericales species, and by the late Preboreal in the lowland of northern Iceland the vegetation cover probably became more or less closed (Rundgren, 1999), although the synchroneity of vegetation change during this period is not as yet based on a very extensive dating record. On Flateyjarskagi (Krossh61sm~ri) east of Eyjafj6rOur both Juniperus communis and Betula sp. pollen are found in sediments with a radiocarbon date indicating late Preboreal age, suggesting either a survival of those species in the mountains/nunataks of Flateyjarskagi or an earlier re-immigration than elsewhere known in Iceland (Hallsd6ttir, 1991). The
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only other alternative explanation would be a problem with the radiocarbon date reinforcing the need for further examination of such key sites to establish the robustness of the record. At the transition between the Preboreal and Boreal (ca 9000 ~4Cyr BP) a 'great' volcanic eruption occurred and a huge tephra layer was spread all over northern Iceland (Tv-4 in Bj6rck et al. (1992) and hereafter referred to as the Preboreal/Boreal tephra marker horizon, the Saksunarvatn ash layer). The air fall component of this tephra layer is in most places about 10 cm thick but the upper re-sedimented, secondary, part can be as thick as ten times that (P6tursson and Larsen, 1992). This event had a drastic influence on vegetational succession and it is evident that the tephra fall was the beginning of several decades, even centuries of unstable environment with sandstorms and mudflows, favouring some plant species adapted to such a hostile habitat, e.g. some within the Poaceae, Salix lanata and Triglochin maritima. Later Cyperaceae, Equisetum (arvense ?) and Potentilla (anserina ?) flourished but the succession of delicate and easily physically damaged species at such conditions was halted. The delayed appearance of Juniperus communis at places other than Flateyjardalur might be the result of this tephra fall. Pollen-analytical work on a peat section with high-resolution stratigraphy (6 cm 100 yr -~) at Hella in Eyjafj6rc3ur implies that it was only after some period of instability due to the tephra fall that Juniperus communis succeeded in establishing itself (Hailsd6ttir, unpub.)(Fig.14.1). 14.2.2 The juniper stage during the Boreal 8800 - 8500/8000 ~4C yr BP
The expansion of Juniperus communis together with Betula nana and Empetrum nigrum on Skagi apparently shaded out Salix (herbacea?) (Rundgren, 1998, 1999). At the inland sites of the fjord bottoms and valleys of northern Iceland east of Skagi, both Salix and Ericales are shaded out as Juniperus, Betula nana and B. pubescens expanded (Hallsd6ttir, 1995) but west of Skagi (Hafratj6rn) Salix vegetation was at first reduced somewhat but then it was restored together with expanding Juniperus and the first appearance of Betula nana (Vasari and Vasari, 1990). This difference may imply that in the west the more shrubby species of Salix (S. callicarpaea, S. lanata, or S. phylicifolia) were already well established. At the same time the herb flora was relatively diverse and rich, particularly the spore plants, indicating the openness of the shrub vegetation acting first and foremost as shelter in the otherwise windy environment. In southern Iceland the vegetational development was similar to that of the west part of northern Iceland (Vasari and Vasari, 1990; Hallsd6ttir, 1987, 1995). A prolonged stage with a prevailing park tundra, characterized at first by Juniperus communis, Salix sp. and later on by Betula nana, came to an end as birch woodland developed in the early Atlantic Chronozone. 14.2.3 Birch w o o d l a n d in the Late Boreal and Atlantic 8500-6000 ~4C yr BP
Birch woodland developed in Skagafj6r6ur and Eyjafj6rOur in the Late Boreal as implied by Betula pollen influx values (Hallsd6ttir, 1996) and macroscopic remains in peat near Ytri-B0egisfi (Bartley, 1973) probably representative of the general picture at
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that time in the inland valleys of central north Iceland. At Faxafl6i birch was expanding in the Late Boreal, as indicated by a rising Betula pollen curve in the peat stratigraphy well above the Preboreal/Boreal tephra marker horizon (cf. Kv/Gr in Sigurgeirsson and Le6sson, 1993). Birch woodland was rather sparse in western north Iceland (H6navatnss~sla district) until Atlantic times, not expanding at Efstadalsvatn on southern Isafj6r6ur until 6750 ~4C yr BP (Caseldine et al., 2003), and as previously described, birch was spreading in southern Iceland at the Boreal/Atlantic boundary with dense woodland first appearing well into the Atlantic Chronozone, at ca 7300 ~4C yr BP (Vasari and Vasari, 1990). In Flateyjardalur the shrubs and scattered birch trees were replaced by birch woodland close to the boundary of the Early and Mid-Atlantic (6900 ~4C BP). A recent study on the upper limit of birch scrub in a valley on Tr611askagi, northern Iceland, indicates that this was reached between 6700 and 6000 x4C yr BP at an altitude of 450-500 m asl. (Wastl et al., 2001). From pollen concentration and pollen influx values it is evident that the birch woodland in Iceland was at its most extensive before 6000 14C yr BP (Hallsd6ttir, 1996). Since then the peatland has been expanding out from the lowland basins where the birch trees have had few possibilities to recover. In many places peat began to accumulate from this time onwards. 14.2.4 Expanding heaths and mires in the Late Atlantic, Subboreal and Early Subatlantic 6000-1200 ~4C yr BP During the latter half of the Holocene there was a retrogressive succession towards more open birch woodland with widespread mires and heathland. On the drier hills, on the well-drained mountain slopes and along valleys and fjords, birch could thrive. On the other hand, mires were expanding across the lowlands and there woodland was overcome and buried in peat. The landscape became more open and the birch woodland more patchy in favour of shade-intolerant herbs and ferns, as indicated by increasing relative values of the pollen and spores of these two groups. The first evidence of Sorbus cf. aucuparia pollen (rowan or mountain ash) is seen during this phase of vegetational development and appears earlier in northern than in southern Iceland. In the pollen diagram from Hegranes in Skagafj6rcSur the pollen curve is almost continuous after 5500 ~4C yr BP, while in the south it is first seen just below the tephra layer Hekla-4 ca 4000 14C yr BP (Hallsd6ttir, 1995) (Fig.14.2). The expansion of mires was not continuous though, as evidenced by wood remains in peat sequences. This is most clearly seen in sections near the edges of the mires, with alternating layers of wood remains and relatively homogeneous Care x peat. Recent research, supported by radiocarbon dates, indicates that oscillations in the mire vegetation, which are reflected in macroscopic remains in the peat, have not necessarily been contemporaneous in all parts of the country (Zutter, 1997). However, in some cases there may indeed have been local fluctuations in the groundwater table, which can blur the general picture. From southern Iceland we can see the lowest birch pollen values, indicating the most drastic opening up of the woodland, in pre-historic times occurring at the same level as the thickest Katla tephra in the Holocene (Kn), which has been radiocarbon dated to 3300 ~4C yr BP (R6bertsd6ttir, 1992). The Betula minimum is more pronounced in the peat stratigraphy than in lakes, sometimes it is seen below the tephra layer and
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sometimes just above it, indicating that factors other than the volcanic eruption itself must have operated to diminish the woodland cover, probably a deterioration in climate. Two, and sometimes three, late Holocene woodland regenerations can be seen in southern Iceland younger than the tephra Ke (ca 2850 ~4C yr BP, R6bertsd6ttir, 1992). The latest is dated to ca 160014C yr BP (Haraldsson, 1981) and appears in most of the socalled landnfim profiles as a transient Betula pollen peak below the Landnam tephra (V6, now dated to AD 871+_2 by means of ice core chronology, Gr6nvold et al., 1995).
14.2.5 Post-settlement history of vegetation Changes in vegetation composition were rapid in the wake of the settlement, and using tephrochronology it can be shown that birch woodland vanished in the vicinity of the farms during only one generation. This happened in the period between the tephra horizons V6-871 and K-920 in southern Iceland, where the influence of settlement has been most intensely studied (Hallsd6ttir, 1987). The woodland could not regenerate itself due to almost unlimited sheep grazing; grass heath, dwarf-shrub heath and mires expanded at the expense of the woodland, as Poaceae, Ericales, Galium, Thalictrum alpinum and Carex pollen types become an important part of the pollen spectra. In the pollen diagrams there are also indications of grain cultivation up to the 16th century AD, probably both barley and oats (I~6rarinsson, 1944; Hallsd6ttir, 1987, 1993), as well as flax (Linum usitatissimum) during the first centuries of settlement (Einarsson, 1962). New pollen types appear and give proxy evidence about cultivation such as the occurrence of weed plants like Polygonum aviculare, Urtica sp. and Spergula arvensis, and some native weed species (apophytes) flourish. Prior to the 15th to 16th century AD iron was extracted from bog-iron in Iceland and this industry required much birch-charcoal. Making this charcoal was devastating for the birch woods (I~6rarinsson, 1974), as is shown by numerous remains of ancient charcoal pits in areas of severe soil erosion, such as Haukadalsheic3i in southern Iceland and B~irc3ardalur in the north. Cooler climate increased the requirements for fuel, which also caused pressure on the woods, although turf and peat cutting for fuel had been undertaken simultaneously for a long time. Grazing all round the year, as it was practised up to the 19th or even 20th century further prevented the regeneration of the woodland. All these adverse influences put together resulted in the woods becoming more and more sparse, and finally the landscape approached the character seen at present, almost treeless and with vegetation covering less than half of that at the time of settlement (Arnalds, 1987). It appears that this change was slower in the highlands than in the more densely populated lowlands, in valleys or by the coast. This has, however, not been thoroughly investigated as yet. In the vicinity of Tjarnarver by the I~6rs~i river, at about 575 m asl., some birch scrub and more abundant dwarf birch survived up to at least 1300 AD, when it was overtaken by willow, which later becomes dominant in the pollen spectrum in the 14th century AD (Fri6riksd6ttir, 1973). Intensive land use began quite early in the lowlands, but in spite of a vegetation more sensitive to interference the interior highlands changed later, albeit responding more quickly after the initiation of intense land use, resulting in many places in desertification (cf. Dugmore and Buckland, 1991). This was accomplished by the unified action of cooler climate, intense utilization of the scrub
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vegetation, and overgrazing, as the sheep had unlimited access to the rangeland in most districts. The rangeland vegetation, dwarf-shrub heath, willow heath, grasslands and mires, was really a better pastureland than the dense birch scrub, which had an adverse effect on wool production by tearing the fleece of the sheep, but wool products were the main merchandise of the farmers in early times, or up to the 14th and 15th centuries AD when they were overtaken by stockfish (Porl~iksson, 1991).
14.3. FUTURE DIRECTIONS IN RESEARCH
From the above review it is clear that considerable progress has been made in developing an understanding of the Holocene vegetation of Iceland but there are still several areas for which further research needs to be undertaken. Underlying current understanding is also a lack of chronological control due to the relative paucity of well dated sequences, although this is perhaps less true of later Holocene profiles which can use well established marker tephra horizons such as the Hekla tephras and the Landn~im series. In concluding the review an attempt is made to identify some central research questions for the future suggesting areas which not only require more work but which would potentially repay the research effort. 14.3.1 The origins of the Holocene flora
The work by Rundgren and Ing61fsson (1999) has highlighted the problem of determining the origins and nature of colonisation of the Icelandic Holocene flora. As yet the sites on Skagi remain the only published sites from which Lateglacial pollen and macrofossil records have been published, but as the work of Nor6dahl and P6tursson (this volume) has shown there must be several locations around the coastal fringe of Iceland at which Lateglacial sites have survived providing potential data on the geographical distribution of taxa at this important period for ecosystem development. The realisation that almost all of the Icelandic flora present before the Younger Dryas/ Preboreal Oscillations quickly recolonised, implying survival through that period of climatic deterioration, adds strength to the view that refugia may have played an important part in vegetation development but until more well-dated sites are examined any discussion will suffer from a lack of primary data to inform the debate. Improvement of our knowledge of the location of the Icelandic ice sheet(s) through the Late Weichselian as a whole will provide a good basis for testing hypotheses of vegetation development. What the data from the Holocene show is that the flora is fundamentally northwest European in character and any debate has to concentrate not on from where the species originated but how they arrived in Iceland. A similar origin can be postulated for the fauna such as Coleoptera (Buckland, 1988) and Chironomidae (Caseldine et al., 2003), and to a lesser extent for microfauna such as testate amoebae (Caseldine et al., in press). The question of hybridisation and the special character of Icelandic species has only attracted limited attention although genetic characterisation of species such as birch has recently provided useful new data (Anamthawat-J6nsson, 1994), and Caseldine (2001) has argued on palynological grounds for Icelandic t ree birches representing possible hybridisation between tree species originating in northern Fennoscandia and
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dwarf birch that was already present in Iceland through the Lateglacial. The taxonomy of tree birch in Iceland and changes in tree birches through the Holocene remains a matter of contention and more detailed macrofossil studies would help to inform the debate. 14.3.2 Palaeoclimatic reconstruction from v e g e t a t i o n records in the H o l o c e n e
Because of the nature of the Icelandic flora there are only limited opportunities for using the record to derive detailed and precise palaeoclimatic data, although change and directions of change i.e. deterioration or amelioration of temperatures can be inferred from some of the pollen and macrofossil data. Because the altitudinal limits of tree birches are closely linked to summer temperature the tree birch record has been used to estimate periods of treeline expansion and regression (Wastl et al., 2001). The lack of analogue data from current treelines in Iceland, due to the heavy influence of grazing, means that any palaeoclimatic inferences rely on extrapolation from Scandinavia where several recent studies have isolated varying temperature parameters (summarised in Caseldine, 2001), but overall uncertainty over the birch data inevitably leads to significant errors in any temperature reconstructions. Nevertheless long-term potential vegetationclimate links have been modelled using birch for the whole of Iceland by Olafsd6ttir e t al. (2001) examining the relative importance of climate and human activity for recent historical landscape change. The importance of [uniperus in the early Holocene and its interrelationship with birch may have a climatic significance, possibly related to snow cover/precipitation rather than temperature but this remains a speculative observation until more detailed comparison with independent climatic information is forthcoming, as from ice core and/or marine data, or perhaps from lake level data, a source of possible palaeoclimatic information as yet virtually untapped in Iceland. Providing a rigorous terrestrial palaeoclimatic record is a major challenge to future palaeoecological research and will require a broad multi-proxy record to be effective, both in establishing the reliability of individual proxies and in understanding the importance of non-climatic factors in determining vegetation change. The first work comparing pollen, lithostratigraphic and chironomid data from a site in NW Iceland has provided an encouraging basis for the future (Caseldine et al., 2003), but also underlines the importance of establishing Icelandic training sets and transfer functions as a foundation for temperature reconstruction, rather than relying on extrapolation from apparently similar environments such as northern Fennoscandia. 14.3.3 Wider correlations - marine and ice core records
The close proximity of high resolution palaeoclimatic records from marine sediments, especially to the north and west of Iceland (Eiriksson et al., 2000a; Jiang et al., 2002) and from the Greenland ice sheet (Johnsen et al., 2001) offers significant opportunities for establishing links between terrestrial, marine and atmospheric systems in and around Iceland. Despite the potential limitations of the floral record it still has an important role in combination with other proxies to understand how systems operated in the past, and how they may develop in the future. Accurate correlation of the records requires a rigorous chronological framework which in Iceland is facilitated by the detailed tephrochronology available for both onshore and offshore records (Eiriksson et al., 2000a).
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Reconstruction of offshore and nearshore water temperatures and the location of currents through time (Eiriksson et al., 2000a; Andrews et al., 2001a, Jiang et al., 2002) now provides a valuable backdrop against which vegetation change can be evaluated leading into questions of the importance of lags and thresholds in the terrestrial ecosystem. Here again the objective needs to be to focused on more specific issues using the marine and ice core records to generate questions for examination in Iceland e.g. was there a terrestrial response to the 8.2 cal. yr BP event (Alley et al., 1997)? Pollen evidence from a lake core in northern Iceland hints at such an evidence, but needs to be followed up by further research at other sites (Hallsd6ttir, 1996 p. 211).
14.3.4 Human record- fleshing out the picture Despite the relatively low resolution of traditional peat and lake records for determining the detailed impact of human settlement on the biota a broad picture has emerged as reviewed above, assisted especially by reconstruction of patterns of soil erosion anchored firmly by the tephrochronological framework available for southern Iceland (Dugmore et al., 2000). In recent years these records have been supplemented by results derived from archaeological sites at which palaeobotanical analyses have been carried out (Zutter, 1997, 2000). Integration of well-dated excavations with local palaeoecological records offers an important opportunity for a much more informative understanding of how communities interacted with their local environments, developing ideas first examined by Hallsd6ttir (1987).
14.4 C O N C L U S I O N Almost 60 years of palaeoecological study in Iceland may not have produced a history of vegetation change comparable to that derived for the more intensively studied areas of North West Europe. Nevertheless it has laid the foundations for future work which will be able to focus on more specific questions, the answers to which will form a key part of our understanding regarding the evolution and future of North Atlantic terrestrial, marine and atmospheric systems. The challenge for researchers in this field is to ensure that the terrestrial record is not overlooked as attention is paid increasingly to the p e r h a p s more attractive high resolution marine and ice core records. Understanding terrestrial responses to climatic change and hence how such systems will evolve in the future cannot be achieved without detail from the terrestrial record itself.