Postglacial history of alder (Alnus glutinosa (L.) Gaertn.) in the British Isles

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  • JOURNAL OF QUATERNARY SCIENCE (1990) 5 (2) 123-1 33 @ 1990 by John Wiley & Sons, Ltd.

    0267-81 79/90/020123-11/$05.50

    Postglacial history of alder (Alms glutinosa (1.) Gaertn.) in the British Isles KEITH DAVID BENNETT Subdepartment of Quaternary Research, Botany School, University of Cambridge, Downing Street, Cambridge CB2 3EA, England H. JOHN B. BlRKS Botanical Institute, University of Bergen, Alldgaten 41, N-5007 Bergen, Norway

    Bennett, K. D. and Birks, H. J. B. 1990. Postglacial history of alder (Alnus glutinosa (L.) Gaertn.) in the British Isles. Journal of Quaternary Science, Vol. 5, pp. 123-133. ISSN 0267-8179. Received 2 January 1990 Revised 3 April 1990

    ABSTRACT: Data from 92 postglacial pollen sequences are used to map the spread and increase of alder (Alnus glutinosa) across the British Isles between 9000 and 5000 years ago. The spread is found to be patchy and erratic in space and time. Consideration of the habitat requirements and reproductive ecology of alder suggest that it spread within Britain and Ireland after about 10 000 yr BP, when suitable habitat for it was scarce. Alder spread across most of Britain and Ireland early in the postglacial but only increased in abundance as (i) suitable habitat became available through changing sea levels, hydroseral successions, and floodplain development, and brd d as (ii) rare weather events produced the necessary conditions for reproduction. Alder is unique among British and Irish trees in its requirement for a suitable habitat isolated among expanses of unsuitable habitats. Because of this, maps of its postglacial population spread and increase do not show the spatial coherence of maps for other forest tree taxa.

    science

    KEYWORDS: British Isles, pollen analysis, Alnus glutinosa, postglacial, distribution change.

    Introduction

    Postgtacial pollen sequences from the British Isles typically include an increase of alder pollen at some point in the early to mid-postglacial as one of their most visually striking features. This increase was used from the earliest days of British and Irish pollen analysis as one of the bases for subdivision of postglacial pollen stratigraphy (Godwin, 1940a; Jessen, 1 949). Godwin (1 940a, b, 1975) consistently argued that alder had been present in small amounts since the beginning of the postglacial but expanded suddenly at the beginning of his Zone VII in response to climatic change. The sudden and presumed synchronous increase could not be due to immigration because many pollen diagrams suggested that alder was already present in low amounts.

    The advent of radiocarbon dating made possible a test of the synchroneity of the alder pollen increase. Smith and Pilcher (1973) showed that the timing of the alder rise varied by about 2000 years across Britain and Ireland, even with the limited number of sites then available. Smith (1970, 1981, 1984) argued that the increase of alder was influenced by the use of fire as part of early postglacial anthropogenic activity. This argument suggests that a local phenomenon (early postglacial human activity) could have caused a broad-scale phenomenon (the increase of alder in Britain and Ireland) just because the two happen to be synchronous at a few sites. However, if humans did cause the alder rise through use of fire, then the effect should be detectable at all sites by microscopic charcoal analyses, but, on the evidence to date,

    this is not the case (e.g. Edwards, 1979, 1985; OConnell et a/., 1988).

    Attention in the last decade has focused on migrational explanations for the alder rise, following the lead of Davis (1976) for tree taxa in eastern North America. Huntley and Birks (1 983) suggested that alder spread across Britain between 8000 and 6000 years ago, moving from south-eastern England. Chambers and Price (1985) presented data from a Welsh site with evidence for an early alder rise (Moel y Gerddi: site 63 on Fig. 1) and argued that a spread of alder into Britain from the west must be seriously considered as an alternative to the south-eastern route proposed by Huntley and Birks (1983). Bush and Hall (1987) argued that the alder rise was due to expansion of populations already present in Britain at the beginning of the postglacial, combined with immigration from continental sources. Chambers and Elliott (1 989) reviewed the data for the spread of alder into and within Britain during the postglacial, as mapped by Huntley and Birks (1983) and Birks (19891, and concluded that this model is not tenable. Rather, they suggested that alder may have survived the last cold stage within the British Isles and increased in response to disturbances, such as those caused by fire (e.g. Smith, 19841, beavers (Worsley, 1978; Coles and Orme 1983) or geomorphological processes. Birks (1 989) suggested that the peculiar ecological requirements of alder would lead, however, to a multiplicity of explanations for the alder rise.

    In this paper we reassess the available data for the timing of the alder increase in Britain and Ireland. We compare the pattern of spread of alder with its modern ecological requirements and consider whether our knowledge of its

  • 124 JOURNAL OF QUATERNARY SCIENCE

    modern ecology is adequate to explain its postglacial fossil record. We follow previous discussion of British and Irish alder in assuming that Alnus glutinosa is the only member of the genus to have been present in the British Isles during the postglacial. This is now supported by early postglacial macrofossil finds of A. glutinosa in Sussex (Waller, 19871, Yorkshire (Bush and Hall, 1987) and Hampshire (Clarke and Barber, 19871, although Heyworth et a/. (1985) suggest that A. incana may have been present during the lateglacial.

    Vascular plant nomenclature follows Clapham et a/. (1981). Dates are given as uncorrected radiocarbon years before AD 1950 (BP).

    Present-day ecology

    Habitats

    Alder is a tree of wet, mildly basic habitats, such as river and streamsides, lake-shores, flushed hillsides, fens, flood-plains, brackish-freshwater transitions in estuaries and sea lochs, and low-lying areas of impeded drainage. The soils in these situations are always wet, often with winter flooding. Surface soil remains wet or very damp even if there is little or no standing water in the summer. Prior to forest destruction and drainage of wetlands, the local and regional distribution of alder within the British Isles would have been primarily limited by soil moisture. In contrast to other trees important in the British postglacial pollen record (Betula, Corylus, Quercus, Ulmus, Tilia, Pinus, Fraxinus, Fagus), suitable potential habitats for alder have always been patchy and can be thought of as wet lowland islands within a sea of better drained soils supporting upland forest. Tansley (1939, p. 460) pictured the flatter lowlands of post-glacial Britain as studded with lakes and meres bordered by wide stretches of swamp, marsh and fen, which were probably largely occupied by a Iderwood .

    Alder can occur as pure stands or mixed with other species, such as ash (Fraxinus excelsior) on fertile, moist soils, willow (Salix) on seasonally flooded sites, birch (Betula) on wet but less fertile sites, and oak (Quercus) and elm (Ulmus) in wet enclaves within upland forests or by streams. Alder can grow on a range of soil types, including peats, humus podzols, moist mull, and mesotrophic or eutrophic swamp peats encompassing a pH range of about 4-7. It has a symbiotic association with the nitrogen-fixing actinomycete Actinomyces alni and this association is most effective at pH greater than 5 (McVean, 1956a). Alder i s one of the British trees most tolerant to waterlogging (Iremonger and Kelly, 1988).

    Accounts of different types of alder-dominated vegetation in the British Isles include McVean (1956b), Mason et a / . (1984), Tucker and Fitter (1981), Kelly (1981), Ranwell (1974), Wheeler (1978, 19801, Pigott and Wilson (19781, and White (1985).

    Establ ishrnent

    The establishment of alder is intermittent in space and time. Fruit production is itself sporadic, limited by a complex of interacting climatic factors such as late frost and desiccating winds (McVean, 1955a). McVean (1955a, 1956b, c) reported that viable seed is not formed above an altitude of 305 m today, although the modern altitudinal limit of alder in Britain

    is 488 m. Seeds are dispersed by wind, moving water, or wind-drift over standing water (McVean, 1953, 1955b). Germination requires moist conditions, high oxygen tension, light shade, and open soil or peat or piles of debris deposited by winter floods (McVean, 1955b). It occurs rarely in dense herbaceous vegetation (Vinther, 1983). Successful establishment requires high light intensity along with abundant moisture for 20-30 days after germination, supplied as precipitation or a high ground-water table. Seedlings are susceptible to drought, late frost, litter accumulation, growth of tall-herb vegetation, and the deposition of silty mud by floods.

    Conditions for successful germination and establishment on a microscale (safe-sites sensu Harper, 1977) thus occur sporadically in space and time. For example, McVean (1 956d) suggests that particular sapling populations originate from chance periods of abnormally high water tables owing to unusually large amounts of precipitation or extensive flooding following heavy snowmelt. It is not unusual to find even- aged alder populations (Pigott and Wilson, 1978; Tucker and Fitter, 1981) that appear to originate from a particular chance combination of favourable conditions. Alder can behave as a pioneer and opportunistic tree given the appropriate conditions of soil moisture, base status, microclimate, and availability of viable seed. On the other hand, the appropriate conditions for establishment appear to coincide only sporadi- cally, at least at the present-day.

    Regeneration

    Once established, alder tends to form a closed, often rather dense canopy, under which there is virtually no self- regeneration. Trees live 25-1 50 years, but usually about 100 years, Seed production does not usually occur until the tree is about 40 years old. With age, alder woods, especially on fen peat, tend to decay in situ, with trees falling over and becoming infected with heart-rot. Older trees often show poor seed production, and the surface peat may degrade owing to aeration associated with root penetration. What regeneration there i s tends to be around the margins of the woods or in areas adjacent to, but not under, the parent trees. It i s likely, but unproven, that at a local scale there may be cycles of episodic establishment, growth, and senescence (Tucker and Fitter, 1981) and that alder dominated stands may be sporadic not only in space but also in time, at least when viewed over a time-scale of centuries.

    Past patterns in time and space

    Methods

    The increase of alder pollen at British and Irish postglacial pollen sites occurs over a considerable period of time. The increase is abrupt at some sites, and gradual at others. Several possible ways exist for quantifying it. Smith and Pilcher (1 973) estimated ages at which continuous records for alder pollen began (empirical limit) and also ages at which sharp increases of alder pollen began (rational limit). After examination of the alder curve from the available sequences we decided that the rational limit was too subjective a criterion for consistent application, and we concluded that the two events that could be aged most repeatably were the onset of a continuous

  • POSTGLACIAL HISTORY OF ALDER (ALNUS CLUTINOSA (L.) GAERTN) 125

    curve for alder pollen (empirical limit), and the age at which the curve reached its maximum values. At nearly all sites, alder pollen frequencies increase steadily to a maximum, and then remain more or less constant at least until after the elm decline (in contrast to frequencies of some other pollen types, such as Corylus, for which early high frequencies are usually followed by a decrease).

    All available pollen diagrams with radiocarbon dates during the early and mid-postglacial were examined. Sites where there appear to be problems with the radiocarbon dates, as well as all sites without dates, have been excluded. For each sequence, ages were obtained for the empirical limit and the age at which alder pollen reached its maximum postglacial frequencies. Ages were obtained from single radiocarbon dates located at either event, or by interpolation between a pair of dates located above and below the event. No ages were estimated as the result of extrapolation above or below a sequence of dates. Values for the duration of the increase of alder pollen were taken as the time interval between the empirical limit and the age at which maximum frequencies were reached. Data were also collected on pollen sums used at each site, together with site characteristics, such as type (lake or bog), size and altitude.

    The data collected were plotted on maps for 500-year intervals from 9000 to 5000 yr BP (Fig. 2). The duration of the increase at each site where age estimates for both events had been obtained was also plotted (Fig. 3).

    Where data on pollen accumulation rates were available, we compared the pattern of increase for alder pollen accumulation rates with an exponential model for its increase (Bennett, 1983a). For 15 sites, six (sites 20, 21, 22, 63, 67 and 81 on Fig. 1 ) fitted an exponential increase, with doubling times ranging from 270 years to 860 years The remaining nine sites (4, 6, 19, 31, 32, 38, 58, 59 and 68 on Fig. 1) showed a poor fit to an exponential: all increased too suddenly, with doubling times of 20-250 years for the period of the step increase. There was no apparent correlation between type of increase (exponential or step) and site type, size, altitude, or geographic location.

    Sources of error

    The validity of the maps as accurate representations of the former distribution of alder in the British Isles depends upon the,quality of the data and the way in which pollen data, especially of low frequencies, are interpreted. Four possible sources of error in the interpretation of the available data are considered: inconsistency in the size of pollen counts (leading to inconsistency in the age of the empirical limit); contamination of samples; long-distance pollen transport; and dating errors.

    Size of pollen counts

    As a pollen sum increases, the chance of encountering scarce pollen types is correspondingly greater, and hence the older i s the apparent empirical limit for such types (Tallantire, 1972). Age at which maximum frequencies are reached is unlikely to be so affected. Where analysts gave pollen sums for the sites used in this analysis, the sums were noted. Most sums were 300-500 pollen grains per level. A few sites had sums that ranged up to 1000 grains per level, and five (sites 51, 58, 67, 68 and 84 on Fig. 1) had sums that were consistently up to 1000 grains per level. No sites had sums

    below 300 grains per level. Of the 14 sites where empirical limits for alder began before 8500 yr BP (Fig. 2), only one (site 51) had 1000 grain counts. It therefore does not seem that sites with high pollen counts distort the distribution of alder by significantly extending the apparent empirical limit to older ages.

    Sample contamination

    Pollen samples can become contaminated in many ways (Faegri and Iversen, 1989). The mechanism of most concern here is contamination during coring of old sediment lacking alder pollen by younger sediment containing abundant alder pollen. Nearly all the studies included here have been made within the last 20 years, during which time the problem of field contamination through the use of devices such as the Hiller peat borer has been widely appreciated. It would be rash to claim that no contamination has occurred, but it seems probable that this is not a significant source of error.

    Long-distance transport of alder pollen

    Pollen of alder is known to be produced in abundance and to be well-dispersed relative to other trees (e.g. Andersen, 1970, 1974). It is possible, therefore, that its pollen could be present at a site before the tree was growing locally, and hence that early low frequencies of alder pollen might not be a reliable indication of early occurrence of alder trees. Parsons et a/. (1980), Prentice (19831, and Prentice et a/. (1 987) show that the frequency of alder pollen in Scandinavian lake sediments today is a poor indicator of the abundance of alder in the landscape around each site. They attribute this to patchy occurrence of alder in the landscape, transport of alder pollen in streams, or dispersal of alder pollen over a wider area than for other tree taxa. Similarly, Chambers and Price (1985) and Chambers and Elliott (1989) argued that alder could be present at scattered sites within a region, but its pollen would still not be recorded at most sites there.

    Most early postglacial pollen spectra are dominated by pollen of trees, such as birch, pine, hazel and oak, that also produce abundant, well-dispersed pollen. With the prevailing vegetation producing abundant pollen, then the effect on pollen spectra of any alder pollen that happens to be dispersed regionally (sensu Jacobson and Bradshaw, 1981) will be minimal. Long-distance dispersal of alder pollen i s only likely to have been significant when there was a substantial source of alder producing the pollen for dispersal. There was certainly none in Britain, and maps of alder pollen abundances on a European scale (Huntley and Birks, 1983) show that sources of alder pollen from the continent did not become general and widespread until 8000 yr BP. If alder on the continent was a significant source for alder pollen in the British Isles during the early postglacial, then we would expect to find low frequencies of alder pollen during the early postglacial at many sites, with most pollen in sites nearest to the European continent (particularly those in south-east England). Figure 2 shows that this is not the case.

    Dating errors

    It is impossible to exclude the possibility that some of the dates used to compile the maps of Figs 2 and 3 are in error, in either direction. The time interval chosen for the maps (500 yr) i s greater than twice the typical standard deviation

  • 126 JOURNAL OF QUATERNARY SCIENCE

    for an early postglacial radiocarbon date (2100 yr). Thus, most events should be mapped at the correct interval, unless radiocarbon dating plateaux, similar to those described from the lateglacial by Arnrnann and Lotter (1989), are found in 9000-8500 yr BP During this period, continuous curves for the postglacial. The conclusions presented here are based on alder pollen began at 14 sites scattered in England, Wales data from 92 sites, so even if the dates for a few are and Scotland, and at one site in western Ireland (92). significantly in error, the general pattern should be unaffected. Macrofossils of alder older than 9000 yr BP are known from

    The maps (Figs 2 and 3)

    Alnus glutinoda . Site locations

    Figure 1 Ubhansen, 1985); 2, Glims Moss (Keatinge and Dickson, 1979); 3, Loch of Winless (Peglar, 1979); 4, An Druim, Eriboll (H. H. Birks, in Birks, 1980); 5, Duartbeg (Moar, 1969a); 6, by Loch Assynt (H. H. Birks, in Birks, 1980); 7, Callanish (Bohncke, 1988); 8, by Little Loch Roag (Birks and Madsen, 1979); 9, Lochan Dubh (Kerslake, 1982); 10, Eilean Mor (Kerslake, 1982); 1 1, Loch Sionascaig (Pennington et a/., 1972); 12, Coire Bog (Birks, H. H., 1975); 13, Eilein Dubh na Sroine (Kerslake, 1982); 14, Eilean Subhainn Lochan (Kerslake, 1982); 15, Eilein Subhainn Bog (Kerslake, 1982); 16, Loch Maree (Birks, H. H., 1972); 17, Coille na Glas Leitire (Kerslake, 1982); 18, Loch Clair (Pennington et a / . , 1972); 19, by Loch Coultrie (H. H. Birks, unpublished); 20, Loch Cleat (Birks and Williams, 1983); 21, Loch Ashik (Birks and Williams, 1983); 22, Loch Meodal (Birks and Williams, 1983); 23, Allt na Feithe Sheilich (Birks, H. H., 1975); 24, Loch Pityoulish (OSullivan, 1976); 25, Abernethy Forest (Birks and Mathewes, 1978); 26, Loch Garten (OSullivan, 1974); 27, Braeroddach Loch (Edwards, 1979); 28, Loch Einich (Birks, 1975); 29, Caenlochan Glen (Huntley, 1981); 30, Coire Fee (Huntley, 1981); 31, Lochan Doilead (W. Williams, in Birks, 1980); 32, by Salen (W. Williams, in Birks, 1980); 33, Oban 1A (Donner, 1957; W. Williams, in Birks, 1980); 34, Dubh Lochan (Stewart et a/., 1984); 35, Craigbarnet Muir (Stewart, 1983); 36, Drimnagall (Rymer, 1974); 37, Loch Cholla (Andrews, 1987); 38 Loch Cill an Aonghais (S . M. Peglar, in Birks, 1980); 39, Loch 6Mhuillin (Boyd and Dickson, 1987); 40, Machrie Moor (Robinson and Dickson, 1988); 41, Din Moss (Hibbert and Switsur, 1976); 42, Cooran Lane (Birks, 1975); 43, Loch Dungeon peat (Birks, 1975); 44, Round Loch of Glenhead (Jones et a/., 1986, 1989); 45, Clatteringshaws Loch (Birks, 1975). 46, Bigholm Burn (Moar, 1969b); 47, Scaleby Moss (Godwin et a/., 1957); 48, Pow Hill (Turner and Hodgson, 1981); 49, Weelhead Moss (Turner et a / . , 1973); 50, Valley Bog (Chambers, 1978); 51, Johnnys Wood (H. J. 8. Birks, unpublished); 52, Burnmoor Tarn (Pennington, 1971); 53, Bishop Middleham (Bartley et a/., 1976); 54, Morden Carr (Bartley et a/., 1976); 55, Neasham Fen (Bartley et a/., 1976); 56, West House Moss (Jones, 1977); 57, Red Moss (Hibbert et a/., 1971); 58, Hatchmere (H. 1. B. Birks, unpublished); 59, Crose Mere (Beales, 1980); 60, Melynllyn (Walker, 1978); 61, Nant Ffrancon (Hibbert and Switsur, 1976); 62, Cors Dolfriog (Edwards, 1980); 63, Moel y Gerddi (Chambers and Price, 1988; Chambers et a / . , 1988); 64, Tregaron (Hibbert and Switsur, 1976); 65, Cefn Cwernffrwd (Chambers, 1982); 66, Wilden Marsh (Brown, 1988); 67, Stow Bedon (Bennett, 1986a); 68, Hockham Mere (Bennett, 1983b); 69, Shippea Hill (Clark and Codwin, 1962); 70, Worlds End (Devoy, 1979); 71, Church Moor (Clarke and Barber, 1987); 72, Cranes Moor (Barber and Clarke, 1987); 73, Gatcombe Withy Bed (Scaife, 1987); 74, Blacklane Brook (Simmons et a/., 1983); 75, Shaugh Moor (Beckett, 1981); 76, Dozmary Pool (Brown, 1977); 77, Newferry (Smith, 1984); 78, Slieve Gallion (Pilcher, 1973); 79, Beaghmore (Pilcher, 1969); 80, Ballynagilly (Pilcher and Smith, 1979); 81, Killymaddy Lough (Hirons, 1983); 82, Meenadoan (Pilcher and Larmour, 1982); 83, Union Wood Lake (Dodson and Bradshaw, 1987); 84, Lough Doo (OConnell et a/., 1987); 85, Redbog (Watts, 1985); 86, Arts Lough (Bradshaw and McCee, 1988); 87, Connemara National Park (OConnell et a/., 1988); 88, Lough Sheeauns (OConnell et a/., 1988); 89, Namackanbeg (OConnell et a/., 1988); 90, Cortlecka (Watts, 1984); 91, Belle Lake (Craig, 1978); 92, Ballinloghig (Barnosky, 1988).

    Location map for sites used in the reconstruction of the distribution of alder in time and space (Figs 2 and 3). 1, Murraster

  • POSTGLACIAL HISTORY OF ALDER (ALNUS CLUUNOSA (L.) GAERTN) 127

    east Yorkshire (Bush and Hall, 19871, Sussex (Waller, 1987) and possibly Hampshire (Clarke and Barber, 1987). Impressions of alder leaves in Yorkshire tufa have been dated to 8600 2 600 yr BP (Pentecost, 1985). The pollen and macrofossil records combine to suggest a scattered presence of alder over much of the British Isles by about 9000 yr BP, with lowest densities possibly in Ireland.

    8500-8000 yr BP Alder pollen frequencies reached maximum values at one site (36) in south-west Scotland. Continuous alder curves occur at sites from the Isle of Wight to Caithness and from East Anglia to Snowdonia, but still at only one Irish site.

    8000-7500 yr BP More sites have continuous alder pollen curves, and four more sites show maximum alder pollen frequencies,. in the Thames Valley (70), west Wales (62, 63), and south-west Scotland (36). A second Irish site (89), also in the west, shows a continuous pollen curve.

    7500-7000 yr BP The distribution of alder now begins to fill in. An increasing number of sites show continuous pollen curves, and several reach maximum frequencies, but none of the four in Ireland. There is no obvious geographic.pattern to the distribution of sites where alder reached maximum frequencies by 7000 yr BP.

    7000-6500 yr BP About half of the sites in England, Wales and Scotland with alder pollen reached maximum frequencies by 6500 yr BP, but only one (89) of 13 Irish sites has maximum frequencies.

    6500-6000 yr BP Maximum alder pollen frequencies have now been reached at most sites in Great Britain, but still at only a minority of Irish sites.

    6000-5500 yr BP In Ireland alder pollen frequencies con- tinue to increase or have reached maximum values at about half of the sites. Most of the sites in Great Britain already show maximum values and a few more are added during this time interval.

    5500-5000 yr BP Only a few sites anywhere in the British Isles had not reached maximum frequencies by 5000 yr BP, mostly sites in the far north or uplands.

    Duration of increase The duration of increase of alder pollen curves varies from less than 500 years to over 3000 years, with little geographic pattern, except that most sites with long increases (over 2500 years) are in northern Scotland.

    Discussion

    The overwhelming impression from the maps (Figs 2 and 3) i s that the spread and increase of alder across the British Isles during the postglacial has been patchy in space and time. None of the maps shows any pattern that could be described as spatially coherent, and it would be difficult to suggest any time-transgressive spread in any direction for alder across the British Isles (see also Chambers and Elliott, 1989). Not only are the first occurrences of alder patchy, but the durations of increase are highly variable., The complexity of the spread and increase of alder at the scale of the British Isles is in

    contrast to the time-transgressive and spatially coherent spread of other tree taxa (Birks, 1989). It i s unlikely, therefore, to be an artefact of the data. Because of the large number of sites available with adequately dated alder pollen increases, the mapped syntheses are a reliable reflection of the way in which alder spread across the British Isles. Its behaviour is much more erratic in space and time than any other tree in the British Isles for which we have adequate data.

    Two factors may contribute to this feature. First, the requirements of alder for establishment and regeneration occur rarely (see above), with rare weather events, of a variety of types, dominating the opportunities for these processes. This leads to sporadic changes in its population sizes on ecological time-scales. Disturbances of any sort might favour alder (e.g. Chambers and Elliott, 19891, but, equally, might restrict it, depending on local hydrological consequences of the disturbance. Second, the habitats available for alder increased during the postglacial in a sporadic and patchy manner depending heavily on local topographic factors. Isostatic uplift, eustatic changes in sea level, the course of hydroseral successions by accumulation of organic sediments in lakes and bogs, and floodplain development in valleys have all tended to increase the amount of habitat potentially available to alder during the course of the postglacial. Even today, this habitat i s patchy in its occurrence in the landscape. It is likely that the erratic pattern of distribution of alder pollen in Scandinavian landscapes today, noted by Parsons et a/. (1 980), Prentice (1983) and Prentice et a/. (1987), i s also due to patchy distribution of alder trees rather than stream dispersal of pollen or a wide pollen catchment.

    Potential alder habitats thus had low connectivity, especially early in the postglacial, and alder has a low frequency of successful reproduction. Turner et a/. (1989), in a discussion of the spread of 'disturbance', show how such a combination of factors should permit only slow spread, i f i t occurs at all, and the way alder spread in the British Isles appears to support that analysis.

    The available record suggests that alder was present in the British Isles early in the postglacial, was probably present as early as 10 000 yr BP, and conceivably present even earlier (Chambers and Elliott, 1989). At that time the habitat suitable for it was scarce and much more isolated than today. It spread across most of these islands within the next 1500 years, but was never abundant. Over the next 5000 years, alder populations increased as habitat became available and as suitable weather events provided opportunities for establish- ment, regeneration and expansion. The combination of sporadically increasing areas of suitable habitat and temporally intermittent conditions for reproduction has resulted in a pattern of spread and increase during the early and mid- postglacial across the British Isles, that is, for all practical purposes, random at these spatial and temporal scales.

    The spread of alder into the British Isles probably occurred at population densities too low for the spread to be followed by current palaeoecological techniques (Bennett, 1985). Since the spread of alder within Britain and Ireland has no coherent spatial pattern, postglacial pollen data cannot be used to suggest any direction of origin (Chambers and Elliott, 1989). Alder may have been present throughout the last cold stage, or it may have spread into the British Isles early in the postglacial. If the latter, then avian dispersal agents may have been important in achieving the necessary rate of spread (Chambers and Elliott, 1989).

    The subsequent increase in population abundance took place in response to changing local conditions and cannot be considered as part of a subcontinental-scale response to changing climate within the postglacial (though the initial

  • 128 JOURNAL OF QUATERNARY SCIENCE

    i ii

    iii iv

    Figure 2 References for sites in caption for Fig. 1. Open circles: sites where a continuous curve for alder pollen began during the map period, or was continuing, but pollen frequencies had not reached their maximum postglacial values. Solid circles: sites where alder pollen frequencies first reached their maximum postglacial values during the map period, or earlier. Crosses: location of finds of macrofossils from the map period (see text).

    Maps of pollen and macrofossil evidence for alder in the British Isles for eight 500-yr periods between 9000 and 5000 yr BP.

  • POSTGLACIAL HISTORY OF ALDER (ALNUS CLUTlNOSA (L.) GAERTN) 129

    V vi

    vii

    ~ .-

    Alms glutinosa

    viii

    I

    Figure 2 Continued

  • 130 JOURNAL OF QUATERNARY SCIENCE

    0 0 - 5 0 0 ~ 0 601 - 1000 yrr 0 1001-150Oyr8 0 1501 - 2000 yrs 0 2001 - 2500 yrs O d o Z k l m 0 >2500yrs

    Figure 3 Duration of postglacial increase of alder pollen frequencies in the British Isles, in radiocarbon years. References for sites in caption for Fig. 1 .

    spread was undoubtedly triggered by the major climatic change that defines the onset of the postglacial). The presumed synchroneity of the 'Zone VII' alder rise provided the basis for believing that the mid-postglacial was climatically wetter than the early postglacial, so the demonstration that the alder rise was not only asynchronous (Smith and Pilcher, 1973) but also patchy and erratic in time and space should encourage revision of that particular long-standing belief. This should not conceal the validity of Codwin's (1940a, b, 1975) inclusion of early postglacial alder occurrences as part of his argument for the explanation of the alder rise.

    Most of the available alder pollen curves with accumulation rates do not increase exponentially, suggesting that factors other than simple population growth over several generations control rates of increase. Many populations may become established in entirety in one or a few favourable seasons by massive seed input from nearby sites, giving the 'step' increases seen at the majority of sites.

    No other major British/lrish tree has the same combination of specificity for an isolated, 'island-like', habitat and such rare opportunities for reproduction. This combination makes the changing spatial pattern of alder spread in the British Isles virtually unmappable, unlike taxa of the more continuous upland forest (Birks, 1989). Patterns of population increase for alder (e.g. Bennett, 1983a) also differ from other taxa, in that most do not follow an exponential increase. The observed 'spread' thus cannot be due to an initial abundance/popuIation density gradient and a rate of population increase (cf. Bennett, 1986b). It is probably due more to the chance aspects of habitat distribution and probability of suitable habitats being colonized (Birks, 1989; Chambers and Elliott, 1989). The

    behaviour of alder could be seen as an extreme of a continuum, at the other extreme of which there is even spread across abundant, constant habitat by taxa with steady and predictable means of reproduction. Most taxa will fall somewhere within these extremes, but the end-points of the continuum, well-exemplified by the behaviour of alder, throw sharply into focus the controlling processes of postglacial tree spread. These processes must operate in a weaker form on taxa nearer the mid-point.

    Acknowledgments We are grateful to Hilary Birks, Mary Edwards, Paul Kerslake, Sylvia Peglar, Leslie Rymer, Martyn Waller and Willie Williams for use of unpublished data or material in theses, and to Sylvia Peglar for drawing the figures. This manuscript has benefited from critical reading by Hilary Birks, Herb Wright, and an anonymous referee. Compilation of the data was facilitated by a NATO grant for international collaboration in research to J.C. Ritchie and us.

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