JOURNAL OF QUATERNARY SCIENCE (1990) 5 (2) 123-1 33 @ 1990 by John Wiley & Sons, Ltd.
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.
KEYWORDS: British Isles, pollen analysis, Alnus glutinosa, postglacial, distribution change.
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
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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).
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).
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.
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
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.
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.
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.
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
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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
Figure 2 Continued
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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.
AMMANN, B. and LOTTER, A. F. 1989. Late-glacial radiocarbon- and palynostratigraphy on the Swiss Plateau. Boreas, 18, 109-126.
ANDERSEN, S. T. 1970. The relative pollen productivity and representation of North European trees, and correction factors for tree pollen spectra. Danmarks Ceologiske Undersngelse, 11(96), 99 PP.
ANDERSEN, S. T. 1974. Wind conditions and pollen deposition in a mixed deciduous forest. II. Seasondl and annual deposition. Crana, 14, 64-77.
POSTGLACIAL HISTORY OF ALDER (ALNUS GLUTINOSA (L.) GAERTN) 131
ANDREWS, M. V. 1987. Loch Cholla, Colonsay. In Excavations on Oronsay: Prehistoric Human Ecology on a Small lsland (ed. P. A. Mellars) Edinburgh University Press, 63-71.
BARBER, K. E. and CLARKE, M. J. 1987. Cranes Moor, New Forest: palynology and macrofossil stratigraphy. In Wessex and the lsle of Wight: Field Guide (ed. K. E. Barber) Quaternary Research Association, Cambridge, 3344.
BARNOSKY, C. W. 1988. A late-glacial and post-glacial pollen record from the Dingle peninsula, County Kerry. Proceedings of the Royal lrish Academy, 888, 23-37.
BARTLEY, D. D., CHAMBERS, C. and HART-JONES, B. 1976. The vegetational history of parts of south and east Durham. New Phytologist, 77, 437468.
BEALES, P. W. 1980. The late Devensian and Flandrian vegetational history of Crose Mere, Shropshire. New Phytologist, 85, 133-161.
BECKETT, S. C. 1981. Pollen analysis of the peat deposits. Proceedings of the Prehistoric Society, 47, 245-273.
BENNETT, K. D. 1983a. Postglacial population expansion of forest trees in Norfolk, UK. Nature, 303, 164-167.
BENNETT, K. D. 1983b. Devensian late-glacial and Flandrian vegetational history at Hockham Mere, Norfolk, England. I. Pollen percentages and concentrations. New Phytologist, 95, 457487.
BENNETT, K. D. 1985. The spread of Fagus grandifolia across eastern North America during the last 18,000 years. lournal of Biogeography, 12, 147-164.
BENNETT, K. D. 1986a. Competitive interactions among forest tree populations in Norfolk, England, during the last 10,000 years. New Phytologist, 103, 603-620.
BENNETT, K. D. 1986b. The rate of spread and population increase of forest trees during the postglacial. Philosophical Transactions of the Royal Society of London, 8314, 523-531.
BIRKS, H. H. 1972. Studies in the vegetational history of Scotland. 111. A radiocarbon-dated pollen diagram from Loch. Maree, Ross and Cromarty. New Phytologist, 71, 731-754.
BIRKS, H. H. 1975. Studies in the vegetational history of Scotland. IV. Pine stumps in Scottish blanket peats. Philosophical Transactions of the Royal Society of London, 8270, 181-226.
BIRKS, H. H. and MATHEWES, R. W. 1978. Studies in the vegetational history of Scotland. V. Late Devensian and early Flandrian pollen and macrofossil stratigraphy at Abernethy Forest, Inverness-shire. New Phytologist, 80, 455-484.
BIRKS, H. J. B. 1980. Quaternary Vegetational History of West Scotland. Excursion Guide, 5th International Palynological Confer- ence, Cambridge.
BIRKS, H. J. 6. 1989. Holocene isochrone maps and patterns of tree- spreading in the British Isles. lournal of Biogeography, 16, 503-540.
BIRKS, H. J. 6. and MADSEN, 6. J. 1979. Flandrian vegetational history of Little Loch Roag, lsle of Lewis, Scotland. lournal of Ecology 67, 825442.
BIRKS, H. J. 6. and WILLIAMS, W. 1983. Late-Quaternary vegetational history of the Inner Hebrides. Proceedings of the Royal Society of Edinburgh, 838, 269-292,
BOHNCKE, S. H. P. 1988. Vegetation and habitation history of the Callanish area, lsle of Lewis, Scotland. In The Cultural Landscape: Past, Present and Future (eds H. H. Birks, H. 1. 6. Birks, P. E. Kaland and D. Moe) Cambridge University Press, Cambridge, 445461.
BOYD, W. E. and DICKSON, J. H. 1987. A post-glacial pollen sequence from Loch aMhuillin, north Arran: a record of vegetation history with special reference to the history of endemic Sorbus species. New Phytologist, 107, 221-244.
BRADSHAW, R. H. W. and McGEE, E. 1988. The extent and time- courseof mountain blanket peat erosion in Ireland. New Phytologht,
BROWN, A. G. 1988. The palaeoecology of Alnus (alder) and the postglacial history of floodplain vegetation. Pollen percentage and influx data from the West Midlands, United Kingdom. New Phytologist, 110, 425-436.
BROWN, A. P. 1977. Late-Devensian and Flandrian vegetational history of Bodmin Moor, Cornwall. Philosophical Transactions of the Royal Society of London, 8276, 251-320.
BUSH, M. B. and HALL, A. R. 1987. Flandrian Alnus: expansion or immigration? lournal of Biogeography, 14, 479481.
CHAMBERS, C. 1978. A radiocarbon-dated pollen diagram from Valley Bog, on the Moor House National Nature Reserve. New
CHAMBERS, F. M. 1982. Environmental history at Cefn Gwernffrwd, near Rhandirmwyn, mid-Wales. New Phytologist, 92, 607-61 5.
CHAMBERS, F. M. and ELLIOTT, L. 1989. Spread and expansion of Alnus Mill. in the British Isles: timing, agencies and possible vectors. lournal of Biogeography, 16, 541-550.
CHAMBERS, F. M., KELLY, R. S. and PRICE, S.-M. 1988. Development of the late-prehistoric cultural landscape in upland Ardudwy, north- west Wales. In: The Cultural Landscape: Past, Present and Future (eds H. H. Birks, H. J. 6. Birks, P. E. Kaland and D. Moe) Cambridge University Press, Cambridge, 333-348.
CHAMBERS, F. M. and PRICE, S.-M. 1985. Palaeoecology of Alnus (alder): early post-glacial rise in a valley mire, northwest Wales. New Phytologist, 101, 333-344.
CHAMBERS, F. M. and PRICE, S.-M. 1988. The environmental setting of Erw-wen and Moel y Gerddi: prehistoric enclosures in upland Ardudwy, North Wales. Proceedings of the Prehistoric Society, 54,
CLAPHAM, A. R., TUTIN, T. G. and WARBURG, E. F. 1981. Excursion Flora of the British Isles, 3rd edn. Cambridge University Press, Cambridge.
CLARK, J. G. D. and GODWIN, H. 1962. The Neolithic in the Cambridgeshire fens. Antiquity, 36, 10-23.
CLARKE, M. J. and BARBER, K. E. 1987. Mire development from the Devensian late-glacial to present at Church Moor, Hampshire. In Wessex and the lsle of Wight: Field Guide (ed. K. E. Barber) Quaternary Research Association, Cambridge, 23-32.
COLES, J. M. and ORME, B. J. 1983. Homo sapiens or Castor fiber! Antiquity, 57, 95-1 07.
CRAIG, A. J. 1978. Pollen percentage and influx analyses in south- east Ireland: a contribution to the ecological history of the late- glacial period. lourndl of Ecology, 66, 297-324.
DAVIS, M. B. 1976. Pleistocene biogeography of temperate deciduous forests. Geoscience and Man, 13, 13-26.
DEVOY, R. J. W. 1979. Flandrian sea-level changes and vegetational history of the lower Thames estuary. Philosophical Transactions of the Royal Society of London, 8285, 355407.
DODSON, J. R. and BRADSHAW, R. H. W. 1987. A history of vegetation and fire, 6,600 B.P. to present, County Sligo, western Ireland. Boreas, 16, 11 3-1 23.
DONNER, J. J. 1957. The geology and vegetation of the late-glacial retreat stages in Scotland. Transactions of the Royal Society of Edinburgh, 63, 221-264.
EDWARDS, K. J. 1979. Environmental impact in the prehistoric period. Scottish Archaeological Forum, 9, 2742 .
EDWARDS, K. J. 1985. The anthropogenic factor in vegetational history. In The Quaternary History of lreland (eds K. J. Edwards and W. P. Warren) Academic Press, London, 187-220.
EDWARDS, M. E. 1980. Ecology and historical ecology of oakwoods in north Wales. Unpublished Ph.D. Thesis, University of Cambridge.
FAEGRI, K. and IVERSEN, J. 1989. Textbook of Modern Pollen Analysis, 4th edn. Wiley, Chichester.
GODWIN, H. 1940a. Studies of the post-glacial history of British vegetation. ill. Fenland pollen diagrams. IV. Post-glacial changes of relative land- and sea-levels in the English fenland. Philosophical Transactions of the Royal Society of London, 8230, 239-303.
GODWIN, H. 1940b. Pollen analysis and forest history of England and Wales. New Phytologist, 39, 370-400.
GODWIN, H. 1975. History of the British flora: a factual basis for Phytogeography, 2nd edn. Cambridge University Press, Cambridge.
GODWIN, H., WALKER, D. and WILLIS, E. H. 1957. Radiocarbon dating and post-glacial vegetational history: Scaleby Moss. Proceed- ings of the Royal Society of London, 8147, 352-366.
HARPER, J. L. 1977. Popufation Biology of Plants. Academic Press, London.
HEYWORTH, A., KIDSON, C. and WILKS, P. 1985. Late-glacial and Holocene sediments at Clarach Bay, near Aberystwyth. lournal of Ecology, 73, 459480.
HIBBERT, F. A. and SWITSUR, V. R. 1976. Radiocarbon dating of Flandrian pollen zones in Wales and northern England. New Phytologist, 77, 793-807.
Phytologist, 80, 273-280.
132 JOURNAL OF QUATERNARY SCIENCE
HIBBERT, F. A., SWITSUR, V. R. and WEST, R. G. 1971. Radiocarbon dating of Flandrian pollen zones at Red Moss, Lancashire. Proceedings of the Royal Society of London, B177, 161-1 76.
HIRONS, K. R. 1983. Percentage and accumulation rate pollen diagrams from east Co. Tyrone. In Landscape archaeology in lreland (eds T. Reeves-Smyth and F. Hamond) BAR British Series, 116, 95-117.
HUNTLEY, B. 1981. The past and present vegetation of Caenlochan National Nature Reserve. II. Palaeoecological investigations. New Phytologist, 87, 189-222.
HUNTLEY, 6. and BIRKS, H. J. B. 1983. An Atlas of Past and Present Pollen Maps for Europe 0-13,000 Years Ago. Cambridge University Press, Cambridge.
IREMONGER, S. F. and KELLY, D. L. 1988. The responses of four lrish wetland tree species to raised soil water levels. New Phytologist, 109, 491497 .
JACOBSON, G. L., Jr. and BRADSHAW, R. H. W. 1981. The selection of sites for paleovegetational studies. Quaternary Research, 16, 80-96.
JESSEN, K. 1949. Studies in late Quaternary deposits and flora history of Ireland. Proceedings of the Royal lrish Academy, 528, 85-290.
j6HANSEN, j . 1985. Studies in the vegetational history of Faroe and Shetland Islands. Annales Societatis Scientiarum Faeroensis Supplernentum, XI.
JONES, R. L. 1977. Late-Quaternary vegetational history of the North York Moors. V. The Cleveland Dales. journal of Biogeography, 4,
JONES, V. j., STEVENSON, A. C. and BATTARBEE, R. W. 1986. Lake acidification and the land-use hypothesis: a mid-post-glacial analogue. Nature, 322, 157-1 58.
JONES, V. I., STEVENSON, A. C. and BATTARBEE, R. W. 1989. Acidification of lakes in Galloway, south west Scotland: a diatom and pollen study of the post-glacial history of the Round Loch of Glenhead. lournal of Ecology, 77, 1-23.
KEATINGE, T. H. and DICKSON, J. H. 1979. Mid-Flandrian changes in vegetation on Mainland Orkney. New Phytologist, 82, 585-612.
KELLY, D. L. 1981. The native forest vegetation of Killarney, south- west Ireland: an ecological account. journal of Ecology, 69, 43 7-4 72.
KERSLAKE, P. D. 1982. Vegetational history of wooded islands in Scottish lochs. Unpublished Ph.D. Thesis, University of Cambridge.
MASON, C . F., MACDONALD, S. M. and HUSSEY, A. 1984. Structure, management and conservation value of the riparian woody plant community. Biological Conservation, 29, 201-21 6.
McVEAN, D. N. 1953. Biological flora of the British Isles. Ahus glutinosa (L.) Gaertn. journal of Ecology, 41, 4 4 7 4 6 6 .
McVEAN, D. N. 1955a. Ecology of Alnus glutinosa (L.) Gaertn. I. Fruit formation. fournal of kology, 43, 4 M O .
McVEAN, D. N. 195513. Ecology of Alnus glutinosa (L.) Gaertn. 11. Seed distribution and germination. lournal of Ecology, 43, 61-71.
McVEAN, D. N. (1956a). Ecology of Alnus glutinosa (L.) Caertn. IV. Root system. lournal of Ecology, 44, 219-225.
McVEAN, D. N. (1956b). Ecology of Alnus glutinosa (L.) Gaertn. V. Notes on some British alder populations. fournal of Ecology, 44,
McVEAN, D. N. 1956c. Ecology of Alnus glutinosa (L.) Gaertn. VI. Post-glacial history. journal of Ecology, 44, 331-333.
McVEAN, D. N. 1956d. Ecology of Alms glutinosa (L.) Gaertn. 111. Seedling establishment. journal of Ecology, 44, 195-21 8.
MOAR, N. T. 1969a. A radiocarbon dated pollen diagram from north-west Scotland. New Phytologist, 68, 209-214.
MOAR, N. T. 1969b. Late Weichselian and Flandrian pollen diagrams from south-west Scotland. New Phytologist, 68, 433-467.
OCONNELL, M., MITCHELL, F. J. G., READMAN, P. W., DOHERTY, T. J. and MURRAY, D. A. 1987. Palaeoecological investigations towards the reconstruction of the post-glacial environment at Lough Doo, County Mayo, Ireland. lournal of Quaternary Science, 2, 149-1 64.
OCONNELL, M., MOLLOY, K. and BOWLER, M. 1988. Post-glacial landscape evolution in Connemara, western Ireland with particular reference to woodland history. In The cultural landscape: past, present and future, (eds H. H. Birks, H. 1. 8. Birks, P. E: Kaland and D. Moe) Cambridge University Press, Cambridge, 267-287.
OSULLIVAN, P. E. 1974. Two Flandrian pollen diagrams from the east-central highlands of Scotland. Pollen et Spores, 16, 33-57.
OSULLIVAN, P. E. 1976. Pollen analysis and radiocarbon dating of a core from Loch Pityoulish, eastern highlands of Scotland. journal of Biogeography, 3, 293-302.
PARSONS, R. W., PRENTICE, 1. C. and SAARNISTO, M. 1980. Statistical studies on pollen representation in Finnish lake sediments in relation to forest inventory data. Annales Botanic; Fennici, 17, 379-393.
PEGLAR, S. M. 1979. A radiocarbon-dated pollen diagram from Loch of Winless, Caithness, northeast Scotland. New Phytologist, 82, 245-263.
PENNINGTON, W. 1970. Vegetation history in the north-west of England: a regional synthesis. In Studies in the vegetational history of the British lsles (eds D. Walker and R. G. West) Cambridge University Press, Cambridge, 41-79.
PENNINGTON, W., HAWORTH, E. Y., BONNY, A. P. and LISHMAN, J. P. 1972. Lake sediments in northern Scotland. Philosophical Transactions of the Royal Society of London, B264,
PENTECOST, A. 1985. Alnus leaf impressions from a postglacial tufa in Yorkshire. Annals of Botany, 56, 779-782.
PIGOTT, C. D. and WILSON, J. F. 1978. The vegetation of North Fen at Esthwaite in 1967-9. Proceedings of the Royal Society of London, 6200, 331-351.
PILCHER, J. R. 1969. Archaeology, palaeoecology, and 14C-dating of the Beaghmore stone circle site. Ulster fournal of Archaeology, 32, 73-91.
PILCHER, J. R. 1973. Pollen analysis and radiocarbon-dating of a peat on Slieve Gallion, Co. Tyrone, N. Ireland. New Phytologist, 72, 681-689.
PILCHER, J. R. and LARMOUR, R. 1982. Late-glacial and post-glacial vegetational history of the Meenadoan Nature Reserve, County Tyrone. Proceedings of the Royal lrish Academy, 828, 277-295.
PILCHER, J. R. and SMITH, A. G. 1979. Palaeoecological investi- gations at Ballynagilly, a Neolithic and Bronze Age settlement in County Tyrone, Northern Ireland. Philosophical Transactions of the Royal Society of London, 6286, 345-369.
PRENTICE, I. C. 1983. Pollen mapping of regional vegetation patterns in south and central Sweden. fournal of Biogeography, 10, 441-454.
PRENTICE, I. C., BERGLUND, B. E. and OLSSON, T. 1987. Quantitative forest-composition sensing characteristics of pollen samples from Swedish lakes. Boreas, 16, 43-54.
RANWELL, D. S. 1974. The salt marsh to tidal woodland transition. Hydrobiological Bulletin, 8, 139-1 5 1 .
ROBINSON, D. E. and DICKSON, J. H. 1988. Vegetational history and land-use: a radiocarbon-dated pollen diagram from Machrie Moor, Arran, Scotland. New Phytologist, 109, 223-251.
RYMER, L. 1974. The palaeoecology and historical ecology of the parish of North Knapdale, Argyllshire. Unpublished Ph.D. Thesis, University of Cambridge.
SCAIFE, R. G . 1987. The late-Devensian and Flandrian vegetation of the lsle of Wight. In Wessex and the lsle of Wight: Field Guide (ed. K. E. Barber) Quaternary Research Association, Cambridge, 156-1 80.
SIMMONS, I. G., RAND, J. I. and CRABTREE, K. 1983. A further pollen analytical study of the Blacklane peat section on Dartmoor New Phytologist, 94, 655-667.
SMITH, A. C. 1970. The influence of Mesolithic and Neolithic man on British vegetation. In Studies in the vegetational history of the British lsles (eds D. Walker and R. G. West) Cambridge University Press, Cambridge, 81-96.
SMITH, A. G. 1981. Palynology of a Mesolithic-Neolithic site in county Antrim, N. Ireland. In Fourth lnternational Palynological Conference Proceedings Volume 111 (eds D. C. Bharadwaj, Vishnu- Mittre and H. K. Maheshwari) Birbal Sahni Institute of Palaeobotany, Lucknow, 248-257.
SMITH, A. G. 1984. Newferry and the Boreal-Atlantic transition. New Phytologist, 98, 35-55.
SMITH, A. G. and PILCHER, J. R. 1973. Radiocarbon dates and vegetational history of the British Isles. New Phytologist, 72, 903-91 4.
POSTCLACIAL HISTORY OF ALDER (ALNUS CLUTINOSA (L.) CAERTN) 133
STEWART, D. A. 1983. The history of alder, Alnus glutinosa (L.) Gaertn., in the Campsie Fells. Clasgow Naturalist, 20, 333-345.
STEWART, D. A,, WALKER, A. and DICKSON, 1. H. 1984. Pollen diagrams from Dubh Lochan, near Loch Lomond. New Phytologist,
TALLANTIRE, P. A. 1972. The regional spread of spruce (Picea abres (L.) Karst.) within Fennoscandia: a reassessment. Norwegian journal of Botany, 19, 1-16.
TANSLEY, A. G. 1939. The British lslands and their Vegetation. Cambridge University Press, Cambridge.
TUCKER, 1. 1. and FITTER, A. H. 1981. Ecological studies at Askham Bog Nature Reserve - 2. The tree population of Far Wood. Naturalist, 106, 3-14.
TURNER, J., HEWETSON, V. P., HIBBERT, F. A., LOWRY, K. H. and CHAMBERS, C. 1973. The history of the vegetation and flora of Widdybank Fell and the Cow Green Reservoir basin, upper Teesdale. Philosophical Transactions of the Royal Society of London, B265, 327-408.
TURNER, J. and HODGSON, j. 1981. Studies in the vegetational history of the northern Pennines. II. An atypical pollen diagram from Pow Hill, Co. Durham. journal of Ecology, 69, 171-188.
TURNER, M. C., CARDNER, R. H., DALE, V. H. and ONEILL, R. V. 1989. Predicting the spread of disturbance across heterogenous landscapes. Oikos, 55, 121-1 29.
VINTHER, E. 1983. Invasion of Alnus glutinosa (L.) Gaertn. in a
former grazed meadow in relation to different grazing intensities. Biological Conservation, 25, 75-89.
WALKER, R. 1978. Diatom and pollen analyses of a sediment profile from Melynllyn, a mountain tarn in Snowdonia, north Wales. New Phytologist, 81, 791-804.
WALLER, M. 1987. The Flandrian vegetational history and environ- mental development of the Brede and Pannel valleys, East Sussex. Unpublished Ph. D. Thesis, Polytechnic of North London.
WATTS, W. A. 1984. The Holocene vegetation of the Burren, western Ireland. In Lake sediments and environmental history, (eds E. Y. Haworth and I. W. C. Lund) Leicester University Press, 359-376.
WATTS, W. A. 1985. Quaternary vegetation cycles. In The Quaternary History of lreland (eds K. j. Edwards and W. P. Warren) Academic Press, London, 155-1 85.
WHEELER, 6. D. 1978. The wetland plant communities of the River Ant valley, Norfolk. Transactions of the Norfolk and Norwich Naturalists Society, 24, 153-1 87.
WHEELER, 6. D. 1980. Plant communities of rich-fen systems in England and Wales 111. Fen meadow, fen grassland and fen woodland communities. journal of Ecology, 68, 761-788.
WHITE, j. 1985. The Cearagh Woodland, Co. Cork. lrish Naturalists
WORSLEY, P. 1978. On the ecology of the beaver and some journal, 21, 391-396.
speculative applications. Quaternary Newsletter, 25, 3-4.