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

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<ul><li><p>JOURNAL OF QUATERNARY SCIENCE (1990) 5 (2) 123-1 33 @ 1990 by John Wiley &amp; Sons, Ltd. </p><p>0267-81 79/90/020123-11/$05.50 </p><p>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 </p><p>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 </p><p>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. </p><p>science </p><p>KEYWORDS: British Isles, pollen analysis, Alnus glutinosa, postglacial, distribution change. </p><p>Introduction </p><p>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. </p><p>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, </p><p>this is not the case (e.g. Edwards, 1979, 1985; OConnell et a/., 1988). </p><p>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. </p><p>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 </p></li><li><p>124 JOURNAL OF QUATERNARY SCIENCE </p><p>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. </p><p>Vascular plant nomenclature follows Clapham et a/. (1981). Dates are given as uncorrected radiocarbon years before AD 1950 (BP). </p><p>Present-day ecology </p><p>Habitats </p><p>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 . </p><p>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). </p><p>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). </p><p>Establ ishrnent </p><p>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 </p><p>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. </p><p>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. </p><p>Regeneration </p><p>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. </p><p>Past patterns in time and space </p><p>Methods </p><p>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 </p></li><li><p>POSTGLACIAL HISTORY OF ALDER (ALNUS CLUTINOSA (L.) GAERTN) 125 </p><p>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). </p><p>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. </p><p>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). </p><p>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. </p><p>Sources of error </p><p>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. </p><p>Size of pollen counts </p><p>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...</p></li></ul>

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