Dendrochronological Studies of Alder ( Alnus Glutinosa ) on Scottish Crannogs

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  • Dendrochronological Studies of Alder

    (Alnus Glutinosa) on Scottish Crannogs

    Anne Crone

    AOC Archaeology Group, UK

    Abstract

    On most Scottish crannogs that have been investigated alder is the species

    most extensively used for construction and therefore has the potential to

    provide fine chronological resolution for these sites. Dendrochronological

    studies of alder have now been undertaken on three crannogs, with mixed

    results. At Buiston the construction of a single comprehensive alder

    chronology has contributed significantly to the overall chronology of the

    crannog, whereas at Oakbank and Cults Loch 3 it was only possible to

    construct numerous small chronologies which have limited value for

    chronological resolution on the sites. Comparison between the datasets

    suggest that factors such as the structure of the parent tree, i.e. whether it

    comes from multi-stemmed coppice or single maiden trees, and the

    presence of multiple sources are likely to be significant factors in the

    successful dendro-dating of the species.

    keywords Scottish crannogs, alder, dendrochronology

    Introduction

    In the British Isles alder (Alnus glutinosa) has seldom been used in dendrochro-

    nological studies, partly because in general it occurs only rarely in archaeological

    and historical structures, but also because it has few of the requisites necessary for

    successful dendrochronological analysis, namely, a clear and reliable ring-pattern

    and long growth sequences, the latter making it unlikely that dated reference

    chronologies could ever be constructed. On the Continent small amounts of alder

    used in the construction of circum-Alpine pile dwellings have been routinely

    analysed as part of much larger assemblages of mixed species (cf. Huber and Merz,

    1962; Billamboz, 2008), but apart from some exploratory studies on assemblages

    from sites in the Somerset Levels (Morgan 1976; 1980a; 1980b) the only major

    studies of alder undertaken in the British Isles have been on assemblages from

    Scottish crannogs.

    journal of wetland archaeology, Vol. 14, September 2014, 2233

    Oxbow Books Ltd 2014 DOI 10.1179/1473297114Z.0000000007

  • This is because, on all prehistoric crannogs in Scotland which have been

    excavated to modern standards (Figure 1), alder has proved to be the favoured

    timber for construction, presumably because it was readily accessible around most

    figure 1 Distribution of all known crannogs in Scotland. Those crannogs that have been

    excavated are named; all other crannogs which have been radiocarbon-dated are identified

    by white crosses.

    STUDIES OF ALDER ON SCOTTISH CRANNOGS 23

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  • lochs. At Cults Loch it accounts for 66% of the wood assemblage (Crone,

    forthcoming) and at Oakbank it formed 62% (Crone, 1988: 56). It was also the

    dominant species at Milton Loch 1 (Piggott, 1953: 152), Loch Arthur (Henderson

    and Cavers, 2011: 108), Ederline (Henderson, 2007: 237) and Erskine Bridge

    (Crone, unpublished). Thus, while alder cannot be used to date these sites

    absolutely, it does offer the potential to develop relative site chronologies,

    providing information on phases of construction and repair which could be vital in

    trying to determine the duration of a settlement, and whether it might have been

    used seasonally.

    Although a dendrochronological study of living alder had been carried out in

    Germany which demonstrated its potential and its drawbacks (Elling, 1966) it was

    considered important that the dendrochronological viability and behaviour of

    alder growing around a Scottish loch should be determined prior to any analysis of

    the archaeological assemblages. The study of the living alder is presented first

    followed by brief summaries of the site studies.

    A study of living alder on the shores of Loch Tay

    A stand of alders fringing the shores of Loch Tay, near Oakbank crannog were

    chosen for study. The stand was a mixture of multi-stemmed trees and standard

    trees, varying in age from 11 to 72 years (Figure 2). This is probably quite a

    characteristic age structure around Scottish lochs; a botanical study of alder

    (McVean, 1953) found that alder generally lives for c. 6080 years, although older

    trees have been found. We might therefore anticipate this age structure being

    mirrored in archaeological assemblages.

    figure 2 The age structure of the living alder trees on the shore of Loch Tay.

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  • The modern study demonstrated that alder could be reliably measured, that

    missing rings were not a problem if multiple radii were measured (but see below),

    and that a chronology could be constructed (Crone, 1988: 114). Some 61% of the

    assemblage produced good visual and statistical correlations at the correct date

    (the year in which the trees were sampled). However, it was not possible to

    correlate all of the sequences even though the correct date was known, and some

    incorrect matches were identified which would have been considered acceptable

    (on the basis of their visual and statistical correlations) in an assemblage of

    unknown date. This was usually caused by the presence of signature years, a strong

    characteristic in alder (Elling, 1966).

    Perhaps the most important feature which the modern study highlighted was

    that some of the correlated sequences were missing as many as five of their outer

    growth-rings, even when the bark was present (Figure 3). As all the trees were

    living when sampled the conclusion was that either the tree had ceased growth or

    that growth had slowed down to the extent that the rings were so narrow as to be

    indistinguishable. Ellings study had identified a similar pattern and he concluded

    that this was probably caused by the overshadowing of dominant trees which led

    to progressive suppression until the tree stopped growing. In the case of the Loch

    Tay alders the dominant stems of multi-stemmed trees may have had the same

    impact. This pattern was also seen in a tree-ring study of 24-year old alder coppice;

    although all the stems had started growth in the same year 41% did not display the

    full complement of rings and many displayed a pattern of suppressed growth,

    presumably because of the presence of more dominant stems (Crone, 1988).

    This clearly has major implications for determining the felling date of the tree,

    and thereby interpreting the chronology of the site. However, suppressed growth

    should be detectable in the ring-pattern so one way of dealing with this problem

    would be to eliminate from further study any sequences displaying this

    characteristic. Also, in larger assemblages it should be possible to place greater

    numbers of sequences in any one phase, and so aberrant sequences should be more

    readily detected.

    Oakbank, Loch Tay

    Oakbank is a submerged crannog which has been partially excavated (Dixon,

    2004). A suite of radiocarbon dates has placed all activity on the crannog

    sometime between 800400 BC (Dixon, et al. 2007) but a high-precision wiggle-

    match date from an oak pile has refined the chronology indicating that the timber

    was felled sometime between 500465 BC (Cook, et al. 2010).

    Alder accounted for 62% of the 258 timbers sampled at Oakbank (Crone,

    1988). It ranged in age from 8 to 75 years but 84% was under 40 years of age.

    Different methodological approaches were explored, a traditional one based

    primarily on visual agreement supported by statistical corroboration, and one

    which used SORT.STRING, a program developed to draw out the largest possible

    groups based only on their internal statistical consistency. Both approaches

    independently produced small groups of sequences with good visual matches

    supported by acceptable statistical agreement; these were mostly groups of four or

    STUDIES OF ALDER ON SCOTTISH CRANNOGS 25

  • five sequences, although there was one larger group of 13 sequences. Block masterswere constructed for the matching groups of sequences and those that compared

    well with each other were combined to form a site chronology, a traditionalchronology 93 years in length containing 66 sequences, and a SORT.STRING

    chronology 78 years in length and containing 68 sequences. However, when the

    figure 3 Examples of missing growth rings on the living alder trees.

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  • two chronologies were compared the majority of the sequences lay in conflicting

    chronological relationships relative to each other, i.e. they would be in one

    chronological position in one chronology and a different position in another. This

    undermined any confidence in either approach and the conclusion was that the

    only reliable correlations were within the small groups. However, this makes very

    little contribution to the overall site chronology; the chronological relationships

    indicated within most of the small blocks are ones that might have been as easily

    surmised on archaeological grounds alone, i.e. that timbers from the same context

    were usually felled at the same time (Figure 4). Nonetheless, the work at Oakbank

    corroborated the study of the modern alder by demonstrating that alder can

    display good visual correlation and that chronological construction might be

    possible.

    figure 4 Oakbank; correlations between some of the alder piles. Note the missing outer

    rings on piles 481, 33 and 30.

    STUDIES OF ALDER ON SCOTTISH CRANNOGS 27

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  • Buiston, Ayrshire

    In contrast to the other Scottish crannogs that have been investigated oak was the

    dominant species used in the construction of Buiston crannog, accounting for 75%

    of the wood assemblage, and consequently it was possible to construct a robust

    tree-ring chronology which provided calendar dates indicating building activity

    starting in the late sixth century and continuing until the latter half of the seventh

    century AD (Crone, 2000).

    Alder comprised the other main structural species; two of the palisades had been

    built solely of squared alder posts, it had been used as undressed roundwood to

    level the crannog surface and converted into planks to floor one of the houses.

    Most importantly for the site chronology, it was used alongside oak in the

    palisaded walkway which encircled the crannog.

    Sixty-eight samples of alder were analysed, ranging in age from 30 (samples

    with less than 30 rings were not analysed) to 135z years, although 95% of the

    timbers were under 80 years of age (Crone, 2000; and see Figure 5). Visual and

    statistical correlations were strong throughout the assemblage and it was possible

    to construct a robust site chronology, 116 years in length and incorporating 40

    sequences (Crone, 2000). The master chronologies of oak and alder were

    compared with each other but did not match; this was tested because Ellings

    study had shown that modern oak and alder can match, although studies in the UK

    have shown otherwise (Groves and Hillam, 1988). However, as alder and oak had

    been used together in the same jointed construction it was reasonable to assume

    that the two species had been felled at the same time and it was therefore possible

    to calibrate the alder using the dated oak chronology. Consequently calendar dates

    could be ascribed to the alder contexts, and the chronology of building activity on

    the crannog refined (Crone, 2000; and see Figure 6).

    Cults Loch 3, Dumfries and Galloway

    On the promontory crannog in Cults Loch dendro-dating of some of the larger oak

    timbers indicated that the main episode of building activity occurred over little

    more than a half-century in the latter half of the fifth century BC (Cavers and

    Crone, forthcoming). Unfortunately, the bark edge had not survived on any of the

    figure 5 The

    age structure

    of the alder

    assemblages

    from Buiston

    and Cults

    Loch 3.

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  • dated oaks and so the building activity could not be phased in any detail. However,

    the timber used to build the crannog was predominantly alder; it comprised 66%

    of the wood assemblage and had been used as piling, foundation material and

    flooring throughout the crannog, thus offering the potential for refining the site

    chronology. Eighty-two samples of alder were analysed, ranging in age from 30

    (again, samples with less than 30 rings were not analysed) to 142 years, although

    81% of the timbers were under 80 years of age (Figure 5). Despite the fact that

    this is a substantial dataset, comparable to Buiston in terms of size and age

    structure, the results have been very disappointing.

    Only one coherent group of any size has emerged from the analysis, along with

    several pairs and trios of sequences (Figure 7). None of these groups correlated with

    each other and so it was not possible to construct a site chronology. However, the

    analysis did produce some information useful in interpreting the crannog. All the

    samples in the largest chronology are from stakes which lie within the overlapping

    footprints of two buildings which must have been built and occupied sequentially.

    The chronological relationships between these stakes, all felled in the same year,

    suggests that these were stakes pinning down the foundations and not stakes relating

    to the superstructures of the two buildings. The chronological relationships between

    another group of three matching sequences suggests that one of the buildings was

    constructed c. 32 years after the foundation of the crannog, thus corroborating the

    short duration of building activity that the oak chronology implies (see above).

    figure 6 Buiston; bar diagram showing the chronological relationships between the oak and

    alder context masters.

    STUDIES OF ALDER ON SCOTTISH CRANNOGS 29

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  • Discussion

    In summary, a single cohesive chronology could be built at Buiston whereas at

    Cults Loch 3 and Oakbank only multiple small chronologies could be built. The

    assemblages from Cults Loch 3 and Buiston are compared below in an attempt to

    figure 7 Cults Loch 3; matching pairs of alder sequences.

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  • define parameters for the successful analysis of alder and what this means in terms

    of the type of woodland being exploited at both sites.

    The size and age structure of the analysed assemblage was very similar; 68 timbers

    at Buiston and 71 at Cults Loch 3, and of those 94% and 87% respectively were

    under 80 years of age (Figure 5). Those sequences that could be dated, or rather be

    correlated with others, were between 30 and 80 years of age at both sites; none of the

    longer sequences were dateable. Thi...

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