Dendrochronological Studies of Alder
(Alnus Glutinosa) on Scottish Crannogs
AOC Archaeology Group, UK
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
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
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
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
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
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
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
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
of the alder
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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
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. This tends to confirm the earlier work at Oakbank
and Erskine Bridge which found that, in general only sequences of 4060 years of
age were reliable (Crone, 1988: 158). The growth-pattern tends to become erratic in
the longer sequences, possibly because of factors like suppressed growth.
The major difference between the Buiston and Cults Loch 3 alders lies in the
morphology of the timbers themselves. At Buiston the samples displayed very little
asymmetry, and this is reflected in the high intra-stem correlations between the
measured radii. At Cults Loch 3 multiple piths and pronounced asymmetry of
growth was a common feature, again reflected in the intra-stem correlations,
noticeably lower than those from Buiston (Figure 8). Furthermore, many of the
Cults Loch 3 samples displayed a dramatic decrease in ring-width in their
outermost rings, to the extent that some could not be measured, but none of the
alders from Buiston displayed a similar growth pattern (Crone, 2000: 53). As seen
in the modern studies this pattern of increasingly suppressed growth generally
reflects stressed competitive conditions and can lead to cessation in growth. These
combined characteristics are suggestive of a differing growth structure. At Buiston
the alder probably came from dominant single-stemmed, maiden trees; indeed the
squared alder posts in two of the palisades were amongst the largest timbers
recorded on the crannog (Crone, 2000: 18). At Cults Loch 3 the alder probably
came from multi-stemmed trees with a more coppice-like form, the kind of
structure observed in carr woodland fringing lochs. These observations suggest
figure 8 Intra-stem correlations Buiston and Cults Loch 3.
STUDIES OF ALDER ON SCOTTISH CRANNOGS 31
that the structure of the parent tree may have a significant bearing on the
dendrochronological viability of alder.
Other factors may also have contributed to the successful analysis at Buiston. The
overall coherence of the assemblage, reflected by the strong internal correlations
between the components, suggests that all the timber was probably coming from the
same source, a woodland that was exploited over a relatively short period of three
decades, and may even have been managed (Crone, 2000: 54). The modern alder
stand on the shores of Loch Tay displayed a similar level of internal coherence,
which was attributed to the fact that it came from a single environmental niche
(Crone, 1988: 119). Conversely, assemblages displaying numerous small chron-
ologies, such as those from Cults Loch 3 and Oakbank, may reflect multiple
woodland sources. Assemblages with wood from multiple sources are always more
problematic, displaying poor internal correlation and hindering chronology
construction. This can also be the case with oak but the climatic signal is so strong
in oak growth that these problems can generally be overcome by direct correlation of
individual sequences against dated master chronologies. The work of Douda et al.
(2009) and others indicates that local environmental factors rather than climate are
more critical to the radial growth of alder and this would explain why we encounter
low correlations within alder assemblages from diverse woodland sources.
Finally, one other possible explanation for the difficulties encountered in
constructing an alder chronology at Cults Loch 3 must be explored. There may be
no correlation because there is no synchroneity amongst the alder sequences; this
would be the case if the intervals between felling episodes was greater than the
length of the sequences. For instance, felling episodes separated by 3040 years
would not be detected amongst sequences of similar span. However, this seems
improbable at Cults Loch 3; the dated oak chronology suggests that all the building
activity occurred within a half-century at most and within that time at least three
structures were built. One might therefore anticipate felling episodes associated with
the construction of the structures of between 1015 years at the very least. As all the
alder comes from deposits associated with the dated oak it seems unlikely that it was
felled at intervals which could not be detected with the length of sequences available.
Comparison of the alder assemblages from Buiston and Cults Loch 3 suggest that the
structure of the parent tree may be a significant factor in the successful dendro-dating
of the species, and that the presence of wood from multiple sources will always be
more problematic for alder dendrochronology than for oak. This latter factor will
only become apparent during analysis but it should be possible to scan samples prior
to analysis to determine whether they represent primarily multi-stemmed or single
stemmed structures. Those samples that display compressed growth in their outermost
rings should also be removed from further analysis. This may all sound a little negative
but the value of the results achieved at Buiston should encourage us to persevere in the
analysis of alder, not least because, in the absence of large assemblages of oak, alder
probably holds the key to chronological resolution on most Scottish crannogs.
32 ANNE CRONE
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Correspondence to: Anne Crone, Unit 7a, Edgefield Industrial Estate, Loanhead,Midlothian EH20 9SY, UK. Email: Anne.Crone@aocarchaeology.com
STUDIES OF ALDER ON SCOTTISH CRANNOGS 33