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, 22–33

� 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

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. 60–80 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.

24 ANNE CRONE

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. Elling’s 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 800–400 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 500–465 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.

26 ANNE CRONE

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

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 Elling’s

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.

28 ANNE CRONE

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

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.

30 ANNE CRONE

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 40–60 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 30–40 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 10–15 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.

Conclusions

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