Dendrochronology, or tree-ring dating, is deceptively simple. Some species of trees add their annual growth increments in two parts: ‘spring wood’ and then ‘summer wood’ cells, so that, when seen on the end-grain, they look like ‘rings’: hence the term. When trees in a given climatic region are similarly affected by yearly changes in the climate (as they are throughout most of the Byzantine world), their rings can be matched (‘crossdated’) with one another so that a given ring can be assigned to a specific calendar year. Sometimes a felling time within a year can be identified. Dendrochronology is the only form of archaeometric dating with this kind of annual or sub-annual resolution. The method works only with species having clear, annual growth rings, and, since the vast majority (99 per cent of monuments where any wood is preserved) of Byzantine and meta-Byzantine buildings were built with oak, this immediately makes tree-ring dating feasible for the Byzantinist. Species in which the annual ring-boundaries are non-existent or indistinct, for example, olive, willow, poplar, and most fruit or orchard trees (whose ring-growth may reflect merely the assiduity or the laziness of the gardener), cannot be crossdated. See Kuniholm 2001 for further discussion and bibliographic references, also Grissino-Mayer 1993 for a list of species which can be crossdated.
Crossdating is the fundamental principle upon which all dendrochronology is based. The researcher has to be assured that rings from two or more specimens were formed in the same year. Simple ring-counts are not sufficient. Neither is a single pattern of co-variation in ring-width (a ‘signature’). In order to avoid the possibility of an accidental (but spurious) ‘match’ dendrochronologists try to compare samples which have at least 100 rings and multiple signatures rather than shorter-lived specimens which may not preserve enough signatures to guarantee the fit. These ring-patterns may be generated by a wide variety of causes (see Schweingruber 1988; Cook and Kairiukstis 1990; Eckstein 1972). The ring-patterns which are most
Usually crossdatable are the trees’ mutual response to some climatic stimulus; in some regions principally rainfall or lack of it; in others principally temperature; in yet others some combination of the two. For the Byzantine world April-May-June rainfall dominates all other stimuli (Hughes and others 2002; Griggs and others
2007). This stimulus-and-response is therefore specific to a climatic region: that is, the south-western USA, the extreme northern timber-line (>~6o°N), northern Europe, the eastern Mediterranean, etc. The climatic boundaries for crossdating have been best determined, in practice, by trial and error. Sometimes they fit the map, sometimes not, and then an explanation for the apparent anomaly must be sought. Wood cut from a forest site in Calabria in southern Italy, for example, crossdates with wood from Greece and Turkey, but it does not crossdate with wood from Spain, or over the Alps, or even Sicily. The first two non-fits are no surprise, but the non-fit with Mt Etna in Sicily, only 80 km away, is, and therefore requires explanation. Sicily appears to belong more to the North African climate system rather than to that of the central/eastern Mediterranean. Similarly, wood from the Black Sea coast of Turkey (the Pontos) does not crossdate with wood from central and western Europe, although forthcoming work in Romania and Bulgaria may help join the chronologies.
Caveats to the dendrochronological method include:
(i) the possibility of reused wood: for example, the Arizona mesas, where wood cut in pre-Columbian times is still in use today (for comments on dendrochronological interpretation see Bannister 1963);
(ii) changing habits of users of wood: for example. Renaissance painters in different centuries tended to let their panels dry out for two, to five, to eight, to ten years before painting on them (see Klein references); for architectural timbers, however, the Byzantine and Ottoman practice seems to have been for the carpenters to cut the wood and use it almost immediately;
(iii) heavily trimmed wood: for example, cut boards or musical instruments;
(iv) wood imported from some other climatic region: Abies (fir) at Herculaneum imported from the Alps, or Quercus (oak) supports for panel paintings in England and the Low Countries which were imported as cut boards from the Baltic (all Klein refs.; Kuniholm 2002; Kuniholm and others 2007);
(v) wood which is so badly degraded that its ring - and cell-structures are not preserved;
(vi) ‘complacent’ ring-sequences: that is, little or no significant change from year to year;
(vii) wood that has such erratic ring-sequences that they appear to fit in more than one place;
(viii) and no wood preserved at all, for example, the Baths of Caracalla or Diocletian in Rome with their hundreds of empty beam-holes.
Lest this long list of caveats seems discouraging, as an addendum to (ii) above, we note the following:
Inscriptional date 1801
1787
1754
1727
1597 spring 1535
Dendrochronological date 1800 winter
1786 winter 1753 winter 1727
1597 spring 1535
Monument
Thessalonike, Moni Vlatadon, roof repair
Ambelakia, Schwartz House Siatista, Nerandzopoulou House Thessalonike, Nea Panaghia Thessalonike, Frourio Vardari Thessalonike, White Tower
Clearly the woodcutters for these buildings must have been following Vitruvius’ dictum (whether they had heard of Vitruvius or not) that one should always use one’s wood fresh while it was still easy to cut.
The standard cautions that govern an archaeologist’s activities in the field apply to dendrochronology as well. One of the reasons for the success of the dendrochronological method has been the history of regular interaction between the archaeologist in the field and the worker in the laboratory. Beware of singleton samples, wood from uncertain contexts, wood that shows signs of reuse, indications of repairs, the wrong kind of nails, traces of machine-sawing where one might expect only axe and adze-marks, etc. For other cautions see appendix III in Baillie 1982.
World History
