DENDROCHRONOLOGY 387 DELTAS See SEDIMENTARY ENVIRONMENTS: Deltas DENDROCHRONOLOGY M Bridge, University College London, London, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction In recent decades tree-ring studies have opened up a whole new area of climatic and other environmental reconstructions at various geographical and temporal scales These generally go under the heading of dendrochronology – although some people reserve this term only for the use of tree rings in dating studies Dendrochronological dating relies on the fact that whilst each tree-ring series reflects peculiarities of the life history of the individual tree, trees of the same species growing at the same time over wide areas respond similarly to the weather experienced both during an individual growth period, and perhaps also to conditioning throughout previous seasons Not all species have clearly distinct ring boundaries, and those without them are of little use to dendrochronology Some species not only have clear rings, but may have clear divisions within the rings, such that the ring can be divided into the ‘earlywood’ which forms during the spring and early summer, and the ‘latewood’ which forms through the summer and early autumn Whilst many studies make use of the whole ring, in recent years many have focused on part of the ring, usually the latewood, which is usually more independent of the influences of previous growth in its variations Most dendrochronological work has been done in temperate zones, although some tropical trees have been used With historical timber, samples of wood from naturally preserved timbers e.g in river gravels, peat bogs and similar environments, usually take the form of cross-sections cut from trunk remains Historical wood in situations such as standing buildings is generally cored down the radius of the trunk Many of the studies outlined below, however, depend on samples from living, or recently felled, trees Living trees can be sampled by extracting radial cores, usually of the order of mm in diameter, taken using especially designed manually operated borers The tree ‘compartmentalizes’ the wound, sealing it off from the rest of the living tissue, and any damage to the tree is generally minimal During the late 1960s and the 1970s, the development of radiocarbon dating and the realization that the formation of radiocarbon in the atmosphere had not been constant through time were the stimuli for several laboratories to attempt to build long (multimillennial) tree-ring chronologies These provided wood samples of known calendar age with which to investigate changes in radiocarbon levels through time, and then to calibrate the radiocarbon time-scale This has made possible ‘wiggle matching’, which in suitable circumstances can provide more accurate radiocarbon dates As the chronologies were being constructed it was noticed that some periods had scarce wood remains which was thought to reflect environmental changes, either in the vigour of tree growth, or in the conditions that favour the preservation of the wood Once several long chronologies became available, it was noticed that certain periods showed common growth responses over wide geographical areas For example, a downturn in oak growth lasting about a decade after 1628 bc was reported in Irish bog-oaks This was subsequently found to occur at the same time in oaks growing in northern England and northern Germany This decade showed the lowest growth rates for many centuries in each locality, and must therefore reflect a large climatic forcing agent It was suggested that the most likely candidate for this particular period of growth reduction was the Santorini (Thera) volcanic eruption, often linked to changes in the Minoan civilization Accepting this as dating that particular event was controversial, but even if some were unable to accept this hypothesis, some major factor had to be responsible for the observed response Throughout the rapid growth of dendrochronology in the late twentieth century, many studies focused