Dendroarchaeology

Dendroarchaeology is a term used for the study of vegetation remains, old buildings, artifacts, furniture, art and musical instruments using the techniques of dendrochronology (tree-ring dating). It refers to dendrochronological research of wood from the past regardless of its current physical context (in or above the soil). This form of dating is the most accurate and precise absolute dating method available to archaeologists, as the last ring that grew is the first year the tree could have been incorporated into an archaeological structure. Dendroarchaeology is a term used for the study of vegetation remains, old buildings, artifacts, furniture, art and musical instruments using the techniques of dendrochronology (tree-ring dating). It refers to dendrochronological research of wood from the past regardless of its current physical context (in or above the soil). This form of dating is the most accurate and precise absolute dating method available to archaeologists, as the last ring that grew is the first year the tree could have been incorporated into an archaeological structure. Tree-ring dating is useful in that it can contribute to 'chronometric', 'environmental', and 'behavioral' archaeological research. The utility of tree-ring dating in an environmental sense is the most applicable of the three in today's world. Tree rings can be used to 'reconstruct numerous environmental variables' such as 'temperature', 'precipitation', 'stream flow', 'drought society', 'fire frequency and intensity', 'insect infestation', 'atmospheric circulation patterns', among others. Tree ring laboratory scientists from Columbia University were some of the first to apply tree-ring dating to the colonial period, specifically architectural timbers in the eastern United States. For agencies like the National Park Service and other historical societies, Dr. Jacoby and Cook began dating historic structures in the lower Hudson River Valley, New Jersey and Eastern Pennsylvania. This was difficult at the time due to a lack of sufficiently long master dating chronology and access to suitable structures. Not until 1998 was a Boston area master dating chronology developed.Today, the effectiveness of tree ring laboratory archaeological dating chronologies covers most of the area that was settled by the first European colonists. The numbers of these are in the hundreds and include historically significant structures such as Independence Hall and the Tuckahoe estate. There are two types of dates that can be assigned to tree specimens: cutting dates, and noncutting dates. Which date is assigned to a specimen is dependent on whether or not there is evidence that the last ring present on the specimen was the last ring the tree grew before it died. Cutting dates can be used for crossdated tree specimens that 'possess evidence that the last ring present on the specimen was the last ring grown by the tree before it died.' Noncutting dates are used for crossdated tree specimens 'if there is no evidence indicating that the last ring present on the specimen is the last one grown before the tree died.' Patterns of tree growth will be similar between trees of the same species, growing in the same climate. These matching patterns align growth rings in different trees formed in the same year. Once aligned, knowing the precise calendar year of any individual tree-ring is the same as knowing the calendar year of all the rings. The goal of a dendroarchaeologist is to determine the year when the last ring was formed. Crossdating, the skill of finding matching ring-width patterns between tree-ring samples, is used to assign the precise calendar year to every ring. This is affected by the climate that the timber was in. It is also important to have enough rings to actually confirm a date. Once the rings are dates, the chronology is measured. The last step is to compare the rings with that of ring-width patterns in sampled timbers and a master dating chronology. For trees to be useful in archaeological analysis, they must 'produce annual growth rings that are uniform around the tree stem', they must 'live for decades and, preferably, centuries' and they 'must have been used extensively by humans either for habitation or fuel.' One of the problems with this evaluation is that it is possible under certain conditions for a tree to miss a growth-ring or produce two growth rings in a season. During extreme drought there can be insufficient growth of xylem to form a noticeable ring. Alternatively, if a defoliating agent (e.g. drought, late frost, or insect damage) can arrest the growth of a tree early in a year, after which there is a secondary growth period of new foliage causing two rings to form. Another difficulty in the use of tree-ring dating as applied to archaeology is the variety and condition of wood used in construction of archaeological sites. Many such samples are encountered wet. Heartwood can normally retain much of its substance and can be dried out and polished for analysis. On the other hand, ancient wet sapwood samples seldom survive drying out. As a result, the sapwood should either be measured wet and then allowed to dry, or it should be frozen or kept wet. In North America, 'millennial-length chronologies have been developed for two species of bristlecone pine (Pinus longaeva in the Great Basin and Pinus aristata in the Rocky Mountains), bald cypress (Taxodium distichum), coast redwood (Sequoia sempervirens), Douglas-fir (Pseudotsuga menziesii), eastern cedar (Juniperus virginiana), juniper (Juniperus sp.), Larch (Larix sp.), lodgepole pine (Pinus contorta), limber pine (Pinus flexilis), mountain hemlock (Tsuga mertensiana), ponderosa pine (Pinus ponderosa), and giant sequoia (Sequoiadendron giganteum) (Jacoby, 2000a).”