Patterns of tree growth, manifest in annual tree-rings, will be similar between trees of the same species, growing in the same climatic region. These matching patterns align growth rings in different trees formed in the same year. Once aligned, crossdated, 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, of a tree cut to provide construction timber, was formed.
In order to determine the precise year in which a tree was cut it is essential that somewhere on the construction timber there still remains a portion of the last ring formed before felling. In the eastern US this is the most difficult challenge. High humidity, clayey soils, shallow depth to bedrock means many basements are chronically damp or wet. Moisture is essential for decay to develop and insects follow decay producing the physical damage that makes the wood frail, especially the outermost, sapwood, layers.
Location of typical construction timbers relative to their position the tree. Below, an enlargement of the last four rings formed before the tree was cut.
The key to successfully dating old buildings is to have a sequence of dated ring width patterns from extant, old, living trees that goes back and a bit before the year when the harvesting of trees for construction occurred. Crossdating, the skill of finding matching ring-width patterns between tree-ring samples, is used to assign the precise calendar year to every ring in a collection of living trees. The ability to find matching ring-width patterns in trees is directly related to the strength and influence of the common climate signal recorded in an individual trees ring-width patterns. In addition, it is also necessary to have enough rings from a sample to provide as many unique patterns as possible as the spacing between patterns is equally important to confirm a date. Depending on the circumstances of any individual tree's climate and life history a specimens may be dated with only 60 years of information, or may require 160. Once dated, the rings are measured and a master dating chronology of standardized ring-width measurements is built.
Visible agreement between samples and the damage made by powder-post beetles.
The final step is to find a match between the patterns in a master dating chronology and the ring-width patterns in sampled timbers. When a match is found, and this match can be verified, then the calendar years of the rings in the master chronology may transferred to their counterparts in the timbers.
The Mt. Wachusetts “living tree” master chronology, dating the timbers from the Rocky Hill meeting House in Amesbury MA, which were then able to date many other first period Boston area buildings, producing the original BeanTown Master Dating Chronology ( ITRDB submission ma003.rwl).