Only a thin outer skin beneath the bark remains alive. A radial sample from the center, or pith, of a tree to the bark will contain a complete series of growth layers making it possible to determine the age of the plant at that height.
The longest record, and the age of the tree, can only be obtained at the base of the tree.
(7) In buildings with a history of repair or construction with re-used material the details of construction, milling or shaping, and juxtaposition of timbers should be examined and noted at the time of sampling.
Ideally as many samples as possible should be obtained, each with a large number of rings and the bark edge present, and each a complete cross section of the timber.
(4) Sufficient number of rings present in a sample for the pattern to be uniquely identified as belonging to a particular period of time, based on comparison with the known standard.
Number 4 is variable depending on the strength of the common growth pattern.
Some of these are available through the International Tree-Ring Data Bank maintained by NOAA (the National Oceanic and Atmospheric Administration) and are available on the World Wide Web.
Sampling historical structures to provide tree-ring dates In order to date the construction of a building with wooden elements the following requirements must be met: (1) Species used in construction must be suitable for dendrochronological analysis (spruce, hemlock, oak, or ash are some examples) (2) Local timber must have been used, or the place of origin of the wooden elements must be known.(3) Samples with an intact bark surface must be obtained (these samples document the year and season of harvest).Milling by sawing or shaping with an ax or adz will remove the outer rings from some or all portions of a timber or board.(4) The samples must have a sufficient number of rings to provide a reliable date.Additionally: (5) Local construction practices including typical season of harvest and the length of the milling and storage process known (this appears vary from a few months to two years for 19th century Vermont).(6) Known history of the structure and area should be noted.Growth patterns useful for dendrochronological dating are produced by yearly variations in climate that favor or disfavor the growth of trees in a region in common with each other.