From: D.J. Keenan
To: Nicola Bell
Sent: 24 November 2008 12:15
Subject: Re: Your FOI request to QUB Ref: FS50163282

Dear Ms. Bell,
I did not find a good, brief, summary of tree-ring science on the internet.  Below is something short that I wrote a while ago; I hope it will be useful.
Douglas Keenan


Most trees grow a tree ring each year. The thickness of tree rings varies from year to year, and is dependent upon the local climate, ecology, and other factors. Trees live for many years, and the tree rings grown over those years form patterns of thick and thin rings. The figure below displays cross-sections from three trees. Imagine that the A rings are from a living tree. Hence the outermost ring (next to the bark) was grown in the last year, the ring next to that was grown the year before, etc. Imagine too that the B rings are from a dead tree, which was found in a field. We do not know a priori the years in which this tree grew, but by matching the outer rings from B with the inner rings from A, we can determine this. Suppose the C rings are from a timber used in a building. The outer rings from C can be matched with the inner rings from B; thus we can determine when the C tree grew. The building must have been constructed after the C tree grew. Hence our tree-ring matching has given us information about the time of the building’s construction.

Figure 1.   Schematic example of tree-ring matching.

Figure 1.   Schematic example of tree-ring matching.

An important goal in tree-ring studies is to build up overlapping tree-ring sequences, extending from the present to the distant past—the figure illustrates. Usually, there will be several trees that grew rings for a particular year; an average ring-width for each year is then calculated. A series of such average ring widths that spans many years is called a “master dendrochronology” for the site at which the trees grew (from Greek, dendron = tree and chronos = time). Constructing a master dendrochronology for a site is essential for tree-ring dating of wooden artefacts from the site and surrounding area.

The field of tree-ring dating (usually called dendrochronology) has two main areas of application. One is to date wood from archaeological sites, as exemplified above with tree C.  The other is to find out about past climates.  Because the thickness rings indicates whether the climate was good or bad for the tree in a given year, the rings tell something about the climate in past years.  Trees from around the world have been studied to give estimates of past climate over large areas of Earth.  In this way, tree rings have been useful in studies of global warming, comparing temperatures of today with temperatures from centuries ago.