Dendrochronology - or the history and uses of tree rings

History
The first document currently known to discuss tree rings was that by Theophrastus in Greece around 322BC. He wrote 9 volumes regarding the History of Plants and mentioned the growth rings in two types of fir trees -  Theophrastus is incidentally frequently quoted as 'The father of botany'.

The discipline of dendrochronology was not formed until around 1928 by Andrew Ellicott Douglass - incidentally frequently quoted as 'The father of dendrochronology'. He was actually an astronomer and found a correlation between tree-rings and the sunspot cycle. The word dendrochronology is a word compounded using three Greek words:

dendron = tree

chronos = time
logy    = the study of

Tree rings

Tree rings are produced by the annual growth of new rings of xylem tissue, which is the outer layer of sapwood. As the xylem matures it dies and becomes part of the heart wood of the tree.

The thickness of the xylem layer is dependent on how favourable the growing conditions were during the year of growth. Warm temperatures and good rainfall are conducive to a wide annual ring, whereas cold and dry conditions will result in a narrow ring. Given this, we can see that growth rings occur best in temperature regions. It is important to note that if there are favourable and unfavourable conditions, for example, a good spring followed by a summer drought and a good autumn - then multiple rings can be produced within a year.

Some trees can live up to a thousand years, for instance, Giant sequoia (Sequoiadendron giganteum) , Coast redwood (Sequoia sempervirens), Alaska yellow-cedar (Chamaecyparis nootkatensis), Douglas-fir (Pseudotsuga menziesii), among many others. These trees can provide a wide knowledge of regional climate by working from the outer rings to the inner rings - effectively working backwards in time - but any tree older than a few decades can provide useful data. The rings from multiple trees can be plotted to see overlaps in tree rings from older wood. This method can establish a continuous history for up to 7000 years. Cores can even be drilled from living trees, similar, in a way, to the drilling of ice cores.

Uses

This science of dating wood by analyzing the growth ring pattern, which is used in many fields including dendro-

• archeology - dating of dwellings
• climatology - record of past climates
• hydrology - past water availability
• glaciology - past movements of glaciers
• pyrochronology - past forest fires
• entomology - past populations of levels of insects