The width of tree rings varies with, among other things, temperature. They can be used to estimate temperature for times before thermometers were in widespread use. When cross sections are taken of trees there is a pattern of annular rings. The width of these rings is, in part, a function of temperature.
Other things which can affect ring width are:
- The age of the tree. The rate of growth varies through the life of the tree.
- Weather. In addition to temperature, ring growth is also affected by precipitation and to a lesser extent by wind speed and sunshine.
- Previous years. If a tree has grown vigorously in one year it is likely to grow vigorously in following years and vice versa.
- Atmosphere. Carbon dioxide is necessary to growth and increased levels of carbon dioxide can lead to enhanced growth.
- Competition. Other trees nearby or other plants can rob a particular tree of nutrients or light.
- Parasites. Infestation by insects or fungi can slow the growth of the tree.
To overcome the above, for temperature reconstruction the sites to be analysed are chosen so that these other factors have limited importance. For example trees might be chosen in areas where rainfall was plentiful so that water stress does not affect growth.
Even in well chosen sites it has to be recognised that ring width is not a uniform function of temperature but is biased toward the temperatures during the growing period. This is sometimes dealt with by analysing early and late growth separately.
It is not necessary to fell the tree first – normally samples are taken by boring into the tree with a hollow bit. Dozens of samples are normally taken from a group of trees.