The Science of Dendrochronology

Dendrochronology is the technique of counting tree rings to determine a tree’s age and measuring the width of these rings to determine characteristics of past climates. This might seem simple: each ring represents one year, and wider rings generally mean better growing conditions-plentiful rainfall, moderate temperatures, and so forth.

But the seasonal growth of a particular tree is affected by factors other than the weather. Trees vary, one from another, just like people do. The genetic makeup of each individual tree is unique, so one particular tree may grow a bit more quickly than another. Highly local conditions can also change over time. It is easy enough to see that [#highlight2]if part of the soil near a tree has been eroded, this will impact the tree’s root system and limit its growth, at least until the situation stabilizes.[/highlight2] Then again, an infestation of insects may affect a tree in one valley more than the same type of tree ten miles away. Or one tree [#highlight3]may suddenly start to get a lot more sunlight when an old, big tree in the neighborhood finally falls.[/highlight3]

These kinds of factors produce significant variations among individual specimens, and that fact means that researchers need to average together samples from many specimens of a single tree species in one region over the same time period. Some dendrochronologists think that measuring an average of twenty-five to thirty tree-ring records in a locale is an [#highlight4]essential[/highlight4] first step in getting around the problem of individual variability. While it may be easy enough to find thirty samples in some locations for particular periods, it obviously becomes less and less likely the more ancient the wood samples are.