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A dendrochronologist measures the creation of ever-widening circles of tree rings in the trunks of trees as a yearly growth phenomenon. Though this only occurs in some tree species and varies depending on the climate, it is seen as an accurate means of dating past events. It is a field of science often used in climatology studies by botanists and foresters, as well as in the field of human archeology.
Dendrochronologist duties include collecting many samples of tree ring data from specific climate regions and species of trees. These rings reflect the amount of growth that a tree undergoes during the standard growing season. This makes tree rings more prominent in trees that have dormant periods in the winter, and is not an effective method of dating for tropical species that don't live through distinct seasons.
To produce accurate dating results, samples of tree ring growth are taken from a region and graphed by computer to produce an accurate reference data set for determining changes in climate. This involves gathering samples of current tree ring growth and working back farther and farther in time towards the early stages of a tree's maturation, as well as gathering tree ring data from old timbers used in buildings and petrified wood or logs preserved from decay in peat bogs. Worldwide associations such as The International Tree-Ring Data Bank, which is maintained in the US by the National Oceanic and Atmospheric Administration (NOAA), use large databases of tree ring data based on stored physical samples to reconstruct changes in the past climate of various regions.
Researching the past in a dendrochronologist job can only be taken so far, however. The oldest living trees are Bristlecone pines, with an ability to survive for 5,000 years, and some living as long as 9,000 years within a narrow habitat range for the species throughout western North America. Extremely old specimens are rare and not used for tree-ring analysis, though preserved timbers from other tree species that were used in human construction can be traced back 4,000 to 5,000 years or more throughout the world. The Sheffield Dendrochronology Lab in the UK has created over 200 reference chronologies of tree ring data in a master sequence that extends back to 5,000 BC, and covers the medieval period in Europe as well.
The research field of dendrochronology has applications in the tracing of social and cultural patterns along with those of the natural sciences, as tree ring data is often taken from wooden artifacts. The rings can preserve the effects of air pollution on growth cycles as a byproduct of industrial processes. This makes the potential for a dendrochronologist job description a diverse one that can span historical research in many fields.
Natural patterns of change in botany, biology, or climatology, such as those caused by glacier movement, ice ages, volcanic activity, or changes in the availability of surface water and rainfall patterns for a particular region, all affect tree growth. Entomologists who focus on the study of insect populations also may have dendrochronologist duties, as certain insect species like the western spruce budworm can suppress tree growth. A dendrochronologist does not merely catalog effects in the past, however; a dendrochronologist is concerned with the long range view of changes recorded in tree rings so that the past can also be a key indicator of trends that may occur in the future.