A tree's growth rate changes in a predictable pattern throughout the year in response to seasonal climate changes, resulting in visible growth rings.
Each ring marks a complete cycle of seasons, or one year, in the tree's life.
European chronologies derived from wooden structures initially found it difficult to bridge the gap in the 14th century when there was a building hiatus, which coincided with the Black Death, Given a sample of wood, the variation of the tree-ring growths provides not only a match by year, it can also match location because the climate across a continent is not consistent.
This makes it possible to determine the source of ships as well as smaller artifacts made from wood but which were transported long distances, such as panels for paintings and ship timbers.
The techniques of dendrochronology are more consistent in areas where trees grew in marginal conditions such as aridity or semi-aridity where the ring growth is more sensitive to the environment, rather than in humid areas where tree-ring growth is more uniform (complacent).
In addition, some genera of trees are more suitable than others for this type of analysis.
Adequate moisture and a long growing season result in a wide ring, while a drought year may result in a very narrow one.
Direct reading of tree ring chronologies is a learned science, for several reasons.
In addition, particular tree species may present "missing rings", and this influences the selection of trees for study of long time spans.These patterns can be compared and matched ring for ring with trees growing at the same time in the same geographical zone (and therefore under similar climatic conditions).When these tree-ring patterns are carried back, from tree to tree in the same locale, in overlapping fashion, chronologies can be built up—both for entire geographical regions and sub-regions.Douglass sought to better understand cycles of sunspot activity and reasoned that changes in solar activity would affect climate patterns on earth, which would subsequently be recorded by tree-ring growth patterns (i.e., sunspots → climate → tree rings).Diagram of secondary growth in a tree showing idealised vertical and horizontal sections.