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Tree rings grow under the bark, and the bark is pushed out while the tree is growing. The growth ring in furniture, walls and other wooden things can be counted, too.
These patterns can be compared and matched ring for ring with trees growing in the same geographical zone and under similar climatic conditions.
Following these tree-ring patterns from living trees back through time, chronologies can be built up, both for entire regions, and for sub-regions of the world.
The outer portion is the "late wood" (and has sometimes been termed "summer wood", often being produced in the summer, though sometimes in the autumn) and is denser.
"Early wood" is used in preference to "spring wood", as the latter term may not correspond to that time of year in climates where early wood is formed in the early summer (e.g.
) was developed during the first half of the 20th century originally by the astronomer A. Douglass, the founder of the Laboratory of Tree-Ring Research at the University of Arizona.
Douglass sought to better understand cycles of sunspot activity and reasoned (correctly) that changes in solar activity would affect climate patterns on earth which would subsequently be recorded by tree-ring growth patterns (, can be seen in a horizontal cross section cut through the trunk of a tree.
Many trees in places with hot summers and cold winters make one growth ring a year.
For the entire life of a tree, a year by year record or ring pattern is formed that reveals the climate conditions in which the tree grew.
The rings are more visible in temperate zones, where the seasons differ more markedly.